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Materials Today Advances 4 (2019) 100025

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Materials Today Advances

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Removal of water-soluble dyes and pharmaceutical wastes bycombining the photocatalytic properties of Ag3PO4 with theadsorption properties of halloysite nanotubes

E. Nyankson a, b, *, R.V. Kumar b

a Department of Materials Science and Engineering, University of Ghana, LG 77, Accra-Ghana, UKb Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB3 0FS UK

a r t i c l e i n f o

Article history:Received 1 July 2019Received in revised form12 October 2019Accepted 21 October 2019Available online 26 November 2019

Keywords:Silver phosphateNano clayPhotocatalysisPhotodegradationWater purification

* Corresponding author.E-mail address: enyankson@ug.edu.gh (E. Nyankso

https://doi.org/10.1016/j.mtadv.2019.1000252590-0498/© 2019 The Author(s). Published by Elsevie

a b s t r a c t

Contaminated water can be best treated by a combination ofzi water treatment technologies such asphotocatalysis and adsorption. A photocatalyst-adsorbent system made up of Ag3PO4 and halloysitenanotubes (HNTs) was synthesized by precipitating Ag3PO4 in the presence of dispersed HNTs. Thesynthesized Ag3PO4eHNTs were characterized by X-ray diffraction (XRD), Fourier transform infra-redspectroscopy (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDX), ther-mogravimetric analysis (TGA), and diffuse reflectance spectroscopy (DRS). The as-prepared Ag3PO4

eHNTs showed high photocatalytic activity in visible light and high efficiency of adsorption for water-soluble dyes, such as methylene blue and rhodamine B, and pharmaceutical chemicals, such as diclofenacsodium, ibuprofen, flurbiprofen, naproxen sodium, and carbamazepine. The efficiency of the photo-catalytic activity and the adsorption capacity of Ag3PO4eHNTs were dependent on the percentage ofHNTs in the photocatalytic-adsorbent material of Ag3PO4eHNTs. The intermediate products for thephotodegradation of ibuprofen and naproxen sodium were identified with liquid chromatography-massspectrometry (LC-MS). The synthesized Ag3PO4eHNT composites showed good potential application inthe treatment of polluted water.© 2019 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Water is the most basic need for the support of life and hence avaluable asset for the human race. However, water shortage,decline of water quality through environmental pollution, emer-gence of water-borne diseases, and unavailability of safe drinkingwater are major problems facing most countries worldwide. Evi-dence is growing that many, if not most, water bodies are pollutedby water-soluble dyes and emerging contaminants like drugs,pharmaceutical chemicals, and personal care products [1e4].

The increased global production of drugs has resulted inincreased pharmaceutical contaminants in our water bodies. Largeamounts of drugs prescribed and consumed by the populace aredischarged and end up in contaminating surface and ground waterbodies. Typical examples of these drugs detected in trace amountsin surface and underground water bodies include ibuprofen and

n).

r Ltd. This is an open access article

diclofenac. The detection of these drugs in surface and groundwaterbodies has been attributed to the fact that they are stable and henceresilient to prevailing wastewater treatment technologies [5e10].Drug stability is very important hence the ingredients used in themanufacturing of pharmaceuticals are aimed at enhancing thestability of prepared drugs. It has therefore been reported thatconventional contaminated water treatment technologies(CCWTTs) are able to abate only a fraction of pharmaceutical pol-lutants present in water bodies [11e13]. Pharmaceutical wastes areonly partly degraded by CCWTTs by strong oxidizers such ashydrogen peroxide, permanganates, and ozone. Application ofthese oxidants leads to the generation of toxic secondary by-products. As an example, ibuprofen is partially removed by CCWTTsand its main metabolite forms remain after treatment and havebeen reported to be toxic to the aquatic environment [14,15].

In addition to pharmaceutical wastes, most surface and groundwater are polluted by water-soluble dyes. The annual global pro-duction of dye is approximately 700,000 tons [16]. Some examplesof dyes are methyl red, rhodamine B, orange G, methylene blue, andCongo red [17]. These dyes are used in the tanning, textile,

under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 1000252

cosmetics, pharmaceutical, leather, printing, and paper-makingindustries. During the dyeing process, approximately 15e20% ofdyes is lost and result in the non-esthetic pollution of our envi-ronment [18,19]. Environmental pollution by dyes has serious effecton aquatic species and human health. Conventional biologicaltreatment methods are ineffective in decolorizing and degradingdyes because of the presence of aromatics and stable dye molecules[20e22]. Treatment technologies such as chlorination and ozona-tion have been adopted recently; however, they have high oper-ating costs and can cause secondary pollution from the productsarising from the treatment [23].

Different remediation strategies such as coagulation, adsorption,reverse osmosis, advanced oxidation processes (AOPs), filtration, andbiodegradation have been developed to manage pharmaceutical anddye wastes. AOPs are effective in degrading non-biodegradable,chemically stable, and persistent pollutants. Photocatalysis, photol-ysis, and photo-Fenton and electrochemical oxidations are some ofthe AOPs used for handling pharmaceutical wastes and dyes [24e27].Reactive oxygen species (ROS) are generated in AOPs and can oxidizea broad range of water contaminants.

