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Sequential leaching of metals from spent re1047297nery catalyst in bioleachingndashbioleaching and bioleachingndashchemical leaching reactorComparative study

Haragobinda Srichandan ab Ashish Pathak a Sradhanjali Singh a Kyle Blight cDong-Jin Kim a Seoung Won Lee b

a Mineral Resource Research Division Korea Institute of Geoscience and Mineral Resources (KIGAM) Gwahang-no 124 Yuseong-gu Daejeon 305-350 South Koreab Nano Engineering Division School of Engineering Chungnam National University Daejeon 305-764 South Koreac Chemistry Department Murdoch University South Street Murdoch 6150 Western Australia Australia

a b s t r a c ta r t i c l e i n f o

Article history

Received 22 January 2014

Received in revised form 4 August 2014

Accepted 28 September 2014

Available online 12 October 2014

Keywords

Acidithiobacilli

Alkali leaching

Bioleaching

Spent catalyst

The effect of sequential leaching such as bioleaching followed by bioleaching and bioleaching followed by chem-

ical leaching is aimed at enhancing metal (Mo Ni V and Al) dissolution from a differently pretreated (acetone

washeddecoked) spent catalyst The X-ray photoelectron spectroscopy characterization of spent catalyst sam-

ples suggested the presence of metals in their oxide and sul1047297de forms Bioleaching followed by bioleaching

with either Acidithiobacillus thiooxidans (Ni-100 Al-55 Mo-81 and V-100) or Acidithiobacillus ferrooxidans

(Ni-94 Al-55 Mo-77 and V-99) signi1047297cantly enhanced removal of Al Ni and V from acetone washed

(AS) spent catalyst compared to decoked spent catalyst (RS) In contrast bioleaching using either

A thiooxidans or A ferrooxidans followed by alkali leaching remarkably enhanced removal of Mo from both AS

and RS although higher yields were achieved using AS Bioleaching using A thiooxidans followed by alkaline

leaching isan optimumstrategy yielding a maximumof 96Mo in 125h from ASA 1047297eld emissionscanning elec-

tron microscopic study revealed only minor stretches of Mo in the treated AS

copy 2014 Elsevier BV All rights reserved

1 Introduction

The petroleum re1047297nery industry use huge quantities of solid catalysts

to convert crude oil into useful products Treating crude oil via reforming

hydrocracking hydrotreating catalytic cracking and alkylation produces

valuable oil products such as fuel oil gas oil kerosene jet fuel gasoline

and naphtha (Furimsky 1996) Catalyst activity decreases with time

and hence they are reactivated reused and 1047297nally disposed of as waste

materials These waste materials are referred as ldquospent catalystrdquo which

contain different metals such as Al Co Fe Mo Ni and V The metals in

the spent catalyst are present in the form of metal ions metal oxides

and metal sul1047297des (Mara1047297 and Stanislaus 2003) Spent catalyst has

been categorized as hazardous by the USEPA and cannot be disposed of

without treatment Various hydrometallurgical processes have been

used to remove metal from spent catalyst The hydrometallurgical pro-

cesses have used high concentration of acid (8 M H2SO4) and alkali

(4 M NaOH) for removing metals from spent catalyst (Ognyanova et al

2009 Park et al 2007) Although hydrometallurgical processes have

shown reasonable metals extraction ef 1047297ciencies the use of high strength

acids and alkali secondary pollution and expensive downstream

processing has restricted their usage on a larger scale Different pyromet-

allurgical techniques such as smelting calcination anhydrous chlorina-

tion have also been employed to recover metals from spent catalyst

(Kar et al 2005) However pyrometallurgical processes suffer with the

use of high energy consumption and emit toxic gases into the atmo-

sphere Therefore the use of relatively benign bio-hydrometallurgical

processes for recovery of metalsfrom spent petroleum catalysts is gaining

attention

Bioleaching has emerged as an ef 1047297cient eco-friendly and cost effec-

tive process to recover metals form spent catalyst Bioleaching is based

on the metabolic activity of various chemoautotrophic bacteria

( Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans) and het-

erotrophic fungi (Aspergilus niger) Various bioleaching studies have

used A ferrooxidans and A thiooxidans to recover metals from spent cat-

alyst (Gholami et al 2011 Pradhan et al 2010) The effects of various

process parameters such as substrate concentration pulp density parti-

cle size and pH on bioleaching have been examined (Kim et al 2010)

The ef 1047297cacy of bioleaching has also been examined using different

types of spent catalysts such as vanadium-rich spent catalyst and

spent nickel oxide catalyst (Mishra et al 2007 Mulak et al 2005)

Treatment of spent catalyst prior to bioleaching is more ef 1047297cacious

than untreated catalyst in terms of metal leaching (Bharadwaj and

Ting 2013)

Hydrometallurgy 150 (2014) 130ndash143

Corresponding author Tel +82 42 8683592 fax +82 42 8683415

E-mail address djkimkigamrekr (D-J Kim)

httpdxdoiorg101016jhydromet201409019

0304-386Xcopy 2014 Elsevier BV All rights reserved

Contents lists available at ScienceDirect

Hydrometallurgy

j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e h y d r o m e t

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Although these studies suggest thepotential of bioleaching for metal

recovery from petroleum re1047297nery spent catalysts the longer reaction

time (up to 70 days) is still a major hindrance to apply the process on

a large scale (Santhiya and Ting 2005) Moreover single stage

bioleaching alone is insuf 1047297cient to remove all Mo present in spent cata-

lyst (Mishra et al 2009 Pradhan et al 2010) due to either the limited

solubility of Mo in a weekly acidic solution or refractory nature of

MoS2 or formation of product layer on the Mo matrix or due to com-

bined effect of all Therefore a second treatment step is required to re-move the remaining Mo present in the bioleach residue Our previous

studysuggested that a sequential reactor employing bioleaching follow-

ed by alkaline leaching signi1047297cantly improves Mo leaching yields from

spent catalyst (Pradhan et al 2013) However that study was conduct-

ed using only A ferrooxidans as a leaching microorganism in the 1047297rst

stage and acetone washed spent catalyst as a feed material

Besides A ferrooxidans another acidophile A thiooxidans is also ca-

pable of leaching metals from spent re1047297nery catalyst using sulfur as an

energy source The use of A thiooxidans during bioleaching can be advan-

tageous due to the comparatively low cost of sulfur Furthermore the

type of pretreatment (decokingroasting) of spent catalyst also signi1047297-

cantly impacts ef 1047297ciency of the bioleaching process (Bharadwaj and

Ting 2013) and the ef 1047297cacy of the process need to be tested using differ-

ently pretreated spent catalyst (decokedorganic washed spent catalyst)

