Adsorption of Arsenic, Phosphorus and Chromium by Bismuth

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  Adsorption of arsenic, phosphorus and chromium by bismuthimpregnated biochar: Adsorption mechanism and depleted adsorbentutilization Ningyuan Zhu  a ,  b , Tingmei Yan  a ,  * , Jun Qiao  a , Honglei Cao  a ,  b a Institute of Soil Science, Chinese Academy of Sciences, No. 71 East Beijing Road, Nanjing 210008, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China h i g h l i g h t s   Bismuth activated carbons derived from wheat straw were fabricated.   Bismuth particles grown within biochar matrix.   BiBC500 showed high adsorption capacity to arsenic, phosphorus and chromium.   Adsorption mechanisms of arsenic, phosphorous and chromium were illustrated.   Phosphate depleted material could photodegrade dye pollutant. a r t i c l e i n f o  Article history: Received 20 April 2016Received in revised form25 July 2016Accepted 7 August 2016Available online 28 August 2016Handling Editor: X. Cao Keywords: BismuthActivated carbonAdsorption mechanismDepleted adsorbents a b s t r a c t Bismuth impregnated biochar were synthesized to deal with wastewater pollution. Nitrogen adsorption-desorption isotherms, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy(FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to determine thecharacteristics of adsorbents and explore the main adsorption mechanism. Results showed that bismuthparticle was carried successfully within the biochar matrix, making contributions to creating microporeand boost speci 󿬁 c surface area. The loaded bismuth, served as the adsorption site, rather than the speci 󿬁 csurface area played an important role in arsenic and phosphorus adsorption. Batch adsorption experi-ments demonstrated a  󿬁 t Langmuir model for arsenic (As) and phosphorus (P) and a suitable Freundlichmodel for chromium (Cr). Thermodynamic parameters depicted the endothermic nature and thespontaneous process for phosphate and arsenic adsorption. Besides, this contaminant-loaded carbonadsorbent was further applied for the removal of methylene blue from aqueous solution. ©  2016 Elsevier Ltd. All rights reserved. 1. Introduction The continuous growth of population, increasing reinforcementofagriculturalactivitiesandrapiddevelopmentofurbanizationandindustrialization worsen water quality of our water resourceswhich is the most essential component for life (Ali, 2012). Theexcessive input of phosphorus in surface water worsened eutro-phication and algae blooms as well as deteriorated the quality of the groundwater severely. Heavy metals, especially the chromiumand arsenic, are more problematic and threatening to ecologicalenvironmentandhumanbeingsbecauseof theirhightoxicity,non-biodegradation and accumulation through food chain. Thus, theWorld Health Organization recommended that the maximumpermissible limit for arsenic should be 10  m g L   1 while themaximumlimitforchromiumbe0.05mgL   1 indrinkingwater(Liuet al., 2008; Tang et al., 2014; Wang et al., 2010).Therefore, it is very imperative to deal with contaminatedwastewater prior to its discharge into soil and water environment.Many researchers have explored various methods such as chemicalprecipitation, gravity separation, solvent extraction, reverseosmosis, ion-exchange, electrocoagulation and electrodialysis, 󿬂 otation and adsorption for removing phosphate or heavy mentalirons from sewage system (Hua et al., 2012). For instance, Afkhamiet al. synthesized DNPH (nano-alumina modi 󿬁 ed with 2,4- *  Corresponding author. E-mail address: (T. Yan). Contents lists available at ScienceDirect Chemosphere journal homepage: ©  2016 Elsevier Ltd. All rights reserved. Chemosphere 164 (2016) 32 e 40  dinitrophenylhydrazine) with a satisfying adsorption capacity to-ward Pb (II), Cr (III) and Cd (II) ions in a multiple-metal solution,indicating that DNPH is a promising adsorbent for heavy mentaltreatment (Afkhami et al., 2010). Nevertheless, few researchesdeveloped effective, ef  󿬁 cient, economical and eco-friendly mate-rials or methods for reduction of arsenic, phosphate and chromiumpollutant which are more dif  󿬁 cult to be removed because of theiranionform (Alemayehuet al., 