Among the AOPs mentioned, photocatalysis is an important,emerging, and currently the most investigated technology[19,28e30]. The mechanism of heterogeneous photocatalysis onthe surface of a semiconductor involves irradiating a semi-conducting material with a light source; photons with energygreater than the band gap are absorbed resulting in the excitationof electrons from the valence band (VB) to the conduction band(CB). This results in the creation of electrons and holes, which areused in the formation of ROS. The ROS created attack organic pol-lutants degrading them into harmless compounds such as H2O andCO2 [31e33]. Renowned researchers have investigated and re-ported on photodegradation of pharmaceuticals such as ibuprofen[34e38], diclofenac [39e42] , and naproxen [43,44]. Treatment ofwater-soluble dyes by photocatalysis has already been reported, forexample, photodegradation of methylene blue [45,46], rhodamineB [47,48], methyl orange [49,50], and Congo red [51,52]. Some of thephotocatalysts that have been explored for their potential appli-cation in water splitting and photodegradation of pharmaceuticalsand dyes include: TiO2 [30,53,54], AgeTiO2 [55], Ag3PO4 [56,57], g-C3N4 [58], TiO2/Ag3PO4/graphene [59], Z-scheme g-C3N4/MoS2/Ag3PO4 [60], Ag3PO4/g-C3N4 [61e63], MOSe2/Ag3PO4 [64], rh-In2O3[33], Ag2Se [65], g-C3N4/Ag3PO4/Ag2MoO4 [66], and Ag3PO4egraphene [67]. The photocatalyst Ag3PO4 has attracted attention asan efficient photocatalyst for environmental remediation because itwas reported that Ag3PO4 can oxidize water and result in theevolution of O2 [68]. Ag3PO4 is visible light active and hasmagnificent photocatalytic properties. The superb photocatalyticactivity of Ag3PO4 is ascribed to the good separation of photo-excited holes and electrons. The excellent photocatalytic proper-ties of silver-based photocatalysts have been explored for thephotodegradation of various dye pollutants [69] and tetracyclinehydrochloride [70]. In addition, Ag3PO4 has exhibited excellentoxidative capacities for the evolution of O2 from water [71]. Aliterature survey has revealed that Ag3PO4 is mainly used for thedegradation of dyes and its potential in degrading pharmaceuticalshas not yet been widely investigated.

In addition to photocatalysis, the adsorption process has beenemployed for the removal of pharmaceuticals and dyes [72].Different adsorbents can be used in this process such as meso-porous silica SBA-15 [73], Zn-zeolite [74], activated carbon [75],microporous coordination polymers [76], cellulosic waste orangepeels [77], carbon nanotubes [78], and halloysite nanotubes [69,79].Among these materials, halloysite nanotubes (HNTs) find wideapplication in different fields [32,69,80e85] because of the fact thatthey are naturally occurring, biocompatible, have high water

dispersity, non-toxic, readily available, and relatively cheapcompared to carbon nanotubes. HNT is a naturally occurring 1:1aluminosilicate clay nanotube, which is mined from various de-posits. HNTs have a rolled tubular structure with the chemicalformula Al2Si2ðOHÞ4,nH2O. HNTs have a pore volume and specificsurface area of approximately 1.3 mL=g and 65 m2=g, respectively[86]. The outer surface of HNTs is slightly negatively chargedwhereas the inner surface of the lumen is slightly positivelycharged [87]. This variation in the surface charge characteristics ofHNTs enables them to selectively adsorb molecules onto theirsurface and in the lumen. The potential of HNTs in adsorbingvarious pollutants such as dyes [79,88,89], heavy metals [90,91],and biopolymers [92] has been reported.

The use of a composite material made up of silver phosphatephotocatalyst and HNT is expected to enhance the removal of phar-maceuticals anddyesbyphotocatalysis and adsorptionprocesses. Theaddition of HNTs may improve the dispersity of the synthesizedAg3PO4 in the composite and also give greater opportunity for ROS toact on adsorbed chemicals, thus enhancing the photocatalytic activityby additional synergy. In this work, Ag3PO4eHNTs composite wassynthesized and characterized by Fourier transform infra-red spec-troscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction(XRD), scanning electronmicroscopy-energy dispersive spectroscopy(SEM-EDX), and diffuse reflectance spectroscopy (DRS). The potentialof Ag3PO4eHNTs in removing pharmaceuticals (ibuprofen, naproxensodium, diclofenac sodium, flurbiprofen, and carbamazepine) andwater-soluble dyes (methylene blue and rhodamine B) by adsorptionandphotocatalysiswasexamined. Thephotodegradationby-productsof ibuprofen and naproxen sodiumwere determined using LC-MS.

2. Experimental procedure

All chemical reagents used in this study were purchased fromSigma Aldrich, UK and used without further purification.

2.1. Ag3PO4eHNT Nanocomposite synthesis

The Ag3PO4eHNTs nanocomposite was prepared by dispersing aknown amount of HNTs in 0.02 M AgNO3 (ca. 150 mL) by sonicatingfor 25min. Then,122.5mL of 0.2MNa2HPO4was added. The solutionwas stirred continuously for 30 min resulting in the formation ofbright yellow precipitates. The resulting bright yellow solution wascentrifuged to collect the Ag3PO4eHNT particles. The collected par-ticleswerewashedwithDIwater, centrifuged (repeated three times),and dried at 70 �C to obtain Ag3PO4eHNTs nanocomposite.Ag3PO4e25 wt% HNT, Ag3PO4e50 wt% HNT, and Ag3PO4e75 wt%HNT nanocomposites were synthesized by varying the amounts ofHNTs used. The above synthesis method was used without theaddition of HNTs to obtain single-phase Ag3PO4 particles.