Moreover the ef 1047297ciency of bioleaching using either A ferrooxidans or

A thiooxidans in the second stage has not been documented or imple-

mented in previous studies Therefore more in-depth sequential reactor

studies using differently pretreated spent catalysts and microorganisms

are required to develop an ef 1047297cient and economical process for leaching

metals from spent re1047297nery catalyst

The aim of this study wasto develop a robust sequential bioleaching

process for enhanced Mo solubilization along with other metals over a

shorter duration Different sequential strategies such as bioleaching

followed by bioleaching and bioleaching followed by alkali leaching

were evaluated using different pretreated spent catalyst samples and

by using different leaching microorganisms In the 1047297rst stage two

types of pretreated spent catalysts (acetone washed and decoked)

were separately bioleached using sulfur ( A thiooxidans) and both sulfur

and iron-oxidizing ( A ferrooxidans) microorganisms The bioleachedresidues obtained from the 1047297rst stage were further subjected to second

stage bioleaching as well as alkali leaching separately to identify the op-

timum strategy in terms of Mo recovery along with other metals

2 Materials and methods

21 Spent petroleum catalyst pretreatment and characterization

The spent catalyst used was received in bulk from a petroleum re1047297n-

ery company located in South Korea The raw spent catalyst (SR) was

coated with oily matter and was subjected to acetone washing in a

Soxhlet apparatus followed by drying in a hot air oven Similarly anoth-

er SR was decoked in a muf 1047298e furnace for 5 h at 500 degC in the presence

of atmospheric oxygen The dried acetone washed (AS) and decoked(RS) spent catalysts were ground separately using a vibrating cup mill

(Fritsh Darmstadt Germany) Particle size distributionwas determined

by using Malvern Laser Mastersizer The particle size distribution for RS

was 0025ndash125 μ m The particle size distribution for AS was 1ndash200 μ m

pH was measured using an Orion portable pH meter whereas redox po-

tential (ORP) was measured using a platinum electrodewith an AgAgCl

reference electrode Ferrous (Fe2+) ion was estimated using the 110-

phenethroline-spectrophotometry method at 510 nm The sulfur and

carbon content was analyzed with a LECO CS-600 analyzer The surface

topography of SR AS RS and the 1047297nal leach residues was examined

using 1047297eld emission scanning electron microscopy (FESEM model Ma-

gellan 400) Metal content (Ni Al Mo and V) of the spent catalyst was

determined by induced couple plasma optical emission spectroscopy

(PerkinElmer Optima 8000 Waltham MA USA) The chemical and

valence states of themetal sul1047297des andoxidesin thespent catalyst sam-

ples were determined using X-ray photoelectron spectroscopy (XPS

Thermo Scienti1047297c model-Sigma probe Indianapolis IN USA) at a

beam voltage of 15 kV and a beam current of 67 mA Monochromatic

AlKα (14867 eV) X-ray radiation was allowed to fall at 30deg on a pow-

dered sample placed on carbon tape The emitted electrons from the

sample were detected by an analyzer placed at an angle of 40deg

22 Microorganism and growth conditions

Pure strains of A ferrooxidans and A thiooxidans were obtained from

the Korea Research Institute of Bioscience and Biotechnology Culture

Collection Center A ferrooxidans was grown in IEM medium (Blight

and Ralph 2004) and provided with 20 g L minus1 of FeSO47H2O The 1047297nal

pH of the medium was adjusted to 168 plusmn 005 using concentrated

H2SO4 A ferrooxidans was allowed to grow in the nutrient medium

leading to complete oxidation of ferrous iron After complete oxidation

of ferrous to ferric ions the growth medium was passed through a

045 μ m membrane 1047297lter to separate the cells After 1047297ltration the cells

were inoculated in a batch reactor for further growth

A thiooxidans was grown in 0 K (9 K medium without 9 g of Fe2+)

medium composed of (NH4)2SO4 (3 g) KCl (01 g) K2HPO4 (05 g)

MgSO47H2O (05 g) Ca(NO3)2 (001 g) dissolved in distilled water at

1047297nal volume of 1 L Final pH was adjusted to 33 plusmn 005 using concen-

trated H2SO4 1 (wv) of S0 was added as an energy source to this me-

dium Due to the oxidation of S0 into sulfuric acid pH of the growth

medium was decreased to 14 The cells during this log growth phase

were separated using a 045 μ m membrane1047297lterand inoculated for fur-

ther growth

23 First stage bioleaching

Experiments were carried out in stirred tank batch reactors (25 L)

All experiments were performed under controlled environmental con-

ditions initial pH 14 plusmn 005 stirring speed 250 rpm temp 35 degC

working volume 1 L Continuous air was supplied to all reactors at a1047298ow rate of 1 LPM to ensure homogenous mixing of the bioleaching

pulp A schematic representation of different strategies employed is

provided in thegraphical abstract Prior to the addingthe spent catalyst

A ferrooxidans cells were suspended in fresh IEM medium supplement-

ed with 1 S0 (wv) and 4 g L minus1 Fe2+ at pH 168 After addition of

A ferrooxidans the planktonic cell count in the reactors was found to

be 9 times 107 mL minus1 A ferrooxidans is capable of oxidizing both S0 and

Fe2+ and generate sulfuric acid and ferric iron as lixiviants during

bioleaching Therefore to utilize the potential of these lixiviants (sulfu-

ric acid and ferric iron) both S0 and Fe2+ were provided as energy

sources during bioleaching When all Fe2+ oxidized to Fe3+ and the

pH of the medium decreased to 14 as a result of oxidation of sulfur to

sulfuric acid AS and RS were (1 wv) added to separate reactors

Similarly in the case of bioleaching with A thiooxidans bacteria weresuspended in 0 K medium supplemented with 1 (wv) S0 at pH 33 plusmn

005 After addition of A thiooxidans the planktonic cell count in these re-

actors was found to be 3 times 107 mL minus1 A thiooxidans can oxidize only S0

and hence S0 was provided as a sole energy source for its growth during

bioleaching experiments When the pH of the medium decreased to 14

AS and RS (1 wv) were added to separate reactors The changes in

pH redox potential and metal solubilization were monitored over time

Two separate control reactors (without cells and sulfur) were also used

by adding AS and RS (pH 14) under similar operating conditions The

changes in pH redox potential and metal solubilization were monitored

over a period of 120 h and samples were withdrawn every 20h The sam-

ples were centrifuged and theliquid was analyzed for metal content The

leaching yield of thedesired element wascalculated based on both the el-

emental content of the feed and the leach liquor

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24 Second stage bioleaching

During 1047297rst stage bioleaching with A thiooxidans and A ferrooxidans

the solubilization of Mo was low for both AS and RS Therefore recovery

of the remaining Mo was the main objective of the second stage After

completing the 1047297rst stage bioleaching the bioleached pulps were

1047297ltered and the solid residues were separately divided into two equal

parts One part was further subjected to second stage bioleaching under

similar operating conditions using A thiooxidans and A ferrooxidans Sim-ilarly the leach residues (AS and RS) obtained from 1047297rst stage control

leaching were also subjected to second stage control leaching

25 Second stage alkali leaching

After completing 1047297rst stage bioleaching the second part of the

bioleach residues was further subjected to second stage alkali leaching

for 5 h (pH 120) The optimum pH for alkali leaching was selected on

laboratory leaching experiments (results not shown) conducted at dif-

ferent alkaline pHs (80ndash130) using 2 M NaOH

26 Sequential extraction study

The study on different binding forms or fractions of the metals was per-

formed in the feed AS and RS A sequential extraction procedure proposed

by Bureau of Community Reference (BCR) was used to determine the dif-

ferent binding forms of the metals (Ni Al Mo and V) present in the

spent catalyst (Ure et al 1993) The BCR process has been successfully ap-

plied to identify the different binding forms of the metals in a variety of

wastes including spent re1047297nery catalyst (Farrell and Jones 2009 Fuentes

et al 2004Pathaket al 2014 Smedaand Zyrcniki 2002) The BCRprocess

identi1047297es thedifferent binding forms of themetals in 4 broad fractionsThe

detailed information about the BCR analysis has been provided in our ear-

lier study conducted with spent re1047297nery catalyst (Pathak et al 2014)