2011;Onnbyet al.,2014).In addition,the removal ef  󿬁 ciency of As (III) is much lower than As (V) foradsorption process was unable to act on the uncharged form of As(III) especially at environmentally relevant pH (Sun et al., 2013).But, arsenite is more toxic, soluble, and mobile than arsenate.Fortunately, metal salt, metal oxides and hydrous metal oxidessuch as granular ferric hydroxide (Banerjee et al., 2008), TiO 2  (Liuet al., 2008), akaganeite (Lazaridis et al., 2005), and Na 2 SO 3 /FeSO 4 (Pan et al., 2014) have been well explored for arsenic, phosphorusor chromium treatment. Adsorption is recognized as one of themost available option because of its low cost and high ef  󿬁 ciency.However, handling and disposal of the waste sludge is still acumbersome problem limiting their actual industrial application.Furthermore, there is still lack of economical way to transformmore agricultural residues like straw resources to valuable andgreenby-product whether inproduction sites or in factory for theirhigh- 󿬁 bre content and low-protein quality. After harvest season,farmers burn crop stalk in great quantities, causing serious envi-ronmental problems such as haze which may pose a serious threatto public health.Biochar, srcinated from biomass pyrolysis under oxygen-limited environment can be used as soil amendment to increasesoil fertility, double or triple plant yields,  󿬁 x carbon and cutgreenhouse gas emissions in a vast scale. Besides, the chemicalmodi 󿬁 ed biochar also adsorb contaminants either by electrostati-cally attractive forces or by ligand exchange mechanism withoutthe adverse waste sludge production (Loganathan et al., 2013;Marris, 2006). Thus, application of the adsorption methods withbiochar based adsorbent is a promising way for arsenic, phosphateandchromiumtreatment.Herein,toenhancetheadsorptionabilityof biochar and probe into the rational straw utilization of strawresources, bismuth oxide and wheat straw were explored to pre-pare a cost-effective but high ef  󿬁 ciency material for wastewatertreatment for the  󿬁 rst time. The speci 󿬁 c objectives of this workwere to: (1) prepare and characterize Bi 2 O 3  impregnated biocharcomposites, (2) test its sorption capacities for As (III), P and Cr (VI),and(3)investigatethepossiblemechanismsinvolvedintheAs(III),P and Cr (VI) adsorption. 2. Materials and methods  2.1. Preparation of adsorbents Bismuth oxide solution (Bi-solution) was prepared by adding10 mmol Bi 2 O 3  to 20 ml hydrochloric acid (0.12 mol) and thendiluted to 100 ml with distilled water. A stock solution of 3000 mg L   1 KH 2 PO 4 , 200 mg L   1 sodium arsenite and 200 mg L   1 potassium dichromate were prepared in a volumetric  󿬂 ask anddiluted to the required concentrations (60 e 1800 mg L   1 ). Bi 2 O 3 ,KH 2 PO 4 , NaAsO 2 , K 2 Cr 2 O 7  and other reagents used in this workwere all at analytical grade.Wheat straw (WS) was obtained from Wuxi city, Jiangsu prov-ince, China. It was milled and sized into particle with diametersbetween 0.60 and 0.80 mm. Bismuth oxide activated carbon (BiBC)and control biochar (CBC) were produced as follow: 10 g WS wasmixed with 100 ml Bi-solution and hydrochloric acid (0.12 mol)respectively, stirred vigorously at 80   C for 3 h, exposed to ultra-sonic treatment to get the targeted particles as small as possibleand then dried at 105   C. The biochar precursors were then heatedto setting temperature (400, 500 and 600   C) and  󿬁 nally main-tained for 60 min in a furnace with the heating rate of 10   C min  1 under nitrogen  󿬂 ow. Finally, the biochar materials were washedwith 0.01 M NaHCO 3  solution and distilled water for three times.Activated carbons were labelled as BiBC400, BiBC500 and BiBC600while the control labelled as CBC400, CBC500 and CBC600.  