2.2. Product characterization

XRD measurement was conducted using a Bruker D8 advancedfocus diffractometer with Cu-Ka radiation (l ¼ 0.15405 nm) and a2q range of 10�e80�. An FEI Nova NanoSem scanning electron mi-croscope fitted with an EDX acquisition detector was used to studythe morphology and elemental composition of the synthesizedsamples. FTIR spectroscopy was carried out using a Bruker Tensor2027 FT-IR instrument with infrared spectra of 400e4000 cm�1. ATA instrument Q500 thermogravimetric analyzer was used toinvestigate the thermal behavior of nanocomposites. The TGA wascarried out in a nitrogen atmosphere and at a heating rate of 10 �C/min. Ag3PO4e25 wt% HNTs were used during the TGA. DRS analysiswas carried out using UVevis spectrometry. Before the DRS

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 3

measurement was done, the synthesized photocatalysts werepressed between two glass plates to form a thin flat film.

2.3. Photocatalytic degradation of rhodamine blue andpharmaceutical drugs

The ability of the synthesized Ag3PO4eHNTs to degrade pol-lutants was examined using rhodamine B dye and pharmaceuticaldrugs (diclofenac sodium, naproxen sodium, ibuprofen, flurbipro-fen, and carbamazepine). In a typical photodegradation test,300 mg of the photocatalysts was added to 300 mL of 5 mg/Lrhodamine B solution. The photocatalysis experiment was con-ducted in a jacketed glass reactor fitted with a 26W Sylvania visiblelamp. Before the photocatalysis experiment, the dye-eAg3PO4eHNTs mixture was stirred in the dark for 30 min. Thiswas done to achieve an adsorptionedesorption equilibrium. Aspecific volume (2 mL) of the visible light irradiated dyeephoto-catalyst mixture was sampled at specific time intervals. Thepowdered photocatalyst was removed from the sampled volume bycentrifuging at 6000 rpm for 5 min. The concentration of the dye inthe sampled volume was estimated by measuring the absorbance(l ¼ 554 nm) using a UVevis spectrophotometer. A similar pro-cedure was used for the photocatalytic degradation of the phar-maceutical drugs. The UVevis absorbance values were measured at276 nm (diclofenac sodium), 230 nm (naproxen sodium), 222 nm(ibuprofen), 247 nm (flurbiprofen), and 285 nm (carbamazepine).The photodegraded by-products from ibuprofen and naproxen so-dium were analyzed with LC-MS.

2.4. Procedure for adsorption of methylene blue dye

The adsorption characteristics of Ag3PO4eHNTs were investi-gated using methylene blue dye. Approximately 100 mg ofAg3PO4eHNTs was added to methylene blue (5 mg/L) dye solution.At specified time intervals, 2 mL of the dye solution was sampled.The nanocomposite was removed by centrifuging at 6000 rpm.UVevis spectrophotometry was carried out to monitor the successof the adsorption process by measuring the absorbance at awavelength of 655 nm. The adsorption equilibrium study wasconducted using different concentrations of methylene blue.

3. Results and discussion

3.1. Characterization of Ag3PO4eHNTs

XRD patterns of the pristine Ag3PO4, HNTs, and Ag3PO4eHNTsare presented in Fig. 1. The peaks for Ag3PO4 depict a well-definedcrystalline Ag3PO4, exhibiting a body centered cubic crystal struc-ture (JCPDS No. 06e0505). On the other hand, from Fig. 1(a) and (b),it can be seen that HNT is very poorly crystallized. As shown inFig. 1(b), the characteristic peak of HNT (110) appeared at a 2q valueof ca. 20.1�. The XRD patterns of all the synthesized Ag3PO4eHNTsare similar to that of the pristine Ag3PO4, implying that the additionof HNTs to Ag3PO4 during synthesis did not have any effect on thecrystal structure of Ag3PO4.

The SEM-EDX images of Ag3PO4, HNTs, and Ag3PO4e50 wt%HNTs are shown in Fig. 2. Ag3PO4 consists of irregularly shapedspherical particles with diameter in the 100e300 nm range. Thisagrees with SEM images of Ag3PO4 reported by Wang et al. [93]. Acloser look at the particles (inserted) indicated that the particlesurface is relatively rough.

The presence of the HNTs in the Ag3PO4eHNTs composite can beobserved in Fig. 2C and D. It is obvious from Fig. 2 that addition ofHNTs did not alter themorphology of Ag3PO4. EDXmapping (Fig. 2Eand F) showed Si and Al bands, which are attributed to the presence

of the HNTs. The elemental composition obtained from the EDXanalysis is consistent with that of HNTs because the mole ratio ofSi:Al in HNTs is 1:1. The chemical formula for HNTs isAl2Si2(OH)4.nH2O.

The FTIR spectra of the pristine Ag3PO4, HNTs, andAg3PO4eHNTs are presented in Fig. 3. For the pristine Ag3PO4, thepeak at 937 cm�1 corresponds to the PeO stretching vibrationmode of PO4 [94,95].

For pure HNTs, sixmajor peaks were identified. The deformationvibrations of the outer surface hydroxyl groups resulted in a peak at905 cm�1. On the other hand, the peak observed at 1000 cm�1 isascribed to the stretching vibrations of SieOeSi. The peaks at1118 cm�1 and 1647 cm�1 resulted from the stretching mode ofepical SieO and deformation vibration of the interlayer water,respectively. The stretching vibrations of the inner surface hydroxylgroups are represented by the peaks at 3691 and 3621 cm�1 [79].The identified peaks for the Ag3PO4eHNTs are nearly identical tothat of Ag3PO4. The intensities of the peaks at 905 and 1000 cm�1 inthe Ag3PO4eHNT photocatalysts were dependent on the amount ofHNTs in the various batch formulations.