3 Results

31 Spent catalyst characterization

311 Chemical composition analysis

The chemical composition of the spent catalyst suggested that car-

bon (C) content was high in the SR (2280) whereas signi1047297cant C

was removed from the RS (148) and AS (983) The removal of C in-

creased thedensity of RSwhich in turn increasedmetalconcentrationsof RS(397Ni 2206Al 282 Moand 1430V) comparedto thosein

the AS (331 Ni 2056 Al 258 Mo and 1140 V) and SR (270 Ni

1531 Al 234 Mo and 876 V)

312 Surface topography and elemental mapping by FESEM

The FESEM analysis of the spent catalyst samples is presented in

Fig 1 The FESEM topographs of only two AS residues (bioleaching

followed by second stage bioleaching with A thiooxidans and

bioleaching with A thiooxidans followed by second stage alkali

leaching) obtained after second stage treatment are presented because

these two processeswere comparatively more effective for leaching the

metals In Fig 1andashd the black and white image shows the topography

and the color images represent Al Mo Ni O S and V in continuous

order The presence of all six elements was quite evident in AS

(Fig 1a) and RS (Fig 1b) A marked difference was observed in the

FESEM topographs of the AS and AS residues Ni and V were absent in

the residue samples due to almost complete leaching of these metals

(90ndash99) during the process Only minor stretches of Mo were ob-

served in the residue obtained after bioleaching followed by alkaline

leaching due to almost complete leaching of this metal As leaching of

Al was signi1047297cantly lower no signi1047297cant decrease in its intensity was

observed in the two residues Sulfur intensity was consistent in the AS

RS and residues The similar sulfur intensity in the residues compared

to feed after thetwo stage treatmentwas dueto externaladdition of sul-

fur during the treatment process

Fig 1 Field emission scanning electron microscopy images of the (a) AS (b) RS (c) AS residue after second stage bioleaching using A thiooxidans (d) AS residue after bioleaching using

A thiooxidans followed by alkali leaching

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Fig 2 a X-ray photoelectron spectroscopy analysis of C Al and V b X-ray photoelectron spectroscopy analysis of Ni and Mo c Binding forms of metals in feed AS and RS

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24 Second stage bioleaching

During 1047297rst stage bioleaching with A thiooxidans and A ferrooxidans

the solubilization of Mo was low for both AS and RS Therefore recovery

of the remaining Mo was the main objective of the second stage After

completing the 1047297rst stage bioleaching the bioleached pulps were

1047297ltered and the solid residues were separately divided into two equal

parts One part was further subjected to second stage bioleaching under

similar operating conditions using A thiooxidans and A ferrooxidans Sim-ilarly the leach residues (AS and RS) obtained from 1047297rst stage control

leaching were also subjected to second stage control leaching

25 Second stage alkali leaching

After completing 1047297rst stage bioleaching the second part of the

bioleach residues was further subjected to second stage alkali leaching

for 5 h (pH 120) The optimum pH for alkali leaching was selected on

laboratory leaching experiments (results not shown) conducted at dif-

ferent alkaline pHs (80ndash130) using 2 M NaOH

26 Sequential extraction study

The study on different binding forms or fractions of the metals was per-

formed in the feed AS and RS A sequential extraction procedure proposed

by Bureau of Community Reference (BCR) was used to determine the dif-

ferent binding forms of the metals (Ni Al Mo and V) present in the

spent catalyst (Ure et al 1993) The BCR process has been successfully ap-

plied to identify the different binding forms of the metals in a variety of

wastes including spent re1047297nery catalyst (Farrell and Jones 2009 Fuentes

et al 2004Pathaket al 2014 Smedaand Zyrcniki 2002) The BCRprocess

identi1047297es thedifferent binding forms of themetals in 4 broad fractionsThe

detailed information about the BCR analysis has been provided in our ear-

lier study conducted with spent re1047297nery catalyst (Pathak et al 2014)

3 Results

31 Spent catalyst characterization

311 Chemical composition analysis

The chemical composition of the spent catalyst suggested that car-

bon (C) content was high in the SR (2280) whereas signi1047297cant C

was removed from the RS (148) and AS (983) The removal of C in-

creased thedensity of RSwhich in turn increasedmetalconcentrationsof RS(397Ni 2206Al 282 Moand 1430V) comparedto thosein

the AS (331 Ni 2056 Al 258 Mo and 1140 V) and SR (270 Ni

1531 Al 234 Mo and 876 V)

312 Surface topography and elemental mapping by FESEM

The FESEM analysis of the spent catalyst samples is presented in

Fig 1 The FESEM topographs of only two AS residues (bioleaching

followed by second stage bioleaching with A thiooxidans and

bioleaching with A thiooxidans followed by second stage alkali

leaching) obtained after second stage treatment are presented because

these two processeswere comparatively more effective for leaching the

metals In Fig 1andashd the black and white image shows the topography

and the color images represent Al Mo Ni O S and V in continuous

order The presence of all six elements was quite evident in AS

(Fig 1a) and RS (Fig 1b) A marked difference was observed in the

FESEM topographs of the AS and AS residues Ni and V were absent in

the residue samples due to almost complete leaching of these metals

(90ndash99) during the process Only minor stretches of Mo were ob-

served in the residue obtained after bioleaching followed by alkaline

leaching due to almost complete leaching of this metal As leaching of

Al was signi1047297cantly lower no signi1047297cant decrease in its intensity was

observed in the two residues Sulfur intensity was consistent in the AS

RS and residues The similar sulfur intensity in the residues compared

to feed after thetwo stage treatmentwas dueto externaladdition of sul-

fur during the treatment process

Fig 1 Field emission scanning electron microscopy images of the (a) AS (b) RS (c) AS residue after second stage bioleaching using A thiooxidans (d) AS residue after bioleaching using

A thiooxidans followed by alkali leaching

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Fig 2 a X-ray photoelectron spectroscopy analysis of C Al and V b X-ray photoelectron spectroscopy analysis of Ni and Mo c Binding forms of metals in feed AS and RS

133H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143

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Fig 2 a X-ray photoelectron spectroscopy analysis of C Al and V b X-ray photoelectron spectroscopy analysis of Ni and Mo c Binding forms of metals in feed AS and RS

133H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143

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313 XPS analysis of raw feed and residue samples