2.2. Characterization of adsorbents Porosity and surface characteristics were measured by N 2 (0.162 nm 2 ) adsorption using a NOVA-2000E (Quantachrome, USA)surface area analyzer. Brunauer-Emmertt-Teller (BET) surface areaand average pore diameter of the BiBCs and CBCs were determinedby multipoint BET analysis of adsorption data points with relativepressures of 0.05 e 0.3. Surface functional groups were detected byFourier transform infrared spectroscopy (FTIR) (Nicolet IS10,Thermo Electron Co, USA) at a spectral range of 4000 cm  1 to400 cm  1 with a resolution of 8 cm  1 . X-ray diffraction (XRD)patterns of the synthesized biochars were measured by a SiemensD-501 diffractometer with Ni  󿬁 lter and graphite monochromator.Furthermore,X-rayphotoelectronspectroscopy(XPS)andscanningelectron microscopy (SEM) were used to further investigate theadsorption mechanism and determine its sur 󿬁 cial morphologycharacteristics.  2.3. Adsorption experiments Adsorption experiments were used to reveal the adsorptionpotential of different biochar samples. Effect of pH on phosphorus,arsenic and chromium adsorption was investigated in a conical 󿬂 ask containing 0.1 g adsorbent and 50 ml of 50 mg L   1 adsorbate(phosphate, sodium arsenite and potassium dichromate). The pHlevel was adjusted at the range of 2 e 10 with hydrochloric acid andsodiumhydroxide.ToevaluatethestabilityoftheBiBCduringthosethree adsorbates adsorption process, the desorbed bismuth con-centration from aqueous solution was also analyzed.Adsorption dosage experiments were executed by adding 0.01,0.05, 0.1, 0.25, 0.5, and 1 g sorbent in a conical  󿬂 ask with 50 mL solution under the optimum pH value at initial 50 mg L   1 con-centrationof phosphate,sodiumarseniteor potassiumdichromate.Themixturewasagitatedat150rpminanorbitalshaker(SHA-C)at25   C for 2 h and then placed for 24 h to reach the equilibriumcondition.Adsorption kinetics experiments were conducted as follows:0.1 g bismuth oxide activated carbon was added in 50 mL phos-phate, arsenic or chromium solution with initial concentration of 300, 10, and 20 mg L   1 under the optimum pH level, respectively.The sorption amount of phosphorus, arsenic and chromium wereinvestigated at different time intervals (0, 0.5,1, 2, 5,10, 30, 60, and120 min). The mixturewas agitated at 200 rpm in an orbital shaker(SHA-C) at 25   C for 24 h to reach the equilibrium condition.Adsorption isotherm experiments were conducted as follows:0.1 g adsorbent was mixed with 50 ml adsorbate solution withconcentration ranging from 60 to 1800 mg L   1 for phosphate andconcentration ranging from 5 to 200 mg L   1 for potassium di-chromate or sodium arsenite under the optimum pH value in thecentrifuge tube respectively. All the vessels above were shaken at150 rpm in the oscillator for 2 h and then placed in the water bathunder constant temperature (15, 25 and 45   C) for 24 h to reachequilibrium.Besides,adsorbatesaturatedmaterialswerepreparedbymixing4gadsorbentwith2L500mgL   1 phosphateor100mgL   1 sodiumarsenite/potassium dichromate solution under the optimum pHvalue and then named as BiPBC500, BiAsBC500 and BiCrBC500 N. Zhu et al. / Chemosphere 164 (2016) 32 e 40  33  respectively.The concentration of phosphate was determined using doubleUV  e vis spectrophotometer (UV-2450 Shimadzu, Japan) at itsmaximum wavelength of 700 nm. Inductive coupled plasmaemission spectrometry (ICP) and Inductive coupled plasma emis-sionspectrometry-massspectrum(ICP-MS)wereusedtodetecttheconcentration of arsenic, chromium and the leaching bismuth. 3. Results and discussion  3.1. Characteristics of biochar  3.1.1. Surface area and pore volume Nitrogen adsorption is a standard procedure for the determi-nation of the porosity of adsorbents. The N 2  adsorption-desorptionisotherms of biochar based materials contained an almost hori-zontal plateau at higher pressures (Fig.1). According to the IUPAC,both BiBCs and CBCs exhibited a type I isotherm and a type H 4 hysteresis loops, indicating relatively small external surfaces andhighly microporous within the materials. Besides, the isotherm of CBC400andCBC600raisedrapidlynearP/P 0  ¼ 1demonstratingthepresence of macropore.Carbonization temperature had a signi 󿬁 cant impact upon thepore development and speci 󿬁 c surface area of the activated agent(Table 1). With temperature increasing from 400 to 600   C, BETsurface areas, pore volumes as well as pore size distribution of allthe materials did showa disciplinarychange. They increased  󿬁 rstlyand then reduced with the continuously increasing pyrolysis tem-perature. It was known that the BETsurface area, pore volume andpore size distribution of activated