DSpace at VNU: Removal of As(V) from aqueous solutions by iron coated rice husk

Fuel Processing Technology 106 (2013) 511–517 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc Removal of As(V) from aqueous solutions by iron coated rice husk E Pehlivan a,⁎, T.H Tran b, W.K.I Ouédraogo c, C Schmidt d, D Zachmann d, M Bahadir d a Department of Chemical Engineering, Selcuk University, Campus, 42079 Konya, Turkey Hanoi University of Science, Hanoi, Vietnam c Laboratoire de Chimie Organique, Structure et Réactivité, UFR-SEA, Université de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso d Institute of Environmental and Sustainable Chemistry, Technische Universitaet Braunschweig, Germany b a r t i c l e i n f o Article history: Received December 2011 Received in revised form 15 August 2012 Accepted September 2012 Available online 30 September 2012 Keywords: Adsorption Rice husk As(V) Fe(III) Isotherms a b s t r a c t A lignocellulosic material extracted from rice husk (Oryza sativa), Vietnam, was modified as a new adsorbent for the removal of As(V) ions from aqueous solution Iron was coated onto this adsorbent by hydrolization of ferric nitrate while adding an alkaline solution drop wise into the batch type reactor The adsorption of As(V) ions from aqueous solution on coated rice husk was then studied at varying pH, As(V) concentrations, contact times, ionic strength, and adsorbent amounts The minimum contact time to reach equilibrium is about h The adsorption of As(V) anions on the coated rice husk was found to be highly pH dependent due to Coulomb interactions between As(V) species in solution and positively charged surface groups RH-FeOOH, as well as formation of chelate complexes with naturally occurring carboxyl and carbonyl functional groups in the matrix As(V) adsorption on Fe(III)-coated rice husk (RH-FeOOH) from aqueous solution was studied in the pH range 2–10 The main effects of pH on adsorption are estimated by considering both the behavior of As(V) ions (hydrolysis and hydroxide precipitation) and the effect of pH on coordination A strong effect of pH was demonstrated at pH 4.0 with a maximum percentage for removal of As(V) ions 94% Although both Langmuir and Freundlich isotherms have been used to characterize the adsorption of As(V), the Langmuir model fitted the equilibrium data better than Freundlich model and confirmed the surface homogeneity of adsorbent The maximum adsorption capacity is determined as 2.5 mg/g of adsorbent at pH 4.0 for the Fe(III)-coated rice husk It is concluded that initial As(V) concentration has an effect on the removal efficiency of RH-FeOOH Higher adsorption of As(V) was observed at lower initial concentrations RH-FeOOH as a low cost material is effective for the removal of As(V) ions and may become a valuable adsorbent to improve the ground water quality in Vietnam © 2012 Elsevier B.V All rights reserved Introduction Arsenic in water streams was reported from over 70 countries to pose serious threat to an estimated 150 million people world-wide [1] Water supply in many countries, e.g., in Bangladesh, India, Taiwan, Vietnam, Burkina Faso, Mongolia, Mexico, Pakistan, France, Italy, Chile, New Zealand and even in the United States contains dissolved arsenic in excess amounts (>10 μg/L), which is the maximum acceptable level recommended by the USEPA [2,3] Arsenic occurs in water stream in several forms depending upon pH value and redox potential The oxidation state of arsenic in dissolved phase plays an important role since it determines the properties of the related chemical species, i.e., toxicity, sorption behavior, and mobility in the aquatic environment [4] Since the pH of the aqueous medium determines the predominant species, it is one of the important parameters for the arsenic removal from drinking and wastewater At typical pH values of natural water (pH 5–8), the two