The XPS analysis for the elements C Al and V has been presented in

the Fig 2a whereas for Ni and Mo has been presented in the Fig 2b

Similar to FESEM XPS analysis of two AS residues has been presentedThe presence of different metal compounds was identi1047297ed based on

their binding energies (eV) For SR AS and RS the presence of all

above 1047297ve elements was con1047297rmed XPS analysis of C1s spectra con-

1047297rmed the presence of C (binding energy of 284 eV) in SR AS and AS

residues As compared to SR signi1047297cant decrease in peak intensity and

peakareawas observed withAS This was dueto thesigni1047297cant removal

of oily organic carbon during pretreatment with acetone The peak in-

tensity and area was almost similar when compared between AS and

AS residues This is because the remaining carbon in AS was insoluble

and hence did not leach during two step treatments In RS no peak

was observed due to the removal of most of carbon during decoking

Analysis of Ni2p spectra in SR con1047297rmed the presence of NiO

(8554 eV) and Ni2O3 (856 eV) A seconds peak of NiO (862 eV) was

also found in AS and RS which may have been due to the removal of

coated oily matter during the pretreatment In both the residues almost

no peak of Ni was observed This is because during the two stage treat-

ment almost complete leaching of Ni (99ndash100) was obtained and

hence was absent in the residues The analysis of Al spectra con1047297rmedthe presence of Al in SR AS RS and residues Al was present in the

form of Al2O3 (binding energy of 734ndash74 eV) In comparison with SR

more intense peaks and large peak area was observed in AS and RS

This may have been due to the removal of organic impurities and com-

plete exposure of Al matrix as a result of pretreatment In comparison

with AS the peak intensity was low in AS residues due to the leaching

of Al during two stage treatment Moreover the residue of two stage

bioleaching showed small peak as compared to the residue of

bioleaching followed by alkali leaching This is because higher amount

of Al (55) was leach out during two stage bioleaching compared to

bioleaching followed by alkali leaching (37)

Mo was found to be present in the form of oxides and sul1047297de in SR

AS and RS The analysis of 3d52 spectra con1047297rmed the presence of

MoO3 (2326 eV) and Mo4O11 (232ndash

233 eV) whereas 3d32 spectra

Fig 2 (continued)

134 H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143

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assured the presence of MoS2 at binding energy of 23294 Analysis of

3d32 spectra assured another peak of MoO3 (2363 eV) The residueob-

tained after bioleaching followed by alkali leaching exhibited low inten-sity of these peaks due to the high leaching of Mo (95) In comparison

small stretches of these mineral forms were still present in the residue

obtained after two stage bioleaching This was due to the comparatively

low leaching of Mo in this process (81) and hence Mo was still present

in this residue Interestingly a newpeak of Mo was observed in residues

samples (227 eV) which corresponded to the 2s spectra of MoS2 Al-

though the reason is not clear the appearance of this new peak can be

attributed to presence of remaining Mo as insoluble sul1047297des The analy-

sis of V2p spectra con1047297rmed the present of V as V 2O5 (517eV) inSR AS

and RS The peak intensity of V decreased in the following order

AS N RS N SR No peaks of V was obtained in the AS residues obtained

after either two stage bioleaching or bioleaching followed by alkali

leaching This is because during both the sequential processes almost

complete leaching of V (100) was observed and hence was absent inthese residues

314 Binding forms of the metals in feed spent catalyst

The different binding forms of metals (Al Mo Ni and V) present in

the ASandRS are shown in Fig 2c Thedifferentbinding forms of metals

correspond to the fractions of metals present in the spent catalyst sam-

ples Al was predominantly present in the residual fraction in the AS

(839) Detailed information on the binding form of metals presented

in acetone washed spent catalyst had also been provided in our earlier

study (Pathak et al 2014) In comparison slight higher amount of Al

was found to beexisted inthe residualfraction in the RS(881) The in-

crease in the residual fraction of RS may be due to the removal of or-

ganics during decoking which exposed the silicates minerals for

reacting with Al thus increasing the residual fraction As per the BCR

scheme the residual fraction represents the most stable binding form

of the metals The metals existed in the residual fraction are largely

bound in crystal lattice of themineral structures Under naturalenviron-mental conditions this fraction is highly stable and dif 1047297cult to release

into the environment The tendency of Al to remain in the residual frac-

tion of soil and sediments has been reported (Khan et al 2013 Walna

et al 2005)

In comparison with Al the higher amount of Ni wasextracted in ex-

changeable fraction (613) followed by oxidizable (190) reducible

(173) and residual (24) fraction in the AS The high concentration

of Ni in exchangeable and reducible fraction suggested that the poten-

tial mobility of the Ni is very high As per BCR scheme the metals pres-

ent in the exchangeable fraction are easily affected by changes in the

ionic composition of water These are weekly adsorbed metals that

retained on the surface by relatively weak electrostatic attraction and

hence exhibit high mobility in the environment The presence of Ni in

the more mobile fractions in soil and sludges has been reported in ear-lier studies (Clementina 2006 Wang et al 2005) Compared to AS the

maximum concentration of Ni was found to be present in the residual

fraction (407) in the RS This was followed by the exchangeable

(296) oxidizable fraction (213) and reducible fraction (84) It

was observed that decoking lead to the signi1047297cant changes in the bind-

ing forms of the metals As a result of decoking at higher temperature

the part of Ni was transformed to residual fraction in the RS This may

have been due to the formation of stable nickel aluminate compound

during high temperature decoking of spent petroleum catalyst

(Eijsbouts et al 2008 Srichandan et al 2013)

In AS V was mainly extracted in the oxidizable fraction (585)

followed by residual (249) exchangeable (84) and reducible frac-

tion (82) The predominant presence of V in the oxidizable fraction

suggested that V is associated with the organic matter in the AS The

Fig 2 (continued)

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association of V with organic matterin other matrixsuch as soil andsed-

iment has been previously reported (Belazi et al 1995 Poledniok and

Buhl 2003) On the contrary lower amount of V was found to exist in

oxidizable fraction (316) in the RS The comparatively lower amount

of V in oxidizable fraction in RS may be due to the fact that decoking re-

moved most of the carbonaceous matter in the RS which caused decrease

in this fraction In RS the highest concentration of V (501) was present

in theresidual fraction whichmight be dueto thetransformation of oxidiz-

able fraction into the residual fraction as a result of decokingIn AS Mo was found mainly in the oxidizable fraction (493) followed

by residual (263) exchangeable (209) and reducible (35) fractions

The higher presence of Mo in oxidizable fraction implies that under highly

oxidizing environment it may release from the AS matrix The tendency of

Mo to remain in organic matter in other solids matrixes such as soil and

sewage sludge has been previously reported (Wichard et al 2009

Zemberyova et al 2010) Moreover signi1047297cant concentration of residual

fraction also indicated that part of the Mo remained in stable form and

hence will not exhibit mobility under normal environmental conditions

On the other hand in the RS the major concentration of the Mo was

found in the residual fraction (485) followed by oxidizable fraction

(251) exchangeable fraction (214) and reducible fraction (50) The

high residual fraction of the Mo in RS suggested that signi1047297cant concentra-