Fig. 4 shows the TGA of (A) HNTs, (B) Ag3PO4, and (C)Ag3PO4e25 wt% HNTs from 25 �C to 1000 �C under flowing N2 gascondition. The change in weight with temperature of the differentsamples is presented in Fig. 4. Fig. 4(A and C) reveal multistagethermal decompositionwith stable intermediates. Ag3PO4 recordedonly a 0.07 wt% loss in the entire temperature range analyzeddepicting that Ag3PO4 is thermally stable.

Two distinct decomposition events were observed for HNTs atca. 60 �C and 450 �C. The weight loss at 60 �C and 450 �C isattributed to the thermal decomposition of adsorbed water mole-cules on HNTs and thermal dihydroxylation of HNTs, respectively. Asimilar observation was made for the Ag3PO4e25 wt% HNTs.Because the TGA curve for Ag3PO4 was straight (with negligibleweight over the entire temperature range), the weight loss inAg3PO4e25 wt% HNTs is due to the presence of the HNTs.

The optical properties of Ag3PO4 and Ag3PO4eHNTs wereexamined with DRS (Fig. 5). The DRS spectra indicate that thesynthesized Ag3PO4 and Ag3PO4eHNTs absorb visible light. Theintensity of the visible light absorption decreased upon the addi-tion of 25 wt% HNTs to Ag3PO4. However, the reflectance ofAg3PO4e50 wt% HNTs is almost the same as that of Ag3PO4 anddecreased when the amount of the HNTs was increased to 75 wt%.The effect of the HNTs on band gap of the composite Ag3PO4eHNTswas investigated using the KubelkaeMunk model [96]. The bandgap estimated from Fig. 5(BeE) are 2.47, 2.47, 2.43, and 2.39 eV forAg3PO4, Ag3PO4e25 wt% HNTs, Ag3PO4e50 wt% HNTs, andAg3PO4e75 wt% HNTs, respectively. An increase in concentration ofthe HNTs therefore resulted in a small reduction of the band gap;thus, the difference is not significant. This is similar to the obser-vation made by Peng et al., who did not observe any significantchange in the band gap of ZnO after the addition of HNTs [97]. Theband gap values calculated from the KubelkaeMunk modelrevealed that both Ag3PO4 and the Ag3PO4eHNTs composite arevisible light active.

3.2. Photocatalytic degradation of rhodamine B dye

The photocatalytic activity of the synthesized Ag3PO4 andAg3PO4eHNTs was examined using the rhodamine B dye and re-sults are reported in Fig. 6(A). Photolysis of the methylene blue dyewithout any photocatalyst revealed negligible degradation ofrhodamine B. The reduction in dye concentration with timeobserved in Fig. 6(A) is therefore solely due to the presence ofAg3PO4 or Ag3PO4eHNTs. Before irradiating with light, the dyee-photocatalyst mixture was agitated in the dark to attain an

Fig. 1. XRD patterns of (A) Ag3PO4eHNTs and (B) magnified XRD pattern for halloysite nanotubes.

Fig. 2. Scanning electron microscopy images of (A) Ag3PO4, (B) halloysite nanotubes, (C and D) Ag3PO4e25 wt% HNTs, and (E and F) EDX mapping of Ag3PO4e25 wt% HNTs.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 1000254

adsorptionedesorption equilibrium. The reduction in the dyeconcentration which resulted from adsorption is reported (the first30 min portion of Fig. 6(A)). Ag3PO4 adsorbed ca. 11% of the cationic

rhodamine B dye. Ag3PO4 possesses a negatively charged surface atneutral pH [98]. The positively charged rhodamine B dye wasattracted electrostatically to the anionic Ag3PO4. This allowed the

Fig. 3. FTIR spectra of Ag3PO4, HNTs, and Ag3PO4eHNTs.

Fig. 4. TGA of (A) HNTs, (B) Ag3PO4, and (C) Ag3PO4e25 wt% HNTs.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 5

cationic dye to be adsorbed onto Ag3PO4. The amount of rhodamineB dye adsorbed increased with increasing amounts of the HNTs inthe Ag3PO4eHNTs (Table 1 and Fig. 6(A)). This implies that HNTsaffected the degree of adsorption of rhodamine B by Ag3PO4eHNTs.This observation is not surprising because a large number of re-searchers have investigated the potential of HNTs in adsorbing dyemolecules and have reported that indeed HNT has a high adsorp-tion potential toward water-soluble dyes [88,99e101]. HNT pos-sesses a negatively charged surface below a pH of 8.5 [86]. Hence,the increase in adsorption can be ascribed to the electrostaticinteraction between the negatively charged surface of HNTs and thecationic rhodamine B. Generally, the photocatalytic activity ofAg3PO4eHNTs was lower when compared to that of Ag3PO4.However, it was deduced that, the degradation rate of Ag3PO4

within the first 2 min is slow (ca. 28%) compared to Ag3PO4e75 wt%HNTs (ca. 36%). The photocatalytic activity was fitted with apseudo-first order (PFO) kinetics and the rate constants (k) arereported in Table 1. The calculated rate constants are 0.3285,

0.2370, 0.1576, and 0.2010min�1 for Ag3PO4, Ag3PO4e25wt% HNTs,Ag3PO4e50 wt% HNTs, and Ag3PO4e75 wt% HNTs, respectively. Anincrease in the adsorption of contaminants on the photocatalyst isexpected to result in an increase in the photocatalyst activity.However, too much coverage will result in a decrease in availableactive sites for the generation of ROS and may influence negativelyon the photocatalytic activity. This may explain why the increase inadsorption was accompanied by a reduction in the photo-degradation performance. However, at higher amounts of HNTs(Ag3PO4e75 wt% HNTs), the HNTs adsorbed most of the dye fromsolution leaving only a small amount to be degraded photo-catalytically and hence the rate constant valuewas obtained. On theother hand, the overall dye removal efficiencies in 10 min (com-bination of adsorption and photocatalysis) were calculated to be 98,94, 84, and 91% for Ag3PO4, Ag3PO4e25 wt% HNTs, Ag3PO4e50 wt%HNTs, and Ag3PO4e75 wt% HNTs, respectively.