predominant forms of ⁎ Corresponding author Tel.: +90 332 2232127; fax: +90 332 2410635 E-mail address: erolpehlivan@gmail.com (E Pehlivan) 0378-3820/$ – see front matter © 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.fuproc.2012.09.021 inorganic arsenic species in aqueous environments are the trivalent arsenite, As(III), and the pentavalent arsenate, As(V) Arsenite mainly exists as fully protonated form and arsenate remains as an anion, which could be found in various ionic forms in dissolved species [4,5] A number of treatment methods have been applied for the removal of arsenic from water such as precipitation, co-precipitation [6], coagulation-microfiltration [7], ion-exchange [8], reverse osmosis and nano-filtration [9] Adsorption has been paid more attention due to its high treatment efficiency and lower process costs compared with the above mentioned ones Industrial and agricultural byproducts play an important role for improving arsenic removal after their simple and cheap chemical modifications that could be of particular interest for developing countries Iron compounds are among the most popular adsorbents being used for the removal of arsenic from aqueous solutions Rice husk (RH) is a promising adsorbent that has interesting properties such as hydrophilic, porous, and high surface area as well as high resistivity Studies on applying RH as adsorbent for arsenic are still scarce Coating RH with iron can increase the removal capacity for As(V) from aqueous solutions Arsenic removal through iron oxyhydroxide (FeOOH)-coated matrices primarily 512 E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 depends on the number of iron sites on the surface of adsorbent Iron used for the modification of adsorbents can occur in various types such as amorphous iron oxide, ferrihydrite, akaganeite, goethite, hematite, etc., depending upon conditions such as pH and temperature [1–3,10] There are a number of different materials for the adsorption of arsenic ions such as rice polish [11], zeolite [12], red mud [13], modified fungi Aspergillus niger [14], activated alumina [15], surface-modified carbon black [16], iron hydroxides and oxides [17,18], open celled cellulose sponge [19] and further adsorbents [20] The aim of this study was to use RH as an agro-waste product that is produced in huge amounts particularly in developing countries in the tropics and subtropics The adsorption capacity of RH should be tested before and after being coated with iron oxyhydroxide (RH-FeOOH) as another cheap material in order to remove As(V) from aqueous solutions The effects of different parameters like pH of the initial solution, As(V) concentration, contact time, amount of RH-FeOOH, and ionic strength should be investigated in easy to run batch experiments that could be easily applied in those countries in order to diminish the arsenic pollution in drinking and process water, e.g., in rural areas Materials and methods 2.1 Treatment of rice husk (RH) Air-dried RH was purchased from the vicinity of Hanoi, Vietnam, and ground in a vibratory mill (BLB Braunschweig) The size fraction of 125–200 μm was collected, washed with deionized water, and airdried in an oven at 60 °C for one day before use All chemicals and reagents used were purchased from Merck (Darmstadt, Germany) and were of analytical grade Aqueous solutions of 0.2 M NaOH and HCl were used to adjust pH MgSO4.7H2O (Fluka, Seelze, Germany), Na3PO4.12H2O (Sigma-Aldrich, Seelze, Germany), and NaNO3 (Merck) were used for ionic strength studies Dissolved As(V) stock solution (X mg/L) was purchased from Merck Iron Nitrate Fe(NO3)3 was used for the modification of adsorbent All glassware was cleaned by soaking in diluted nitric acid and washing with deionized water 2.2 RH treatment with ferric nitrate Fe(NO3)3 solution The milled and air-dried RH (30 g) was pretreated (1) with mol/L H2SO4 (1:1 