tion ofMowasin stable form and only extremeharshand longer conditions

can release Mo from the RS

The results indicated that metals in the present study (Ni V Mo and

Al) exhibited different binding forms in the different fractions in the dif-

ferently pretreated spent catalysts (AS and RS) In theAS and RS Al was

mainly presented in the residual fraction Ni was existed mainly in the

exchangeable fraction in the AS whereas in the RS it was presented in

the residual fraction Mo and V were present in the oxidizable fractions

in theAS whereas these metalswere existed mostly in theresidual frac-

tion in the RS Overall it was found that decoking caused signi1047297cant

changes in thebinding forms of themetals and allthe metalsshowed el-

evated concentration of residual fraction in the RS as compared to AS

32 Bioleaching studies

321 Changes in pH and ORP during bioleaching

AS and RS were acid consuming In the case of abiotic (controls)

leaching of RS and AS pH (Fig 3a) increased from an initial value of

14 to 215 and 220 respectively The acid consuming property of AS

and RS was associated with elemental forms of the spent catalyst RS

and AS were comprised of metal oxides and metal sul1047297des based on

the XPS analysis Metal oxides are acid soluble and solubilized as

metal sulfates when reacted with acid as per the common equation

given below

MeO thorn H2SO

4rarrMeSO

4 thorn H2O eth1THORN

where MeO represents oxides of Ni Al Mo and V and their dissolution

can be explained individually as follows

NiO thorn H2SO4rarr NiSO4 thorn H2O eth2THORN

Al2O3 thorn 3H2SO4rarrzAl2ethSO4THORN3 thorn 3H2O eth3THORN

MoO3 thorn H2SO4rarrMoO2SO4 thorn H2O eth4THORN

V2O3 thorn H

2SO

4 thorn O2rarrethVO2THORN2SO4 thorn H

2O eth5THORN

The resulting acid consumption by metal oxides increased the pH of

the solution The increase in medium pH as a result of addition of both

decoking and acetone washed spent catalyst has been reported in

earlier studies (Bharadwaj and Ting 2013 Srichandan et al 2013)

Acid consumption was lower during second stage abiotic leaching

(data not shown here) This is because most of the acid consuming

metal oxides were already solubilized from the spent catalyst during

1047297rst stage treatment of the control

The initial increase in pH was less during bioleaching compared to

that in the control (Fig 3a) The oxidation of sulfur by A ferrooxidans

and A thiooxidans produced acid duringbioleaching which compensat-

ed for the acid consumed by the spent catalyst In fact due to bacterialacid generation pH decreased to b14 suggesting that both bacteria

were effective oxidizing the sulfur in the presence of spent catalysts

During second stage bioleaching of residues the decrease in pH was

greater compared to that during 1047297rst stage bioleaching This might be

due to the low metal oxide content of the residues after 1047297rst step

bioleaching which consumed less acid The ferric iron immediately

was reduced to ferrous iron by addition of spent catalyst during

bioleaching with A ferrooxdians (Fig 3b) The resulting ferrous iron in

solution was further oxidized by A ferrooxdians to ferric ion which ox-

idized the spent catalyst matrix Fig 3b presents the interrelationship

between ferrous and redox potential as a result of A ferrooxidans activ-

ity The redoxpotentialvalues are shown as white symbols and the cor-

responding ferrous concentrations as black symbols At the start of 1047297rst

stage bioleaching the ferrous concentration was very low (0 h prior to

spent catalyst addition) After adding the spent catalyst ferric iron oxi-

dized the spent catalyst surfacematrix and was reduced to ferrous ion

The increase in ferrous concentration was less during second stage

bioleaching of the AS and RS residues compared to that during 1047297rst

stage bioleaching of AS and RS This might be becausepart of the metals

was leached during1047297rst stage bioleaching and hence ferric iron reduc-

tion was less during secondstage bioleaching The initial increase in fer-

rousconcentration was inversely related with redox potential Bacterial

oxidation of ferrousion caused an increase in the redox potential during

1047297rst and second stage bioleaching of AS after the 1047297rst 20 h After 60 h

both ferrous and redox potential were stabilized in the AS The ferrous

concentration increased continuously in RS until 100 h and 60 h during

1047297rst and second stage bioleaching respectively The ferrous concentra-

tion and redox potential were stabilized during 1047297rst stage bioleaching

whereas a decrease in ferrous ion was observed during the secondstage until the end

322 Leaching yields of metals during 1047297rst stage bioleaching with

A ferrooxidans

The leaching pro1047297le of Ni Al Mo and V from RS and AS is shown in

Fig 4a b The leaching yields of Al were less compared to that of Ni Mo

andVinRS(Fig 4a)andAS(Fig 4b) The leaching yields of Ni V and Mo

were almostsimilar in the RSwhereas higherleachingwas observed for

Ni and V in the AS compared to that for Mo The higher leaching yield of

Ni and V compared to Mo hasbeen reported in earlier bioleaching study

(Mishra et al 2007) The leaching yields of all metals from RS and AS

were higher during the 1047297rst 20 and 40 h respectively After the initial

faster leaching leaching remained constant throughout the period

Bioleaching resulted in higher 1047297nal solubilization of Mo (58) and V inthe RS (57) compared to that in thecontrol (Mo-42 V-32) whereas

no signi1047297cant differences were observed in the case of Ni and Al Signif-

icantly higher leaching wasobtained for Al (34) Ni (90) andV (97)

in the AS compared to those in the control (Al-19 Ni-50 and V-40)

Signi1047297cant differences were observed for Ni and V leaching when AS

and RS were compared The 1047297nal leaching yield of Al (34) Ni (90)

and V (97) was signi1047297cantly higher from the AS compared to that

from the RS (Al-20 Ni-50 and V-57) Leaching yield of Mo was al-

most similar in both RS (58) and AS (65)

323 Leaching yields of metals during 1047297rst stage bioleaching with

A thiooxidans

Unlike A ferrooxidans A thiooxidans only produces sulfuric acid as

lixiviant Fig 4c and d represent the metal leaching yield from the RS

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and AS respectively Similar to bioleachingwith A ferrooxidans lower Al

leaching yields were observed in both theRS and AS The leaching yield

of metals was in the order of V N Ni N Mo N Al for AS whereas it was in

the order of Mo N V N N N Al for RS Similar to bioleaching using

A ferrooxidans Ni and V followed the similar leaching pattern in the

AS and RS Bioleaching resulted in signi1047297cantly higher leaching yieldsfor all metals in RS (Ni-58 Al-20 Mo-70 and V-58) and AS (Ni-

87 Al-34 Mo-73 and V-92) compare to those in the control The

higher leaching yield during bioleaching was due to the low pH

achieved in the bioleaching reactors as a result of microbial oxidation

of sulfur Similar to bioleaching with A ferrooxidans comparatively

higher leaching yields of Ni and V were observed from the AS whereas

no signi1047297cant differences in the leaching yield of Mo was observed in

the RS or AS

324 Leaching yields of metals during second stage bioleaching with

A ferrooxidans vs second stage alkali leaching

The leaching yields of metals obtained during 1047297rst and second stage

bioleaching ofthe RSand ASare shown in Fig5a Theleachingyields ob-

tained during 1047297rst stage bioleaching followed by second stage alkali

leaching of the RS and AS are shown in Fig 5b Leaching of Ni Mo and

V increased substantially (25ndash31) during second stage bioleaching of

the RS whereas no signi1047297cant enhancement was observed in the

leaching yields of these metals from the AS This is because the majority

of Niand V werealready leached out during1047297rst stage bioleaching of AS

Leaching of Al was enhanced more in the case of AS (21) compared tothat of the RS (13) The 1047297nal leaching yields after second stage

bioleaching were higher using AS (Ni-94 Al-55 Mo-77 and V-

99) compared to the RS (Ni-75 Al-33 Mo-87 and V-88)