The photostability of Ag3PO4 and Ag3PO4e25 wt% HNTs forvisible light photodegradation of rhodamine B was examined byrecycling the used photocatalyst. From Fig. 6(B), after the 4th cycle,the photodegradation performance of Ag3PO4 and Ag3PO4e25 wt%HNTs has reduced drastically from 98% to 66% and 94% to 64%,respectively. It can, however, be noticed that at lower cycles, thereduction in the photostability was higher for Ag3PO4e25 wt%HNTs when compared with Ag3PO4. This can be attributed to therelatively smaller amount of Ag3PO4 in Ag3PO4e25 wt% HNTs whencompared with Ag3PO4 because equal amounts of material wereused. In addition, at lower cycles (1 and 2) and lower irradiationtime (2 and 4 min), the photocatalytic activity of Ag3PO4 increasedat the 2nd cycle, from 27.5% to 36.4% and from 60.4% to 63.5%. Thisagrees with the published results in the literature [93]. Using SEMand XRD, Henry et al. confirmed the formation of metallic Ag afterfour cycles of Ag3PO4 photocatalysis [102]. Two prominent peaks ofmetallic Agwere observed at 38.1� and 64.2� representing (111) and(220) crystallographic planes, respectively. SEM imaging revealedthe formation of metallic Ag nanocrystals on the surface of Ag3PO4[102]. The photocorrosion of Ag3PO4 is due to its small water sol-ubility (20 mg/L) in solution and its CB energy (0.45 eV) [68,103].With this CB, photoinduced electrons cannot be captured by H2O inthe absence of electron scavengers. Hence the electrons can only becaptured by the Agþ dissolved in solution from Ag3PO4. At theinitial stage of the photocatalytic activity, part of the Agþ in Ag3PO4is reduced to metallic silver nanoparticles [68,104]. These Agnanoparticles serve as electron trapping site during photocatalysisand hence reduce the electronehole recombination [105]. Theholes can then oxidize the rhodamine B dye resulting in its pho-todegradation, thus being responsible for the enhancement of thephotodegradation performance at lower cycles and shorter irradi-ation time. As the cycles are increased, more of the Ag nanoparticlesare formed and this will result in entire surface of Ag3PO4 beingcovered with Ag. As a result, the Ag nanoparticles shield Ag3PO4from the irradiated light and result in a reduction in the photo-degradation efficiency.

HNTs have been reported to possess superb dispersion proper-ties [106]. Based on this, Ag3PO4eHNTs are expected to possess anenhanced dispersion property. To evaluate this, the photocatalyticactivities of Ag3PO4 and Ag3PO4e25 wt% HNTs were examinedwithout stirring the photocatalystedye solution. The results arepresented in Fig. 7 and Table 1. The photodegradation performanceof Ag3PO4 was lower than that of Ag3PO4eHNTs. The addition of theHNTs enhanced the dispersion of Ag3PO4eHNTs and as a resultincreased the contact between the photocatalyst and the dyeresulting in the enhanced photodegradation. This result impliesthat HNT can be used to significantly enhance photodegradation inscenarios where there are limited stirring/mixing.

Fig. 5. (A) DRS of Ag3PO4, Ag3PO4e25 wt% HNTs, Ag3PO4e50 wt% HNTs, and Ag3PO4e75 wt% HNTs; band gap estimation using the KubelkaeMunk method for (B) Ag3PO4, (C)Ag3PO4e25 wt% HNTs, (D) Ag3PO4e50 wt% HNTs, and (E) Ag3PO4e75 wt% HNTs.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 1000256

3.3. Adsorption of methylene blue dye

When examining the potential of the synthesized Ag3PO4 pho-tocatalyst in degrading methylene blue, we noticed that all themethylene blue was adsorbed from the solution within the 30 minof adsorptionedesorption equilibrium duration. Hence, the authors

deemed it prudent to conduct detailed adsorption studies usingmethylene blue and Ag3PO4e25 wt% HNTs. The results for theadsorption studies are presented in Fig. 8 and Table 2. Theadsorption process and mechanism of the adsorption were inves-tigated using the Langmuir and Freundlich equilibrium isotherms.The equation for the Langmuir isotherm is:

Fig. 6. (A) Photocatalytic activity of Ag3PO4 and Ag3PO4eHNTs and (B) photostability of Ag3PO4 and Ag3PO4e25 wt% HNTs.

Table 1Rate constants and overall removal efficiency of Ag3PO4 and Ag3PO4eHNT.

Sample Rate Constant (K/min) Overall Removal Efficiency (%) Percentage of Dye Adsorbed (%)

Ag3PO4 0.3285 98 14Ag3PO4e25 wt% HNTs 0.2370 94 18Ag3PO4e50 wt% HNTs 0.1576 84 26Ag3PO4e75 wt% HNTs 0.2010 91 34

Investigating the effect of HNTs on the dispersion of the nanocomposites

Ag3PO4 0.0748 55 11Ag3PO4e25 wt% HNTs 0.1957 90 17Ag3PO4e50 wt% HNTs 0.1344 81 23Ag3PO4e75 wt% HNTs 0.1490 83 27

Fig. 7. Photocatalytic activity of Ag3PO4 and Ag3PO4eHNTs without stirring thephotocatalystedye solution.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 7

CeQe

¼ CeQm

þ 1KLQm

(1)

In this equation, Ce ¼ equilibrium concentration (mg/L),Qe ¼ equilibrium capacity of adsorption (mg/g),Qm ¼ maximum capacity of adsorption (mg/g), andKL ¼ Langmuir constant (L/mg).

