Photochemistry and Photobiology, 2005, 81 : 636-640

Photooxidative Treatment of Sulfurous Water for Its Potabilizationq

Fernando Hernandez* and Gunther Geissler lnstituto de Ciencias, Benemerita Universidad Autonoma de Puebla, Ciudad Universitaria, Puebla, Mexico

Received 13 December 2004; accepted 24 February 2005

ABSTRACT The feasibility of potabilization of sulfurous water was investigated by photochemical oxidation processes using a batch photoreactor and a continuous-flow photoreactor, equipped with UV lamps of 1000 W and 1500 W, respectively. Additionally, two advanced processes of oxidation were applied i.e. with a use of a UV 1ightMzOdair and UV light/ H202/03/air. These two processes were compared for their efficiency to the direct oxidation process where ozone is used in the absence of UV light. Results obtained for both advanced processes showed better oxidation than takes place by ozone in the absence of UV light. After the photooxidation processes, different processes for the absorption or precipitation of sul- fates were investigated to comply with the World Health Orga- nization (WHO) norm that demands a limit of 5250 mgL-' of SO:- in drinking water. Additionally, reverse osmosis was simulated using Osmonics Inc. software to predict the feasi- bility of lowering the salt concentration below WHO limits.

INTRODUCTION The underground water in volcanic zones frequently contains a lot of different sulfurous compounds such as sulfides (which have a pungent smell and are toxic), sulfates (which are laxatives), sul- fites and colloidal sulfur. Therefore, it is impossible to use such water for human consumption without proper treatment. The con- ventional technologies to eliminate these substances use chemicals such as oxidation agents or precipitation.

Various natural autopurification processes, such as the processes that take place in the troposphere and in superficial layers of the seas, lakes and rivers, are photochemical reactions induced by UV light. Starting from ozone and water molecules, these processes form hydroxyl radicals ('OH), which are strong oxidation agents (1).

O3 -%02 + 0' h < 310 nm O* + H20 A 2 'OH

In ideal cases these 'OH radicals lead to a total mineralization of the polluting agents by radical chain reactions. Depending on their

TPosted on the website on 3 March 2005 *To whom correspondence should be addressed Instituto de Ciencias,

Benemkrita Universidad Authoma de Puebla, Ciudad Universitaria, Edif. 76, Col. Jardines de San Manuel, PueblaPue., C.P., 72570, M6xico. Fax: 52-222-229-5500 ext. 7056; e-mail: fhernand@siu.buap.mx

Abbreviations: ZNM-4, Mexican Natural Zeolite. 0 2005 American Society for Photobiology 0031-8655/05

chemical composition, organic compounds are transformed in water, carbon dioxide, nitrogen (N2) and inorganic ions (2). Through these processes the inorganic species of lower oxidation state are also oxidized (3).

One of the consequences of these observations is the simulation of these natural autopurification processes by various modem technologies for water treatment, where UV light is applied and an oxidation agent like H20z or O3 is used as a source of 'OH radicals (homogenous photochemical processes). In some cases a semicon- ductor such as Ti02 (in photocatalytic heterogeneous processes) is used as a catalyst to induce the photooxidation.

There are several sulfurous water reserves in different regions of the world. The use of advanced processes of oxidation for its purification could be a solution to reduce the existing drinking water deficit in these parts of the world. There are several publi- cations on the heterogeneous and photocatalytic oxidation of sulfur compounds, transforming them into sulfates, for example: H2S in gaseous phase (4,s) or in aqueous phase and alkaline conditions (6,7), sulfites in aqueous phase (8,9), sulfur in aqueous phase (lo), thiosulfates (l l) , sulfur dioxide in water (12) and other evil- smelling compounds (13).

The observed disadvantages of these processes are that (1) the presence of solids and a catalyst induce the precipitation of sulfates and carbonates, inactivating the oxidation process; (2) in some processes the reaction time is very long; (3) the experiments are done in artificially prepared samples and not with natural sulfurous water; (4) the catalysts need to be eliminated at a later stage; and (5) the photochemical process has never been applied for the puri- fication of sulfurous water for potability purposes.