w/w of dry matter at 80 °C for 0.5 h) for removing starch, proteins, and carbohydrates, and (2) with 0.5 mol/L NaOH (RH/NaOH 5:1, stirring for 24 h at room temperature) for removing the low molecular weight lignin type compounds After filtration, the adsorbent was air-dried in an oven at 50 °C for 24 h The prepared adsorbent was then coated with a ferric nitrate Fe(NO3)3 solution A mixture of 800 mL of 0.05 M ferric nitrate Fe(NO3)3 in ultrapure water and 25 g RH was given in a L beaker A mol/L NaOH solution was slowly added into the reaction vessel at a velocity 10 mL/h under continuous stirring The mixture was left for day during this procedure Thereby, pH was kept between 2.8 and 3.5 and readjusted if necessary The suspension was then filtered using a cellulose acetate membrane of 0.45 μm pore size and washed with ultrapure water several times until pH neutral, air-dried in an oven at 50 °C, and stored in closed bottles at room temperature until use 2.3 Determination of iron amount loaded onto RH The content of iron loaded onto RH, an important factor influencing the As(V) adsorption capacity, was determined by soaking RH in M HCl for recovering all the loaded iron, and the solution was analyzed for iron concentration using HG-AAS The amount of iron loaded in RH was calculated as 1.2 mg/g of adsorbent 2.4 Spectroscopic studies for raw and coated RH In order to identify the main functional groups of the raw and Fe-coated adsorbent responsible for As(V) adsorption FT-IR spectra were recorded (Buker Tensor 27, diamond ATR at 4000–520 cm −1, 32 scans) (Fig 1) The broad and strong bands at 3340 cm −1 are the hydroxyl (\OH) and the absorbance at 2890 cm −1 are due to \CH groups of the adsorbents The absorption at 1741 cm −1 is attributed to stretching vibration of the carboxyl groups The bands observed at 1024 cm −1 were assigned to C\OH stretching of alcohols and carboxylic acids The coating of RH with FeOOH leads to some important changes of the IR spectrum especially in the decrease of the ratio of COOH (1741 cm −1) and COO − (1677 cm −1) group's intensities, which can be explained by the complexation of Fe 3+ ions with carboxylate groups in the matrix Intensity for lignin structure was observed to be decreased at 1263 cm −1 This can be ascribed to a partial oxidation of lignin by Fe 3+ ions [10] 2.5 Preparation of standards, reagents and analyses Ultrapure water was used for all experiments All chemicals used for the coating process and batch equilibrium studies were of analytical grade and purchased from Merck (Darmstadt, Germany) The AAS standard solution of 1000 mg/L As(V) was prepared by transferring the contents of a Titrisol ampoule with As2O5 in H2O (Merck, Germany) into a L volumetric flask, which was filled up to the mark at 20 °C according to the instructions by Merck Arsenic solutions with different concentration used in the batch studies were prepared by diluting the main stock solution The As analyses were performed with a Hitachi Atomic Absorption Spectrophotometer (Series Z-2000; Hitachi Corporation, Japan) which was connected to a hydride formation system (model HFS-3; Hitachi) For hydride generation the following solutions were used: (i) 1.2 M HCl (p.a., Merck); (ii) NaBH4–NaOH solution: solute 10 g NaBH4 (p.a., Fluka) in L H2O (Seralpure) by adding g NaOH (p.a., Merck); the solution was prepared immediately before use; (iii) KI-solution as a reduction agent; 20% (w/v; reduction to As(III)) All standards, reference solutions, and sample solutions were adjusted to 0.24 N HCl and 2% KI The reduction agent was added at least 30 before analysis In general a 5-point calibration was run before starting the analyses (0–20 μg/L) Argon was used as carrier gas with a flow rate of 0.3 L/min for constant transfer of As-hydride from the reaction cell to the cuvette The 193.7 nm emission line of the As-hollow-cathode