No signi1047297cant enhancement was observed in the second stage alkali

leaching yield of Niand Al using either the AS or RS(Fig5b) However a

signi1047297cant enhancement (30) wasobservedin theleachingyieldof Mo

from both theRS and AS There wasalsosigni1047297cant enhancement (28)

in leaching of V from the RS As almost all of the V (97) from AS was

already leached out during the 1047297rst stage the yield of V remained con-

stant after second stage alkali leaching Overall the total leaching yield

(including 1047297rst and second stages) of Mo was best by employing

bioleaching in the 1047297rst stage followed by alkali leaching in the second

stage of AS About 95 Mo was leached out along with 90 Ni 38 Al

and 98 V from the AS

Fig 3 a Evolution of pH during bioleaching of the AS and RS using A thiooxidans and A ferrooxidans b Changes in ferrous concentration and redox potential during bioleaching with

A ferrooxidans

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and AS respectively Similar to bioleachingwith A ferrooxidans lower Al

leaching yields were observed in both theRS and AS The leaching yield

of metals was in the order of V N Ni N Mo N Al for AS whereas it was in

the order of Mo N V N N N Al for RS Similar to bioleaching using

A ferrooxidans Ni and V followed the similar leaching pattern in the

AS and RS Bioleaching resulted in signi1047297cantly higher leaching yieldsfor all metals in RS (Ni-58 Al-20 Mo-70 and V-58) and AS (Ni-

87 Al-34 Mo-73 and V-92) compare to those in the control The

higher leaching yield during bioleaching was due to the low pH

achieved in the bioleaching reactors as a result of microbial oxidation

of sulfur Similar to bioleaching with A ferrooxidans comparatively

higher leaching yields of Ni and V were observed from the AS whereas

no signi1047297cant differences in the leaching yield of Mo was observed in

the RS or AS

324 Leaching yields of metals during second stage bioleaching with

A ferrooxidans vs second stage alkali leaching

The leaching yields of metals obtained during 1047297rst and second stage

bioleaching ofthe RSand ASare shown in Fig5a Theleachingyields ob-

tained during 1047297rst stage bioleaching followed by second stage alkali

leaching of the RS and AS are shown in Fig 5b Leaching of Ni Mo and

V increased substantially (25ndash31) during second stage bioleaching of

the RS whereas no signi1047297cant enhancement was observed in the

leaching yields of these metals from the AS This is because the majority

of Niand V werealready leached out during1047297rst stage bioleaching of AS

Leaching of Al was enhanced more in the case of AS (21) compared tothat of the RS (13) The 1047297nal leaching yields after second stage

bioleaching were higher using AS (Ni-94 Al-55 Mo-77 and V-

99) compared to the RS (Ni-75 Al-33 Mo-87 and V-88)

No signi1047297cant enhancement was observed in the second stage alkali

leaching yield of Niand Al using either the AS or RS(Fig5b) However a

signi1047297cant enhancement (30) wasobservedin theleachingyieldof Mo

from both theRS and AS There wasalsosigni1047297cant enhancement (28)

in leaching of V from the RS As almost all of the V (97) from AS was

already leached out during the 1047297rst stage the yield of V remained con-

stant after second stage alkali leaching Overall the total leaching yield

(including 1047297rst and second stages) of Mo was best by employing

bioleaching in the 1047297rst stage followed by alkali leaching in the second

stage of AS About 95 Mo was leached out along with 90 Ni 38 Al

and 98 V from the AS

Fig 3 a Evolution of pH during bioleaching of the AS and RS using A thiooxidans and A ferrooxidans b Changes in ferrous concentration and redox potential during bioleaching with

A ferrooxidans

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325 Leaching yields of metals during second stage bioleaching with

A thiooxidans vs second stage alkali leaching

No signi1047297cant increase in metal leaching yield was observed during

second stage bioleaching (Fig 5c) of the RS except V (23) and Ni

(9) The remaining Ni (13) and V (8) were completely leached out

from the AS Al yield increased by 21 from the AS whereas only an

8 increase was observed from the RS Total leaching yields (after sec-

ond stage bioleaching) obtained using the AS were (Ni-100 Al-55

Mo-81 and V-100) higher than those achieved with the RS (Ni-67Al-28 Mo-76 and V-81)

Fig 5d represents the second stage alkaline leaching of residues ob-

tained after 1047297rst stage bioleaching using A thiooxidans The results

showed no enhancement in leaching yields of Ni and Al from either

the AS or RS although signi1047297cant enhancement in the leaching yield

of Mo and V was observed Bioleaching using A thiooxidans followed

by alkali leaching from AS produced a signi1047297cantly higher yield of Mo

(96) along with Ni-87 Al-37 and V-100 compared to those in

the RS (Mo-89 Ni-60 Al-24 and V-83)

Overall the sequential strategy involving bioleaching with A thiooxidans followed by alkali leaching produced maximum recovery

of Mo (96) The signi1047297cant enhancement in the recovery of Mo during

second stage alkali leaching produced a conducive environment for

leaching of Mo The EhndashpH diagram also suggested that an alkaline pH

favors solubilization of Mo

4 Discussion

We observed that thedifferent metalsfollowed differentleachingki-

netics The possible reasons related to difference in leaching behavior

are discussed brie1047298y

41 Leaching behavior of metals during 1047297rst stage bioleaching

The leaching yields of Ni and V were high and both exhibitedsimilar

leaching patterns from the AS and RS (Fig 4andashd) The leaching yield of

Mo wasless than that of Ni and V in AS Al exhibited the lowest leaching

Fig 4 andash

b Leaching yield of metals from (a) RS and (b) AS using A ferrooxidans cndash

d Leaching pro1047297le of metals from (a) the RS and (b) AS using A thiooxidans

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yield among all metals Thevariations in leaching yieldsof NiV Moand

Al can be explained on the basis of EhndashpH diagrams (Fig 6) and by the

different binding forms of the metals (Fig 2c) presented in the AS and

RS The redox potential values obtained in the present experimentswere converted into Eh values (+200 mV)with respect to standard hy-

drogenelectrode for obtaining the EhndashpH diagramsThe low leaching of

Al from both the AS and RS was associated with the form in which Al is

present in these spent catalysts As per the results of sequential extrac-

tion study Al was predominantly existed in the residual fraction of the

AS (839) and RS (881) As per BCR scheme residual fraction is the

most stable fraction and metals present in this fraction exhibit low or

no mobility This explains the low leaching yield of Al during

bioleaching either from AS or RS Moreoverthe amount of residual frac-

tion was higher in the RS as compared to AS which resulted in slight

lower Al leaching yield from the former Lower solubilization of Al was

also observed in an earlier bioleaching study conducted with decoked

spent catalyst (Bharadwaj and Ting 2013) The higher leaching of Ni

and V as compared to Al from the AS and RS in the present study was

in agreement with a study performed with AS (Pradhan et al 2010)