This article presents the results of research efforts made to find the best conditions for applying an advanced process of homogenous photochemical oxidation for sulfurous water treatment, involving total oxidation of the sulfur compounds. Two different systems of oxidation were used: UV light/H202/air and UV light/H202/03/ai to compare them with direct oxidation with ozone but without UV light. The work also included a search for the means of reducing salt concentrations to acceptable standards and the established norms for drinking water as well as the simulation of a reverse- osmosis process.

MATERIALS AND METHODS In a first stage of the photochemical experiments, a batch-type photoreactor was used. The reactor has a 1 L capacity and is equipped with a mercury medium-pressure steam UV lamp (model PW-1022, Heraeus, Frankfurt, Germany), which is 110 mm length and uses 1000 W, 145 V and 7.5 A. The lamp is protected by a quartz tube, The reactor has a distilled water cir- culating system to regulate the temperature. The reaction vessel is made of Pyrex glass, which in the upper part has two inlets and in the lower part an inlet for the air or aidozone flow.

636

Photochemistry and Photobiology, 2005, 81 637

Table 1. Characterization of sulfurous water

1 0.54 18.4 729 0.1 174 60 0.5 100 0 2 6 29.4 520 0.2 600 135 0.1 104 16 3 1.22 1.5 717 0.2 440 91 0.9 100 31 4 3.15 0.9 548 1.8 266 108 0.5 120 3 5 4.5 6.5 920 0.3 352 260 0.7 160 24 6 0.020 ND* 470 8.1 1395 920 0.5 100 30

*ND = not determined.

The air feeding is done by means of a little bomb at the rate of 2000 mL min-I, which equals a supply of 400 mL m i d of 0 2 . To generate ozone, we used a King Ozone Hydrozon K-40 generator (Puebla, Mbxico), which produces 40 mg of ozone per hour.

In the second stage of the photochemical experiments, an Aquionics flow reactor (Erlanger, ICY) provided by Germ-Ex (Mbxico City D.F., Mbxico) was used. The flow reactor has a free volume of 10 L with an adjustable continuous progression flow between 0 and 37.85 L m i d (0 to 10 gallons per minute) with the possibility of recirculation, and it is equipped with a 1.5 kW UV lamp protected by a quartz tube. The reactor has a 1 hp supply pump and 2 feeding pumps, one for the purpose of adding H202 and the other to regulate the pH when necessary. Samples of the sulfurous water from six different natural sources in the City of Puebla, MCxico were collected and characterized (Table 1).

The characterization of the sulfurous water and the experimental monitoring were conducted using cell tests for the SPECTROQUANT SQ118 photometer (Merck, Dmstadt, Germany). Table 2 presents the parameters and the method number (from the manual for the photometer SQ118) used in the analysis.

Water hardness was determined using a complexometric valuation method Aquamerck 1.1 1104.0001 from Merck (Darmstadt, Germany). After the photooxidation processes, the residual hydrogen peroxide content (or other peroxides that formed when ozone was used) were determined using the paper indicator, peroxide test 25 (0.5-25 mg L-I) from Merck. Measurements of pH were determined on a potentiometer PC-18 from Conductronic (Puebla, MCxico). Al(OH)3, MgO and A1203 (Baker, Phillipsburg, NJ) were used in the sulfate-elimination experiments. The concentration of the hydrogen peroxide used is 50% vol/vol.

Ecofloc 6700 and 6708 flocculants from Ecotec S.A. de C.V (Naucalpan, Mbxico) were used in a 0.15% (vol/vol) aqueous solution. The zeolites used were provided by the Zeolite Research Department of the Institute of Sciences at the Bentmerita Universidad AutQoma de Puebla. For the statistical analysis, an analysis of covariance (ANCOVA) test used the Statgraphics, version 4.1 program (Manugistics Inc., Rockville, MD).

The W lightlH202lair batch-reactor system. The batch reactor was filled with 800 mL of sulfurous water, and hydrogen peroxide (6 X mL L-') was added as a source of OH radicals to oxidize the sulfur compounds present to produce sulfates. A flow of air was allowed to bubble through the liquid to agitate the reacting system and at the same time to act as a source of oxygen (400 mL min-' of oxygen). The irradiation time was 15 min, and after every 5 min interval, the pH value, the concentrations of the sulfur compounds and the quantity of the remaining hydrogen peroxide were measured.