lamp was used For the reduction of As(V) into As(III), 2.5 mL of 30% HCl and 2.5 mL of 20% (w/v) KI was added to 25 mL of the standard or sample solution and left for 15 2.6 Batch adsorption experiments Batch adsorption experiments were carried out in order to evaluate the performance of the adsorbent for As(V) removal Batch experiments were performed in triplicate in sealed glass beakers by adding RH-FeOOH in 50 mL of aqueous As(V) solution of desired initial pH, As(V) ion concentration, and temperature The pH of working solutions was controlled and adjusted by adding 0.2 M HCl or 0.2 M NaOH as required The beakers were shaken on a horizontal shaker at 200 rpm for certain periods (15 min–24 h) The adsorbents were then separated through filtration and the remaining filtrates were analyzed for As(V) concentration by hydride generation atomic absorption spectrometry with a Zeeman correction (HGAAS-Hitachi Z-2000 AAS) The amount of As(V) adsorbed per unit mass of the RH-FeOOH (mg/g) was calculated using following Eq (1): Q e ¼ ðC i −C e ÞV=W ð1Þ E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 513 Fig FTIR spectra of raw RH and coated RH-FeOOH (black: raw RH, red: RH-FOOH) where Ci and Ce are the As(V) concentrations in mg/L initially and at equilibrium, respectively; V is volume of the arsenic solution in mL; and W is the weight of RH-FeOOH in mg For studying the effect of initial pH (2–10) on arsenic uptake by RH-FeOOH, adsorption experiments were performed using 50 mL of solution with initial As(V) concentration of mg/L and adsorbent dose of g/L at 23 °C Effect of variation of initial As(V) concentration was studied with different initial arsenic concentration of 1, 3, 5, 7.5, 10, 15, 20, 30, 50, and 75 mg/L; adsorbent dose of g/L; pH 4; temperature 23 °C Effect of contact time (15 min–24 h) and adsorbent amount (0.1–0.3 g) was studied with initial As(V) concentration of mg/L; pH 4; temperature 23 °C Results and discussion 3.1 Effect of pH on As(V) removal Iron oxides have been considered already as effective materials for removal of As(V) in water streams and sediments [21] The adsorption process of As(V) on these materials takes place at the hydrous oxide/water interface The adsorption capacity for As(V) strongly depends on the chemical species and characteristics of the solid supports RH consists of cellulose, hemicellulose and lignin The cellulose and hemicellulose are bound to lignin both by hydrogen and covalent bonds Cellulose is a common material in plant cell walls Hemicellulose consists of different monosaccharide units such as glucose, xylose, mannose, galactose, and arabinose Lignin comprises a variety of functional groups including aliphatic and phenolic hydroxyl-, methoxyl-, and carbonyl groups, which are able to transfer electron pairs from oxygen atoms and forming coordination complexes with toxic metals Raw RH also contains some polar functional groups such as alcoholic, carbonyl, carboxylic and phenolic ones, which are potentially able to complex As(V) pH value is considered as an important parameter in arsenic removal [22] The effect of pH on adsorption process was studied at 22 °C in the range from 2.0 to 10.0 It was observed that the initial pH of all solutions increased slightly after shaking the samples for h Fig shows the effect of equilibrium pH on the As(V) ion adsorption of RH-FeOOH The percentage of adsorption increases slightly at the pH range of 2.0–4.0 and maximizes at pH 4.0 which shows high selectivity of the modified adsorbent for As(V) After pH 6.0, the adsorption of As(V) decreases significantly until pH 10.0 The mechanism for the removal of As(V) from solution phase is attributed to Coulomb interactions and formation of chelate complexes between the As(V) ions and the charged functional groups on the adsorbent's surface At lower pH, the positive charge density on surface sites is