Fig. 8(A) represents the plot of Ce/Qe vs. Ce. The Langmuir con-stant was calculated from the intercept whereas the maximumadsorption capacity was computed from the slope. The computedvalues are presented in Table 2. The R2 value obtained from theLangmuir isotherm is 0.9391. To examine the favorability, orotherwise, of the adsorption process, an equilibrium parameter RLwas computed. RL was calculated from equation (2) and the valuesare presented in Table 2.

RL ¼1

1þ KLCO(2)

Co ¼ initial dye concentration (mg/L)KL ¼ Langmuir constant (L/mg)

RL and Qm of Ag3PO4e25 wt% HNTs were computed to be0.1355e0.4393 and 9.470 mg/g, respectively. The computed valuessuggest that Ag3PO4e25 wt% HNTs have a homogeneous surface.The homogeneous surfacewas covered with a monolayer of the dye

molecules. The adsorption of methylene blue by Ag3PO4e25 wt%HNTs was therefore favorable.

The Freundlich model (equation (3)) in addition to the Lang-muir equilibrium model was used to probe further the adsorptiondata.

lnQe ¼ lnKF þ1nlnCe (3)

Fig. 8. (A) Langmuir and (B) Freundlich adsorption isotherms, and (C) pseudo-first order and (D) pseudo-second order kinetics data for the adsorption of methylene blue byAg3PO4e25 wt% HNTs.

Table 2Calculated adsorption isotherm and equilibrium kinetics parameters for the adsorption of methylene blue by Ag3PO4e25 wt% HNTs.

Adsorption Isotherm and Equilibrium Kinetic Models Parameters Computed from the Adsorption Isotherm and Equilibrium Kinetic Models

Langmuir Qm ¼ 9.470 (mg/g)KL ¼ 0.6383 (L/mg)R2 ¼ 0.9391RL ¼ 0.1355e0.4393 (L/mg)

Freundlich KF ¼ 1.682 (mg/g (mg/L)1/n)n ¼ 3.352R2 ¼ 0.895

Pseudo-First Order Qe ¼ 2.161 (mg/g)K1 ¼ 0.0314(min�1)R2 ¼ 0.4741

Pseudo-Second Order Qe ¼ 8.977 (mg/g)K2 ¼ 0.2776 (g/mg min)R2 ¼ 0.9996

Interparticle Diffusion ModelKP ¼ 1.73858 (mg/g min ½)R2 ¼ 0.9117

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 1000258

Qe ¼ equilibrium capacity of adsorption (mg/g)Ce ¼ equilibrium dye concentration (mg/L)KF ¼ Freundlich constant (mg/g (mg/L)1/n)1/n ¼ adsorption intensity

Higher value of KF depicts higher affinity of Ag3PO4e25 wt%HNTs for adsorption. 1/n (0.1 < 1/n < 1) determines how favorablethe adsorption process is [107,108]. The Freundlich constants werecalculated from Fig. 8(B) and presented in Table 2. The adsorption ofmethylene blue by Ag3PO4e25wt% HNTs is favorable because 1/n is

less than 1. However, a relatively lower KF and R2 of 0.895 show thatthe adsorption process follows the Langmuir isotherm model.

The rate controlling step and the mechanism of the entireadsorption process were examined using the following adsorptionkinetic models: intra-particle diffusion (IPD), pseudo-second order(PSO), and pseudo-first order (PFO) models. The PFO equation is:

logðQe �QtÞ¼ logQe � K1

2:303t (4)

Qe ¼ equilibrium adsorption capacity (mg/g)

Fig. 9. Intra-particle diffusion (IPD) model for the adsorption of methylene blue ontoAg3PO4e25 wt% HNTs.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 9

Qt ¼ the capacity of adsorption at a specific time t (mg/g)K1 ¼ PFO rate constant (min�1)

The parameters K1 and Qe were calculated from Fig. 8(C) and arepresented in Table 2. The PFO kinetic model is not suitable becauseof the low R2 value of 0.474, for describing the adsorption ofmethylene blue by Ag3PO4e25 wt% HNTs.

The adsorption kinetics was then investigated using the PSOkinetic model. The PSO kinetic model is presented by the followingequation;

tQt

¼ 1

K2Q2e

þ tQe

(5)

Qe ¼ equilibrium capacity of adsorption (mg/g)Qt ¼ capacity of adsorption at a specific time t (mg/g)K2 ¼ PSO rate constant (g/mg min)

Qe and K2 were computed from Fig. 8(D). The high correlationcoefficient of 0.9996 suggests that the adsorption process follows aPSO kinetics model. This implies that the adsorption of the dye byAg3PO4e25 wt% HNTs is inclined toward chemisorption. Thechemisorption arose from the electrostatic interaction between thenegatively charged outer surface of HNTs and the cationic methy-lene blue.