Table 2. The methods used in the SPECTROQUANT SQ118 program

Parameter Method number

so,-2 so3-2 Mg+' Cat' HS- NO3- N&+ Turbidity

133 134 151 15

99, 100 117 126 113

The UV lightlH202 jlow-reactor system. A tank was filled with 66 L of sulfurous water and hydrogen peroxide (6 X 10" mL L-') was added. The water was irradiated under conditions of recirculation and regulated flow. The pH value, the concentrations of the sulfur compounds and the quantity of the remaining hydrogen peroxide were determined at 5 min intervals.

The UV lightlH~O2/O31air batch-reactor system. The batch reactor was filled with 800 mL of sulfurous water and hydrogen peroxide (6 X lo4 mL L-'). A flow of air and ozone was allowed to bubble through the liquid to agitate the reacting system as well as to act as a source of oxidants. The liquid was irradiated for 3 min. The irradiated water was characterized at 1 min intervals to determine the pH and the remaining amount of peroxide.

The W IightlH~O~IO~lair $ow-reactor system. Hydrogen peroxide (1.5 X lo4 mL L-') was added to a tank filled with 66 L of sulfurous water. A diffuser was connected by pipe to the ozone generator and introduced into the feeding tube of the flow reactor. The sulfurous water was irradiated, allowing only a single passage through the reactor. The water obtained was characterized, and the remaining amount hydrogen peroxide and the pH were determined.

Table 3. Results of the photooxidative treatment with the UV light/€I'Od air batch-reactor system

The photooxidative treatment,

H202 6 X mL L-' irradiation time*

Sulfurous water (800 mL) before

Source treatment 5 min 10 min 15 min

s2- (mg L-') 1 0.540 0.004 0.001 0 2 6.0 0.008 0.002 0 3 4 s 0.028 0.006 0

1 18.4 ND ND 0.4 2 29.4 ND ND 1.4 3 6.5 2.4 1.4 1 .o

so3'- (mg L-')

sod2- (mg L-'1 1 730 ND ND 815 2 520 ND ND 580 3 920 935 940 953

1 6.50 6.13 6.81 6.83 2 6.35 6.5 1 6.70 6.90 3 6.45 6.48 6.52 6.70

PH

Turbidity (UNF) 1 ND ND ND ND 2 31 18 5 2 3 24 12 3 1

*ND = not determined.

638 Fernando Hernandez and Gunther Geissler

Table 4. Results of the photo-oxidative treatment with the UV light/H202 flow-reactor system

~~ ~~ -

Time of irradiation (min)

5 10 15 20 Flow H202

(L min-') L min-') HS-' (mg L-')

9.5 100 0.020 0.013 0.013 0.009 11.4 50 0.010 0.007 0.007 0.005 13.6 150 0.020 0.015 0.010 0.006 26.5 7.5 0.008 0.005 0.005 0.001 26.5 6 0.006 0.005 0.005 0.003 26.5 15 0.006 0.005 0.005 0.005

The 03/air system. Sulfurous water (800 mL) was placed in the batch reactor and without any irradiation a flow of air and ozone was allowed to bubble through the liquid using a gas diffuser. The water obtained was char- acterized at 5 min intervals to determine the amount of sulfide remaining.

Experiments to reduce the sulfate concentration. The concentration of sulfates in the sulfurous water before and after irradiation was always higher than the concentration allowed by drinking water standards. To comply with those standards, several procedures were applied to reduce the sulfate concentration.

Experiments with zeolite and hydrotalcite. Irradiated sulfurous water (1 L) was allowed to pass through a column filled with solid material. Fractions of 20 mL were collected, and the concentration of sulfates was determined.

Experiments with AI(OH)j, MgO and AI2O3. The solids were added to the irradiated sulfurous water. The mixture was agitated for different lengths of time. The sulfate concentration was determined after filtration.