carbons were closely related tothe  󿬁 nal pyrolysis temperature, raw material and retention time.With the same raw material and retention time, increasingcarbonization temperature within a certain temperature range in-creases the evolution of volatile matters from the precursor, ben-e 󿬁 ts pore development and even creates new pores. Thispyrolyzationeffect,whenoutoftherange,willcausethecollapseof micropore and widen a signi 󿬁 cant amount of micropore to meso-pore, quality just drop (Demiral and Gunduzoglu, 2010).BiBCs had larger BET surface area and pore volume, especiallythe micropore area and micropore volume, but a lower averagepore diameter than that of CBCs under the same carbonizationtemperature (Table 1). This could be attributed to the fact that theactivatingagent(bismuthoxide)helpedincreatingnewmicropore,inhibited the formation of tars which jam up the pores and, thus,decreasedmicroporecollapseof thesamples.Theaveragediameterof 2.00 nm indicated that the synthesized BiBC500 was in themicropore region according to the IUPAC classi 󿬁 cation implyinghigh adsorption potential. Highly developed micropore structuremeans high speci 󿬁 c surface area and high adsorption potential tocertain substances.  3.1.2. Morphology study The morphology and particle size of the materials wereanalyzed by SEM. The pore structure was observed obviously forthe CBC500 and CBC600 (Appendices Fig. 1). However, the porestructure of CBC500 was inerratic while the pore structure of CBC600 was unordered. The surface of CBC400 was rough butapparently absence of porosity. The  󿬂 ourishing pore structure wasobserved for all the bismuth activated carbon. Besides, when thebismuth oxide impregnated biochar precursor was carbonized at400, 500, and 600   C, globular particles were inlaid on the poroussurface of biochar, with a diameter between 0.5 and 1  m m (seeAppendices Fig. 1). The size of the pores on the surface of biochardecreased with the increasing carbonization temperature. Besides,few larger particle size with an uneven surface was also observedfortheBiBC600.Theverysmallwhiteglobularparticleswere 󿬁 rmlyattached within the biochar matrix which may represent the bis-muth serve as the adsorption site on the surface of activatedcarbon.  3.2. The main adsorption mechanism BiBC500 was used to investigate the adsorption performance of bismuth activated carbons because of their high BET surface areaand abundant pore volume. The raw biochar (CBC500) removednegligible phosphorus, arsenic and chromium while BiBC500showed high adsorption ability to those kinds of anion.  3.2.1. FTIR FTIR was used to identify the functional groups and the rolesthey played during the modi 󿬁 cation and adsorption procedure(Fig. 2). Functional groups of BiBC500 were not changed greatlyafter being doped with bismuth (Fig. 2b). The new peaks at513 cm  1 and 622 cm  1 were observed which demonstrated theexistence of Bi e O bond on BiBC500 (Fruth et al., 2006). The strongnewbandcenteredat1027cm  1 ,593cm  1 and536cm  1 werealsonoticed (Fig. 2c). Those new peaks were linked with the stretchingvibration of PO 4  groups and the bend vibration of O-P-O (Luwanget al., 2011). The bands at 3435 cm  1 which represented the hy-droxyl groups were all existed in CBC500, BiBC500 BiAsBC500,BiPBC500 and BiCrBC500. But this peak for BiAsBC500, BiPBC500and BiCrBC500 was much more intense than BiBC500, implying O-H of the water molecules were adsorbed on the activated carbonsurface with the adsorption process as well (Xue et al., 2009).It was reported that the characteristic absorption bands of adsorbed arsenate was 650 e 1050 cm  1 for As-OH or As-Ostretching vibration (Hu et al., 2015). Peak at 1045 cm  1 Fig. 1.  N 2  adsorption e desorption isotherms of various activated carbons.  Table 1 Surface areas and pore volumes of activated carbons.Sample S BET  (m 2 /g) S Micro  (m 2 /g) D p  (nm) V  Micro  (cm 3 /g) V  Total  (cm 3 /g)CBC400 6.34  e  7.03  e  0.011CBC500 124.44 46.82 2.58 0.019 0.080CBC600 38.49  e  5.54  e  0.053BiBC400 87.42 52.15 2.49 0.023 0.054BiBC500 190.40 124.55 2.00 0.047 0.095BiBC600 106.70 22.26 2.95 0.010 0.079 N. Zhu et al. / Chemosphere 164 (2016) 32 e 40 34  represented a weak As e O bond suggesting As (III) surface complexwas non protonated (Pena et al., 2006) while peak at 876 cm  1 implied the As-OH vibration(Fig. 2d). The FTIR results suggestedthat electrostatic attraction contributed slightly to As (III) adsorp-tion. However, no any new peak was observed after Cr adsorption(Fig. 2d). The chromium adsorption mechanism should be studiedby other technological means.  