The Langmuir adsorption isotherm confirms the monolayeradsorption of methylene blue onto Ag3PO4e25 wt% HNTs. The dyemolecules were probably transported from the dye solution to theAg3PO4e25 wt% HNTs. It is possible that the transfer of the dyemolecules occurred through diffusion. Thus the IPD model [109]presented below (equation (6)) was therefore used to examine theadsorption process;

Qt ¼Kpt1=2 þ C (6)

Qt ¼ the capacity of adsorption at time t (mg/g)C ¼ the intercept (constant)Kp ¼ IPD rate constant (mg/g min ½)

The IPD rate constant (Kp) was computed from Fig. 9. Thecalculated Kp value can be found in Table 2. Fig. 9 shows a multi-stage plot. Within the first 3 min of adsorption, methylene blue wastransported to the external surface of the Ag3PO4e25 wt% HNTsthrough the IPD process [79]. It can be deduced from Fig. 9 that theadsorption process reached equilibrium within 9 min. Theadsorption capacity remained almost constant after 9 min.

3.4. Photocatalytic degradation of ibuprofen, diclofenac sodium,naproxen sodium, flurbiprofen, and carbamazepine

Ag3PO4e25 wt% HNTs and Ag3PO4 visible light photo-degradation of different pharmaceuticals was examined. Thedisappearance of the drugs was monitored by measuring UVevisabsorbance. The application of UVevis absorbance in monitoringthe destruction of pharmaceuticals has been reported [110,111]. Thedecrease of the various drugs was first investigated under visiblelight irradiation without photocatalyst. There was insignificantdecrease in the concentration of the drug within the time frame ofthe experiment. This implies that light irradiation alone is notenough to cause the degradation of the pharmaceuticals. Upon theintroduction of Ag3PO4 and Ag3PO4e25 wt% HNTs, gradual

disappearance of the drugs was observed (Fig. 10 and Fig. S1 in thesupporting document). For the same photocatalyst, the degradationrate varied from one pharmaceutical to the other. This can beattributed to the different stabilities of the pharmaceuticalsinvestigated in this study. The degradation of some of the phar-maceuticals resulted in the formation and appearance of interme-diate compounds and this was observed in the degradation ofibuprofen (Fig. 10C) and naproxen sodium (Fig. 10D). The absor-bance of these intermediates occurred at 312 and 260 nm fornaproxen sodium and ibuprofen, respectively. The shift in the ab-sorption peak of ibuprofen from 220 nm to 260 nm during photo-degradation has been reported by Tian et al. [112]. Plots comparingthe efficiency of Ag3PO4 and Ag3PO4eHNTs in removing the phar-maceuticals from solution are presented in Fig. S1. The first portionof the plots in Fig. S1 represents removal of the pharmaceuticals byadsorption. As was observed for rhodamine B, the removal of thepharmaceuticals through adsorption was pronounced forAg3PO4e25 wt% HNTs when compared with Ag3PO4. This is due tothe presence of HNTs. The photodegradation process was fittedwith a PFO kinetic model.

lnCCO

¼ � kt (7)

Co ¼ initial concentration (mg/L)C ¼ dye concentration at time t (mg/L).k ¼ PFO rate constant (min�1)

The half-life (t1/2) of the entire degradation process was esti-mated from the equation

t1=2 ¼ln 2k

The calculated k and t1/2 values are presented in Table 3.The photodegradation products of ibuprofen and naproxen so-

diumwere identified using LC-MS. Ag3PO4e25 wt% HNTs degradedibuprofen and this was observed by a reduction in the peak level ofibuprofen (m/z 205; retention time (rt) of 6.874 min) and the

Fig. 10. Ag3PO4e25 wt% HNTs photodegradation of (A) 50 mg/L diclofenac sodium, (B) 10 mg/L flurbiprofen, (C) 15 mg/L ibuprofen, (D) 8 mg/L naproxen sodium, and (E) 20 mg/Lcarbamazepine.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 10002510

subsequent appearance of new peaks. The identified intermediateproducts are presented in Fig. 11.

The intermediate products identified from the photo-degradation of ibuprofen are presented in Fig. 11. Thus, it can bededuced that the action on ibuprofen by the ROS resulted in thedecarboxylation of ibuprofen and the subsequent formation of anintermediate product with value of 161 (rt ¼ 5.037 min). The peakthat appeared with value of 221 (rt ¼ 3.280 min) represents theformation of a mono-hydroxylated ibuprofen product (2-[4-(1-hydroxyisobutyl)phenyl]propionic acid). This compound isformed by the hydroxylation of ibuprofen. 4-Ethylbenzaldehydewith value of 133 (rt ¼ 3.931) was also identified. This product mayhave been formed as a result of ,OH-induced degradation ofibuprofen and has also been identified by other researchers

[113,114]. Madhavan et al. [115] also observed these intermediatesafter the sonolysis of ibuprofen. The degradation product ofibuprofen with value of 175 (rt ¼ 3.931 min) has been identified as4-isobutylacetophenone [116]. The product with value of 166(rt ¼ 4.207) may be formed by the hydroxylation of the compoundwith value of 149. From the intermediate compounds identifiedfrom the photodegradation of ibuprofen by visible light irradiationof Ag3PO4e25 wt% HNTs, a probable degradation path was pro-posed and is presented in Fig. 12.

The photodegraded naproxen sodium was also analyzed withLC-MS and four major photodegraded by-products were identified(m/z 158, rt ¼ 4.414 minm 202, rt ¼ 3.372 minm 358,rt ¼ 8.091 min; and m/z 402, rt ¼ 4.277 min). Decarboxylation ofnaproxen sodium will result in the formation of 1-ethyl-5-

Table 3Calculated rate constants (k) and half-life (t1/2) for the photodegradation of different pharmaceuticals using Ag3PO4 and Ag3PO4e25 wt% HNTs.