rising which results in higher electrostatic attraction between (FeOH2+) and As(V) ions This provides a higher adsorption capacity for As(V) In contrast, while increasing the pH, the electrostatic repulsion is increasing due to the decrease of positive charge density of proton on the adsorption sites Moreover, the electrostatic attraction between the positively charged surface groups (FeOH2+) and As(V) species H2AsO4− and HAsO42− decreases and hinders the formation of surface complexes resulting in lower adsorption capacity for As(V) The pH also affects the presence of various As(V) species in aqueous solution It was reported that As(V) occurs in solution at different pH in form of H3AsO4, H2AsO4−, HAsO42−, and AsO43− oxo anions The predominance of various As(V) species as a function of pH is shown in Fig The different species of As(V) are present in solution based on the following three equilibriums and their respective stability constants [22] (Eqs (2)(4)): ỵ K H3 AsO4 H2 AsO4 ỵ H ; P1 ẳ 2:3 ỵ K H2 AsO4 HAsO4 ỵ H ; P2 ẳ 6:7 ỵ K HAsO4 AsO4 ỵ H ; P3 ẳ 11:6 2ị 3ị 4ị The pH influences the protonation or deprotonation of the adsorbent surface The interactions of As(V) ions with the coated RH surface are due to Coulomb interactions, ligand exchange phenomena, and formation chelate complexes Narasimhan reported that goethite, ferrihydrite and FeOOH loaded bio-sorbent have the same adsorption mechanisms for As(V) [23] Iron oxides can form oxy-hydroxides 514 E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 As(V) 100 Sorption % 80 60 40 20 0 10 12 pH Fig pH effect on the adsorption of As(V) on RH-FeOOH (initial As(V) concentration: ppm; solvent volume: 50 mL; adsorbent amount: 0.2 g; temperature: 22 ± °C, contact time: h) which can be protonated or deprotonated depending on the pH value of solutions and as a result, a positive or negative surface can be formed as the following [24] (Eqs (5) and (6)): ỵ ỵ `Fe\OH ỵ H `Fe\OH2 5ị ỵ 6ị `Fe\OHH ỵ `FeO In the pH range 2–7, the major arsenate species in aqueous solution is H2AsO4− An adsorption takes place by the reaction between the active hydrolyzed form of `FeOH on the surface of RH-FeOOH and As(V) ions that leads to the formation of surface complexes according to the following equilibriums [24] (Eqs (7) and (8)): ỵ ỵ 2`FeOH2 ỵ H2 AsO4 ẵ`FeOị2 \AsOOHị ỵ H3 O ỵ H2 O ỵ `FeOH2 ỵ H2 AsO4 ẵ`Fe\Oị2 \AsOOHị þ H3 O þ ð7Þ ð8Þ In the first reaction, non-specific Coulomb interactions (outer-sphere adsorption) or ligand exchange reaction on the surface (inner-sphere adsorption) with As(V) can occur [25] as the following (Eqs (9)(13)): ỵ ỵ `Fe\OH2 ỵ H2 AsO4 `Fe\OH2 O4 AsH2 `Fe\OH ỵ H2 AsO4 `Fe\OAsO3 H2 ỵ OH Fig Distribution (%) of various As(V) species as a function of solution pH [21] 3.2 Effect of agitation time on the removal of As(V) The results of time effects on As(V) adsorption process on RH-FeOOH matrix are given in Fig This graph demonstrates that the As(V) adsorption reached an equilibrium 96–99% after shaking for h It seems that there is not much difference in adsorption percentage thereafter The adsorption of As(V) takes place at a pH below the pHpzc that was found to be about 5.8–6.3 (Fig 5) For that reason, the experiments were carried out at pH below pHpzc of adsorbent The scanning electron microscopic pictures (SEM) reveal the surface textures and porosities of RH and RH-FeOOH (Fig 6) It shows very fine particle sizes having pores within the particle of varying size 3.3 Effect of initial As(V) concentration The influence of the initial concentration on the adsorption on RH-FeOOH was studied at pH 4.0 after h shaking with As(V) with the concentrations in the range of 1–75 ppm (Fig 7) The results demonstrated that at higher concentrations, more As(V) ions remained in dissolved phase due to the saturation of binding sites (`Fe\OH) towards As(V) As(V) uptake by RH-FeOOH was 99.6 % at a