The EhndashpH diagram suggested that V has a wide range of solubility

and exists as soluble VO2+2 in solution supporting its higher yield Sim-

ilarly Ni was readily soluble under the EhndashpH conditions of the presentinvestigation (pH 2ndash09 and Eh 650ndash850 mV) and existed as NiOH+3

This con1047297rmed the higheryield of Ni in the present study However sig-

ni1047297cantly lower amount of Ni andV were leached from the RS compared

to the AS using either A ferrooxidans or A thiooxidans whereas Al and

Mo showed little differences (Fig 4andashd) The decrease in leaching

yield of Ni from the RS can be explained based on the binding forms of

Ni In the AS Ni was mainly presented as exchangeable fraction

(613) which is considered to be the most mobile fraction This caused

higher leaching yield of Ni during bioleaching from the AS On the con-

trary exchangeable fractionwas signi1047297cant less in the RS (296) and most

of the Ni was present as a residual fraction in the RS (407) This caused

low leaching yield of Ni from the RS A comparatively lower yield of

Ni during bioleaching has been reported in the decoked catalyst

(Bharadwaj and Ting 2013) Similar to Ni most of the V (501)

Fig 4 (continued)

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was present as residual fraction in the RS as compared to only 249 in the

AS This explains the low leaching yield of V from RS as compared to AS

The leaching yield of Mo in AS and RS was higher than that of Al

Similar observations were also reported in an earlier bioleaching study

conducted with spent catalyst (Kim et al 2010 Mishra et al 2009 )

The higher leaching yields of Mo compared to Al was expected as signif-

icantly higher concentration of Mo was present in the exchangeable re-

ducible and oxidizable fraction compared to Al During 1047297rst stagebioleaching with both A ferrooxidans and A thiooxidans lower leaching

yield of Mo from the AS and a similar and lower yield from the RS was

observed compared to that of V This was largely due to the presence

of Mo as insoluble MoO3H2O in the present EhndashpH conditions of the

bioleaching study When comparing between AS and RS slightly

lower leaching yield of Mo was observed from the later This was due

to the fact that in the AS most of the Mo was present in the oxidizable

fraction (493) which was susceptible to the highly oxidizing condi-

tions of thebioleaching Part of theMo wasalso existed in theexchange-

able fraction (209) which was expected to solubilize easily under the

acidic conditions of bioleaching On the contrary in the RS compara-

tively lower amount (251) of oxidizable fraction was present More-

over signi1047297cant higher amount of Mo (485) was presented in the

residual fraction which leads to its comparatively lower solubilization

42 Leaching behavior of metals during second stage bioleaching and alka-

line leaching

The leaching patterns were different between second stage

bioleaching using either A ferrooxidans (Fig 5a) or A thiooxidans

(Fig 5c) with corresponding alkali leaching (Fig 5b and d) Ni was not

leached out whereas low Al yield was obtained using alkali leaching

in comparison with bioleaching from both the AS and RS The low Niyield during alkalileachinghas also been reported in another study con-

ducted in alkali solution (NaOH) using spent sulfuric acid catalyst

(Ognyanova et al 2009 Villarreal et al 1999) In contrast leaching of

Mo increasedsubstantially in both theAS andRS duringsecond stage al-

kali leaching V yield increased in both second stage bioleaching as well

as in second stage alkali leaching of RS

The different solubilities of metals during second stage treatment

can be explained based on the EhndashpH diagram (Fig 7) The EhndashpH dia-

gram shows that under alkaline conditions Al and Ni exists as solid

phase (insoluble form) in the form of AlO(OH) and Ni(OH)2 respective-

ly which accounted forlower yields of Ni and Al during second stage al-

kali leaching of the AS and RS Furthermore Mo exhibited higher

solubility in alkaline pH and existed as soluble MoO4minus2 whereas it exists

as insoluble MoO3H2O at acidic pHs The higher solubility of Mo under

Fig 5 andashb Final leaching yields of metals from RS and AS (a) after two stage bioleaching using A ferrooxidans and (b) after 1047297rst stage bioleaching using A ferrooxidans and second stage

alkalileaching cndashdFinal leachingyieldsof metalsfromRS andAS (c)after1047297rstand secondstagebioleachingusing A thiooxidans(d) after1047297rst stagebioleaching using A thiooxidansfollow-

ed by second stage alkali leaching

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alkaline conditions explains the higher yield of Mo during second stage

alkali leaching usingthe ASand RS V has a wideEhndashpH range of solubil-

ity and hence resulted in almost similar leaching yield during second

stage bioleaching and alkali leachingFrom these results we conclude that two stage bioleaching with

A thiooxidans using the AS resulted in higher yields of metals compared

to those in the RSSimilarly bioleaching using A thiooxidans followed by

alkali leaching resulted in higher metal yields from the AS compared to

those in the RS The higher leaching yield of metals from AS in compar-

ison with RS was based on the binding forms of the metals present in

above spent catalyst samples In general higheramount of residual frac-

tion was observed for all the metals (Al-881 Ni-407 V-501 and

Mo-485) in the RS as compared to AS (Al-839 Ni-24 V-249

and Mo-263) This might have caused lower leaching yields of metals

from RS during 1047297rst stage bioleaching The decoking of spent catalyst

compared to acetone washing produced a negative impact on leaching

yieldsof metals and hence and decoking can be avoided for better econ-

omy and eco-friendless

Two stage bioleaching was more effective in terms of leaching yields

of Ni and Al whereas bioleaching followed by alkali leaching was found

to bethe optimum strategy for Moleachingusing either ASor RS Noap-

parent difference in leaching yield was observed between bioleachingusing A thiooxidans followed by alkali leaching and bioleaching using

A ferrooxidans followed by alkali leaching with the AS However

bioleaching with A thiooxidans is economically feasible due to the low

cost of sulfur as an energy source Hence two stage bioleaching of AS

with A thiooxidans is better in terms of leaching yield of Ni and Al

Leaching yield of Mo (96) can be best achieved with bioleaching

using A thiooxidans followed by alkali leaching

5 Conclusion

The present results suggest that different sequential leaching pro-

cesses signi1047297cantly enhanced the removal of metals (V Ni Mo and Al)

from both the AS and RS although higher yields were achieved using

the AS The lower leaching yields of metals from RS were due to

Fig 6 EhndashpH diagram for (a) Ni (b) Al (c) Mo and (d) V in the presence of other co-elements at 35 degC and the solution metal concentration after the 1047297rst stage experiment

141H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143

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the comparatively higher presence of metals in the residual fraction in

the RS as compared to AS The decoking of spent catalyst has a negative

impact on leaching yields and can be avoided for better economy and

eco-friendless Bioleaching followed by bioleaching using either

A thiooxidans (V-100 Ni-100 Mo-81 and Al-55) or A ferroxidans

(V-99 Ni-94 Mo-77 and Al-55) signi1047297cantly increased the yields

of all metals from the AS although the use of A thiooxidans is favored

due to the use of only one substrate (sulfur) and lower processing

cost Furthermore bioleaching with A thiooxidans followed by alkaline

leaching was an optimum sequential strategy in terms of Mo recovery

(96) from the AS

Acknowledgment

This study was supported by Leading Foreign Research Institute Re-

cruitment Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Science ICT and Future Planning