Drug Rate Constant (K/min) Half Life (t1/2-min)

Ag3PO4 Ag3PO4e25 wt% HNTs Ag3PO4 Ag3PO4e25 wt% HNTs

Diclofenac Sodium 0.02364 0.033 29.3 21.0Naproxen Sodium 0.16367 0.68184 4.2 1.0Ibuprofen 0.00315 0.00224 220.1 309.4Flurbiprofen 0.00105 0.00217 660.1 319.4Carbamazepine 0.00274 0.00328 253.0 211.3

Fig. 11. Structure of intermediate compounds formed from the photodegradation of ibuprofen.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 11

methoxynaphthalene. Oxygen trapping and break down will resultin the formation of an unstable hydroperoxide in the presence ofoxygen. The hydroperoxide formed is responsible for the formationof 1-(6-methoxynaphthalen-2-yl)ethanol (m/z 202) [44,117,118]. Inthis study, the intermediate compounds formed through thedecarboxylation and the resulting unstable hydroperoxidewere notidentified. The detachment of the CeC bond from the naphthalenering of 1-(6-methoxynaphthalen-2-yl)ethanol resulted in the for-mation of 2-methoxynaphthalene (m/z 158). Decarboxylation ofnaproxen sodium followed by subsequent oxidation may lead tothe formation of a dimer with value of 402 [117]. Kanakaraju et al.has reported that a photoproduct m/ 402) may be formed byhydroxyalkylation of the product formed by the reaction of 2-methoxynaphthalene (m/z 158) and 1-(6-methoxynaphthalen-2-yl)ethanol (m/z 202) [44]. Dimerization of 1-(6-methoxynaphthalen-2-yl)ethanol (m/z 202) may lead to the

formation of a product with value of 358. A possible degradationpathway is provided in Fig. 13.

A possible explanation for the observations made is presentedbelow. HNTs have a negatively charged surface in the pH range of2e8. Hence dispersing of HNTs in AgNO3 solution resulted in theattachment of Agþ onto the surface of HNTs. The growth of Ag3PO4therefore occurred on the surface of HNTs upon the addition ofNa2HPO4 resulting in the structure of Ag3PO4eHNTs presented inFig. 14. A heterojunction was therefore created between the HNTsand Ag3PO4. Because the outer surface of HNT is anionic, cationicmolecules such as methylene blue dye can easily adsorb onto thesurface of HNT through electrostatic interactions. Such anadsorption will enhance the degradation of the molecules becausethe Ag3PO4 photocatalyst is also in contact with HNTs. Li et al.reported that the unique structure of HNTs enhances the absorp-tion of visible light [119]. As was observed from the DRS,

Fig. 12. Proposed degradation path for the photocatalytic degradation of ibuprofen by Ag3PO4e25 wt% HNTs.

Fig. 13. Proposed Ag3PO4e25 wt% HNTs degradation pathway of naproxen sodium under visible light irradiation.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 10002512

increasing the amount of HNTs beyond 25 wt% resulted in anincreased visible light absorption. This may enhance the photo-catalytic activity. HNTs may retard recombination of holes andelectrons and as a result enhance the photodegradation perfor-mance [120]. Because the surface of HNTs is anionic, compositematerials formed with HNTs have been reported to possessexcellent hydrophilicity [121]. Hydrophilic Ag3PO4eHNTs com-posite will have high dispersity, which will improve the contactamong the Ag3PO4eHNTs, water, and the dissolved pollutants. Theadsorbed species on HNT surfaces distributed uniformly over theAg3PO4 photocatalysts will offer additional action of ROS on the

pollutants. These factors can account for the enhanced degrada-tion efficiency as observed in this study. The ROS responsible forthe photodegradation of rhodamine B dye by Ag3PO4eHNTs wasexamined using EDTA-Na2, benzoquinone, and t-BuOH. EDTA-Na2served as a scavenger for photogenerated holes, whereas benzo-quinone and t-BuOH served as scavengers for hydroxyl radical andsuperoxide radical, respectively [122]. During the photocatalysistest, EDTA-Na2, t-BuOH, and benzoquinone was added to therhodamine B-Ag3PO4-HNTs mixture. The effect of each of theabove chemicals on the photodegredation efficiency of Ag3PO4-HNTs was then examined.

Fig. 14. Schematic representation of Ag3PO4eHNTs and the photocatalytic process of Ag3PO4eHNTs.

E. Nyankson, R.V. Kumar / Materials Today Advances 4 (2019) 100025 13

4. Conclusion

In summary, Ag3PO4eHNTs composite was synthesized in thepresence of dispersed HNTs. Ag3PO4eHNTs showed high photo-catalytic activity and adsorption for rhodamine B, methylene blue,ibuprofen, diclofenac sodium, naproxen sodium, flurbiprofen, andcarbamazepine. The high adsorption capacity of Ag3PO4eHNTs wasascribed to the presence of HNTs whereas the photodegradationunder visible light was due to Ag3PO4. The photodegradation effi-ciency and the adsorption efficiency were dependent on theamount of HNTs used in the synthesis. The presence of the HNTs didnot enhance the photostability of Ag3PO4 present in the composite.The as-prepared nanocomposites also showed high aqueous dis-persity and an enhanced photocatalytic activity when compared toAg3PO4 without agitation. The combination of photocatalytic ac-tivity of Ag3PO4 and the adsorption capacity of HNTs has endowedthe Ag3PO4eHNTcomposites with good potential application in thetreatment of polluted water.

Data availability

The raw/processed data required to reproduce these findingscannot be shared at this time as the data also form part of anongoing study.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.

Acknowledgment

This work was financially supported through CommonwealthAcademic Fellowship funded by the UK Government, the AlboradaTrust Fund, and the CAPREx Fellowship.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.mtadv.2019.100025.

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