concentration of ppm The data obtained from equilibrium isotherm that provides information on the sorption capacity are the most important factor in any ð9Þ 120 As (V) 100 10ị ỵ `Fe\OH2 ỵ H2 AsO4 `Fe\OAsO3 H2 ỵ H2 O 11ị Sorption % 80 60 40 `Fe\OH ỵ HAsO4 `Fe\OAsO3 H ỵ OH 12ị 20 2`Fe\OH ỵ HAsO4 `Fe2 \AsO2 H ỵ 2OH 13ị The protonation of active surface sites at low pH plays a significant role in reducing the free As(V) ions in the dissolved phase Above pH 7.0, adsorption of As(V) decreases because of the competition between H2AsO4− ions and OH− ions for the reactive sites of RH-FeOOH [26,27] 0 10 15 20 25 30 Contact time (hours) Fig Sorption isotherm of As(V) on RH–FeOOH as a function of contact time (initial As(V) concentration: ppm; solvent volume: 50 mL; pH: 4; adsorbent amount: 0.2 g; temperature: 22 ± °C) E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 12 Arsenic uptake q(mg/g) 10 pH final 515 1,6 1,2 As(V) 0,8 0,4 0 0 10 11 pH initial adsorption system The As(V) adsorption capacity of RH-FeOOH was calculated using Langmuir and Freundlich models [28] These isotherms are related to As(V) uptake per unit weight of RH-FeOOH, qe, and the equilibrium As(V) ion concentration in the dissolved phase, Ce The Langmuir isotherm has been widely applied for adsorption processes to separate an analyte from aqueous solutions [29] Langmuir isotherm model assumes that adsorption process forms a monolayer and occurs at specific homogeneous adsorption sites Intermolecular forces decrease rapidly with the distance from the surface The Langmuir model is more popular since it contains the two reasonable parameters (Kb and As) that are easy to interpret the adsorption [30–32] The general form of Langmuir model is: ð14Þ where As (mol/g) and Kb (L/mol) are the coefficients, qe is the weight adsorbed per unit weight of adsorbent and Ce is the analyte concentration in solution at equilibrium Freundlich equation can be used as another model for determination of As(V) adsorption capacity as shown below: Freunlich equation : x ¼ kC e 1=n m 10 15 20 25 Fig Sorption isotherm of As(V) on Fe-loaded rice husk (RH) as a function of initial As(V) concentration (initial As(V) concentration: 1–75 ppm; solvent volume: 50 mL; pH: 4; adsorbent amount: 0.2 g; temperature: 22 ± °C; contact time: h) Fig The pH point zero of charge (pHpzc) of RH-FeOOH Langmuir equation : Ce Ce ẳ ỵ qe As As K b Arsenate concentration Ce (ppm) ð15Þ where 1/n is the intensity of adsorption; k is the adsorption capacity, x/m is the weight adsorbed per unit weight of adsorbent and Ce is the analyte concentration at equilibrium in solution The modified formula of this equation, Eq (16) was also obtained log x m ẳ logk ỵ logC e n ð16Þ Langmuir and Freundlich constants and correlation coefficients (R 2) are shown in Table For the determination of these coefficients, R value was calculated from the linear form of Langmuir isotherm as 0.995 for As(V) ion adsorption This result indicates that the As(V) ion adsorption onto RH-FeOOH fits well the Langmuir model Thereby, the adsorption of As(V) ions onto RH-FeOOH is considered forming a monolayer that takes place at the functional groups or binding sites on the sorbent surface The maximum adsorption capacity (mg/g) of RH-FeOOH for As(V) was found to be 2.5 mg/g (Table 1) Comparison of As(V) adsorption (mmol As/g adsorbent) of different adsorbents reported in the literature is given in Table It appears that RH-FeOOH has a reasonable potential as adsorbent for the removal of As(V) from aqueous solutions 3.4 Desorption studies The desorption of As(V) ion from the adsorbent (RH-FeOOH) was investigated as well Desorption studies can help to regenerate the adsorbents for further reuse Desorption efficiency of As(V) ions from RH-FeOOH was studied with 30% HCl and M NaOH It was concluded that the desorption percentage of As(V) from adsorbent is higher