3.2.2. XRD study and XPS investigation There was no obvious peaks found for the raw biochar (Fig. 3a).Diffraction peaks were observed at 2 q  ¼  27.165, 37.949, 39.618,44.553, 46.007 and 48.689 which were corresponding to 012, 104,110, 015, 113 and 202 planes respectively, marching bismuthstructure (Fig. 3b). Those diffraction peaks could also be observedafter arsenic, phosphorus or chromium adsorption. The newdiffraction peaks at 2 q  ¼  14.605, 20.068, 25.472, 29.033, 29.504,31.316 and 41.866 which were assigned to 100, 101, 110, 111, 200,102 and 211, respectively (Fig. 3c) demonstrating the existence of bismuth phosphate compound (Nithya et al., 2015). However, noobvious new peaks were found after arsenic or chromiumadsorption, suggesting that there was no crystal generated.XPS was used to further investigate the mechanism of arsenic,phosphorus and chromium sorption by the bismuth modi 󿬁 edbiochars (Fig. 4). Bismuth peaks corresponding to Bi4f were foundalmost same in the surface of Bi-impregnated biochar and otheradsorbed materials. The binding energy of 159.6 and 164.9 eV represented the existence of bismuth structure. Prominent peakcorresponding to P2p and As3d were observed in the surface of BiPBC500 and BiAsBC500 respectively demonstrating phosphorusor arsenic was  󿬁 xed successfully on the material after adsorption.The binding energy of 134.5 eV demonstrated the PO 4  groupstructure (Lo et al.,1994). The binding energy was 48.75 suggestingAs (III) oxidation was not happen. XPS investigation also displayedthe major adsorbed chromium was Cr (III) (Fig. 4). This resultdemonstrated that reduction of hexavalent chromium washappened in the process of Cr adsorption.  3.2.3. The mechanism of adsorption of arsenic, phosphorous andchromium Bismuth, with the electronic con 󿬁 guration of (Xe)4f  14 5d 10 6s 2 6p 3 , is one of the most thoroughly investigated maingroup elements, which has been known as  ‘ the wonder metal ’ owing to the easy involvement in chemical combinations for theelectrons in its p orbital (Sun et al., 2014). Thus, the bismuthimpregnated biochar may have the great potential in adsorbingsome inorganic anions contaminant through ligand exchange orelectrostatic incorporation.XPS results demonstrated phosphorus, arsenic and chromiumwere immobilized  󿬁 rmly on the surface of bismuth activated car-bon. The FTIR detected new peaks after arsenic and phosphorusadsorption suggesting these two species were adsorbed by chem-ical adsorption. The As (V) could be adsorbed through the elec-trostatic force (ion-exchange on the protonated surface based onCoulombic force) and Lewis acid e base interactions (ligand ex-changereactions) while theAs(III)adsorptionwas mainly involveda Lewis acid e base (ligand exchange) reaction (Munoz et al., 2002).XPS results also indicated the adsorbed arsenic was As (III). Thus,the main arsenic adsorption mechanism could be attributed to theLewis acid e base reaction between the bismuth atom and arsenite.Based on the results above, the main phosphorus sorption processcould be described as follow: bismuth atom was loaded on thesurface of activated carbon, served as the sorption site and thenbecome hydroxylated under the acidic condition. Then the hy-droxylated bismuth attracted phosphate group and formed BiPO 4 which has been reported as a high effective photocatalyst fordegradation methylene blue ef  󿬁 ciently under the ultraviolet irra-diation (Pan and Zhu, 2010). This is our ongoing word for the uti-lization of phosphate exhausted adsorbent. FTIR detected no new Fig. 2.  FTIR graph of a) CBC500, b) BiBC500, c) BiAsBC500, d) BiPBC500 and e)BiCrBC500. Fig. 3.  XRD pattern of a) CBC500, b) BiBC500, c) BiAsBC500, d) BiPBC500 and e)BiCrBC500. N. Zhu et al. / Chemosphere 164 (2016) 32 e 40  35
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