Experiments with jocculants. An aqueous solution (1 mL) of the floc- culants (0.15% vol/vol) was added to 1 L of irradiated sulfurous water and the mixture was agitated slowly. The sulfate concentration was determined after decantation and filtration.

Simulation of reverse osmosis. The simulation was realized using Winflows version 2.0.33 software (GE Osmonics, Minnetonka, MN). An AG8040F membrane (GE Osmonics) was used for the simulation. The concentration of used for the simulation corresponds to the results of the photochemical experiments, whereas the concentrations of Cazf, Mg2+, Naf, NH4+, C1-, HC03+, and CO2- were provided by a TINEP Company (Puebla, MCxico) potabilization plant, which uses a nonphotooxidative method of potabilization of sulfurous water.

RESULTS The UV light/H202/air batch-reactor system

Table 3 shows the best results obtained from the treatments of sulfurous water of three different sources.

The UV light/H202 flow-reactor system

Table 4 shows the results of the photooxidation of sulfide obtained in the treatment of sulfurous water with samples from the same source. The initial concentration of sulfide was 6.34 mg L-'.

Table 5. Results of the photooxidative treatment with the UV light/HzOd 03/air batch-reactor system

Irradiation HS- so32- time (min) (mg L-') (mg L-9 PH

0 6.34 1.5 6.35 1 0.017 6.40 2 0.003 6.48 3 0.000 1 .o 6.52

Table 6. Results of 03/air treatment without UV light -~ ~~

Irradiation HS- Turbidity time (min) (mg L-') (UNF)* PH

0 5

10 15

~~

0.022 4 6.35 0.0 18 6 6.37 0.017 ND 6.40 0.010 28 6.42

*ND = not determined.

The UV light/H202/03/air batch-reactor system

Table 5 shows the best result of several experiments made. The initial concentrations of the sulfurous water were: 6.34 mg L-' of HS-; 1000 mg L-' of S042- and 1.5 mg L-' of SO$-. The amount of hydrogen peroxide added was of 6 X lo4 mL L-I.

The UV light/H202/0&ir flow-reactor system

The concentrations of the sulfur compounds in the sulfurous water before the treatment were 4.5 mg L-' of HS-; 780 mg L-' of S04-2 and 1.0 mg L-' of S 0 3 - 2 . The concentration of hydrogen peroxide added was 1.5 X lo4 mL L-' of the sulfurous water. The water passed through the reactor only once (20.8 L mir-'). The treated water shows a turbidity of 5 turbidity units (UT(F)). The water was filtered and characterized, and the HS- concentra- tion decreased to zero.

The 0Jir system without UV light

Table 6 shows the best results obtained.

Elimination of sulfates

Table 7 shows the best results obtained in such trials.

Simulation of reverse osmosis

The simulation of reverse osmosis with Winflows 2.0.33 software was used to anticipate the elimination of salts. The results of this

Table 7. Elimination of sulphates

SO,-* initial final concentration concentration

Material (mg L-') (mg L-') Observations

Zeolite ZNM-4

Nitrated hydrotalcite

MgO/A1203 1 : 1

Ca(OH)2/Al(OH), 1 :1

ECOFLOC 6708 ECOFLOC 6700/

ECOFLOC 6700 + AI(OH),

coagulant

998

500

823

823

823

800 990

600

543 Increase of sulphate (fracction 30) concentration by

desorption alters the 30th fraction

synthetic, expensive and has to be activated

for 8 h

for 5 h

9 Hydrotalcite is

550 Has to be agitated

580 Has to be agitated

548 Instantaneous

520 Needs agitation 600 Needs a treatment

in two steps 230

Photochemistry and Photobiology, 2005, 81 639

Table 8. Results of the simulation effect by reverse osmosis

Initial concentration 14007 5007 2177 1.397 546t 10007 1.877 5803 Final concentration 71211 25511 1155 0.808 115s 501 11 0.9411 247 8

?Data from a sulfurous water treatment plant before the reverse osmosis. $Results of the phototreatment. §Meets the World Health Organization (WHO) regulations. /(Not regulated.

simulation are shown in Table 8. To obtain 300 L s-' of drinking water, the simulator calculated required reverse osmosis with 1008 eight inch AG8040F membranes with an array of 112 housings with six membranes each in the first bank and 56 housings in the second bank as shown in the Fig. 1. The efficiency of the membranes was 75%. The size of the pump to move 350 L s-' was 425 kW (550 hp) and required 0.35 kW mP3 of electrical energy.