(2014)

References

Belazi AU Davisdon C Keating GE John DL 1995 Determination and speciation of heavy metals in sediments from the Cumbrian coast NW England UK J Anal AtSpectrom 3 (10) 233ndash240

Bharadwaj A Ting YP 2013 Bioleaching of spent hydrotreating catalyst by acidophilicthermophile Acidianus brierleyi leaching mechanism and effect of decoking

Bioresour Technol 130 673ndash

680

Blight KR Ralph DE 2004 Effectof ionic strengthon ironoxidation with batch culturesof chemolithotrophic bacteria Hydrometallurgy 73 325ndash334

Clementina OO 2006 Biosurfactant Enhanced Remediation of a Mixed ContaminatedSoil(Masters thesis) Concordia University Canada

Eijsbouts S Battiston A Vanleerdam G 2008 Life cycle of hydroprocessing catalystsand total catalyst management Catal Today 130 361ndash373

FarrellM Jones DL 2009 Heavy metal contamination of a mixed waste compost metalspeciation and fate Bioresour Technol 100 (19) 4423ndash4432

Fuentes A Llorens M Saez J Soler A Aguilar M Ortuno F 2004 Phytotoxicity andheavy metals speciation of stabilized sewage sludges J Hazard Mater 108 161ndash169

Furimsky E 1996 Spent re1047297nery catalysts environment safety and utilization CatalToday 30 223ndash286

Gholami RM Borghei SM Mousavi SM 2011 Bacterial leaching of a spent MondashCondashNire1047297nery catalyst using Acidthiobacillus ferrooxidans and Acidithiobacillus thiooxidansHydrometallurgy 106 26ndash31

Kar BB Murthy BVR Misra VN 2005 Extraction of molybdenum from spent catalystby salt-roasting Int J Miner Process 76 143ndash147

Khan S Kazi TG Arain MB Kolachi NF Baig JA Afridi HI Shah AQ 2013 Evalu-ation of bioavailabilityand partitioningof aluminumin sediment samplesof differentecosystems by modi1047297ed sequential extraction methods Clean Soil Air Water 41 (8)808ndash815

Kim DJ Pradhan D Ahn JG Lee SW 2010 Enhancement of metals dissolution fromspent re1047297nery catalysts using adapted bacteria culture mdash effects of pH and Fe(II) Hy-drometallurgy 103 136ndash143

Mara1047297 M Stanislaus A 2003 Option and process for spent catalyst handling and utili-zation J Hazard Mater 101 123ndash132

Mishra D Kim DJ Ralph DE Ahn JG Rhee YH 2007 Bioleaching of vanadium richspent re1047297nery catalysts using sulfur oxidizing lithotroph Hydrometallurgy 88202ndash209

Mishra D Ahn JG Kim DJ Chaudhury GR Ralph DE 2009 Dissolution kinetics of spent petroleum catalyst using sulfur oxidizing acidophilic microorganism J HazardMater 167 1231ndash1236

Mulak W Miazga B Szymczycha A 2005 Kinetics of nickel leaching from spent cata-

lyst in sulphuric acid solution Int J Miner Process 77 231ndash

235

Fig 7 EhndashpH diagram for (a) Ni (b) Al (c) Mo and (d) V in the presence of other co-elements at 35 degC and the solution metal concentration after the second stage experiment

142 H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143

7232019 1-s20-S0304386X14002163-main

httpslidepdfcomreaderfull1-s20-s0304386x14002163-main 1414

Ognyanova A Ozturk AT Michelis ID Ferella F Taglieri G Akcil A Veglio F 2009Metal extraction from spent sulfuric acid catalyst through alkaline and acidicleaching Hydrometallurgy 10 20ndash28

Park KH MohapatraD Nam CW 2007 Twostage leachingof activated spent HDS cat-alyst and solvent extraction of aluminium using organo-phosphinic extractantCyanex 272 J Hazard Mater 148 287ndash295

Pathak A Srichandan H Kim DJ 2014 Fractionation behavior of metals (Al Ni V andMo) during bioleaching and chemical leaching of spent petroleum re1047297nery catalystWater Air Soil Pollut 225 (1893) 1ndash10

Poledniok J Buhl F 2003 Speciation of vanadium in soil Talanta 59 (1) 1ndash8Pradhan D Mishra D Kim DJ Ahan JG Chaudhury GR 2010 Bioleaching kinetics

and multivariate analysis of spent petroleum catalyst dissolution using two acido-phile J Hazard Mater 175 267ndash273Pradhan D Patra AK Kim DJ Chung HS Lee SW 2013 A novel sequential process

of bioleaching and chemical leaching for dissolving Ni V and Mo from spent petro-leum re1047297nery catalyst Hydrometallurgy 131ndash132 114ndash119

Santhiya D Ting YP 2005 Bioleachingof spent re1047297nery processing catalyst using Asper- gillus niger with high-yield oxalic acid J Biotechnol 116 171ndash184

Smeda A Zyrcniki W 2002 Application of sequential extraction and the ICP-AES meth-od for study of the partitioning of metals in 1047298y ashes Microchem J 72 9ndash16

Srichandan H Kim DJ Gahan CS Singh S Lee SW 2013 Bench-scale batchbioleaching of spent petroleum catalyst using mesophilic iron and sulfur oxidizingacidophiles Korean J Chem Eng 30 (5) 1076ndash1082

Ure AM Quevauviller PH Muntau H Griepink B 1993 Speciation of heavy metals insoils and sediments An accountof the improvementand harmonization of extractiontechniques undertaken under the auspices of the BCR of the commission of EuropeanCommunities Int J Environ Anal Chem 51 135ndash151

VillarrealSM Kharisov BI Martinez LMT Elizondo VN 1999 Recovery of vanadiumand molybdenum from spent petroleum catalyst of PEMEX Ind Eng Chem Res 384624ndash4628

Walna B Siepak J D rzymala S Sobczynski T 2005 Research on aluminum speciationin poor forest soils using the sequential extraction method Pol J Environ Stud 14(2) 243ndash250

Wang C Hu X Chen ML Wu YH 2005 Total concentrations and fractions of Cd Cr

Pb Cu Ni and Zn in sewage sludge from municipal and industrial wastewater treat-ment plants J Hazard Mater B119 245ndash249Wichard T Mishra B Myneni SCB Bellenger JP Kraepiel AML 2009 Storage and

bioavailability of molybdenum in soils increased by organic matter complexationNat Geosci 2 625ndash629

Zemberyova M Hagarovaacutea I Zimovaacutea J Bartekovaacutea J Kussb HM 2010 Determina-tion of molybdenum in extracts of soil and sewage sludge CRMs after fractionationby means of BCR modi1047297ed sequential extraction procedure Talanta 82 (2) 582ndash586

143H Srichandan et al Hydrometallurgy 150 (2014) 130ndash143