DISCUSSION In short periods of irradiation, the photochemical treatment of sulfurous water provided satisfactory results. No additional chemical compounds are added to water except H202 andlor ozone. The results obtained are independent of the source of the sulfurous water and of the concentration of the sulfur compounds. No catalysts needed to be added or eliminated, as is the case with photocatalysts. The best results are obtained from use of the UV light and a mixture of ozone and hydrogen peroxide, both at very low concentrations. Approximately 99.7% of the sulfides were oxidized during the first minute of irradiation.

In the treatments in which ozone was used, the turbidity increased because of the formation of a solid substance that was filtered, dried and identified as carbonate. In quantity, it weighed 0.lg L-l of treated water. Table 9 shows the best results obtained for the final concentration of the sulfide from the different treatments.

ANCOVA statistical calculations of the results of all photoox- idation experiments show that the photooxidation takes place efficiently and independent of the flow and the initial concen- trations of sulfurous water. The characterizations of the water irradiated during different time intervals show significant variations among the different concentrations of sulfur compounds (P < 0.05). With respect to the initial concentrations of hydrogen peroxide and the flow of sulfurous water, the results are independent of the photooxidation process ( P < 0.05).

Table 9. The best results obtained in the photooxidation treatment

In the elimination of sulfates, the use of different absorption or precipitation processes did not provide satisfactory results except for the use of hydrotalcite. However, the hydrotalcite used needed to be nitrated and that increases the concentration of undesired nitrate in the water. Natural hydrotalcite can also be used for this purpose, but it has to be activated by heating to 250°C. The reverse-osmotic simulation predicts a satisfactory elimination of the dissolved salts, but a pilot plant would have to be constructed to test those results.

CONCLUSIONS The photooxidative process was used for the first time to oxidize sulfur compounds in naturally sulfurous waters. In a batch reactor it was possible to demonstrate that the sulfur compounds of the sulfurous waters could be oxidized to sulfate in a UV light/H20d air system with very small concentrations of hydrogen peroxide (6 X mL L-I). In a flow reactor it was possible obtain the same results by adding only 6 X lo4 mL L-' of hydrogen peroxide.

In the flow reactor it was possible to increase the efficiency of the photooxidation process by using a mixture of ozone (40 mg/h) and 1.5 X lo4 mL L-' of hydrogen peroxide. In 1 min of irradiation and at a flow of 20.8 L min-' approximately 99.7% of the sulfur compounds were oxidized.

The photooxidation process provides good results independent of factors such as the concentration of sulfur compounds, the source of sulfurous water and the water flow through the reactor. A simulation of the reverse osmosis was used to predict the type and magnitude of a plant needed for elimination of sulfates and other salts to obtain drinking water by this process.

Acknowledgements-We are grateful to the companies Germ-Ex (Mtxico City, Mkxico) for lending us the experimental flow reactor, Clear Water Technologies (Puebla, Mtxico) for helping us with the reverse-osmosis simulations and to the nonphotochemical desulfuration plant of the com- pany TINEP (Puebla, Mtxico) for giving us access to typical data on ion

Hz02 Flow Irradiation (mL L-') HS-' final Photooxidation Type of

system reactor (L min-') time (rnin) x (mg L-') Observations

UV/H20z/air Batch 10 60 0.001 uv/H*o* UV/H20J03/air Batch 3 6.0 0.000 UV/H20J03/air Flow 20.8 1.5 0.000 Without recirculation

Flow 26.5 6.0 0.006 10 min of recirculation

640 Fernando Hernandez and Gunther Geissler

Resyclmg [SO Lr ' I

Figure 1. Array of the membrane system.

concentrations of desulfurated water before being treated by reverse os- mosis. F. Hemhdez thanks Conacyt, Mexico, for scholarship 122996.

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