when using NaOH than HCl Table shows that desorption of As(V) enhanced by the increase of pH Maximum desorption was Fig SEM of RH (a) and RH-FeOOH (b) 516 E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 Table Langmuir and Freundlich isotherm constants Langmuir isotherm parameters Freundlich isotherm parameters As (mg/g) Kb R2 Kf (mg/g) N R2 2.5 2.0 0.995 1.15 3.26 0.982 Table Maximum adsorption capacities of some adsorbents reported in the literature Adsorbent Max As(V) adsorption capacity (mmol As/g) pH Reference Gibbsite Fly ash Hematite Feldspar Sulfate-modified iron oxide-coated sand RH-FeOOH 0.073 0.40 0.003 0.003 0.0017 3.0–7.0 4.0 4.2 6.2 [33] [34] [35] [35] [36] 0.033 Present study observed at a pH range of 12–14 90% of As(V) is recovered under these conditions These results demonstrate that adsorbed As(V) can be desorbed from the RH-FeOOH using M NaOH and thus successfully applied for the regeneration of the RH-FeOOH while shaking for 20 h 3.5 Effect of adsorbent amount on the As(V) removal The amount of RH-FeOOH used for adsorption experiments was varied from 0.1 to 0.3 g in 50 mL volume at an initial As(V) concentration of ppm; contact time of h at 22 ± °C and pH The As(V) equilibrium concentration in dissolved phase decreased when increasing adsorbent quantity (Fig 8) Thus the optimum adsorbent amount (RH-FeOOH) was found as 0.25 g by the given As(V) content 3.6 Effects of ionic strength on As(V) removal Ionic strength is one of the important factors influencing aqueous phase equilibrium The effects of the interfering sulfate, phosphate and nitrate anions were evaluated for the As(V) adsorption Adsorption process decreases when increasing ionic strength of the dissolved phase The results showed that there was significant decrease in As(V) adsorption when 50 ppm phosphate ions were contained together with As(V) in the solution However, the adsorption of As(V) was slightly decreased by addition of 50 ppm nitrate and sulfate ions The competition of phosphate with arsenate for the same sorption sites is very likely due to similar molecular structures of the two anions in the periodic system of elements This fact has to be considered if one wants to remove As(V) from natural waters that contains also phosphate residues from wastewater or fertilizers Conclusion The RH-FeOOH adsorbent was prepared using rice husk from Vietnam The characteristics of RH and RH-FeOOH were identified by Fig Sorption isotherm of As(V) on Fe-loaded rice husk (RH) as a function of adsorbent amount (initial As(V) concentration: ppm; solvent volume: 50 ml; pH: 6; adsorbent amount: 0.1 g–0.3 g; temperature: 22 ± °C) using FTIR technique The adsorption capacity of prepared RH-FeOOH was investigated by the batch adsorption experiments which revealed that RH-FeOOH adsorbent was effectively removing As(V) The As(V) removal capacity of the prepared RH-FeOOH material is 99.6% at pH This proves that the coated adsorbent has a remarkable capacity for removing As(V) from aqueous solutions The kinetic studies indicated that equilibrium of As(V) adsorption on RH-FeOOH was reached after h As(V) adsorption increased with an increase of As(V) in the solution The optimum pH corresponding to the maximum adsorption rates was found to be about pH for RH-FeOOH The As(V) adsorption on RH-FeOOH was best described using Langmuir isotherm model only It was found that the desorption percentage of As(V) from adsorbent is high at pH above 12 The presence of high phosphate concentrations decreases the As(V) adsorption due to the competition for the same sorption sites Meanwhile, the adsorption capacity for As(V) was not affected when adding nitrate and sulfate ions to the solution at the same amounts This laboratory investigation was performed under conditions that can easily be scaled up and applied for the removal of As(V) in developing countries of tropics and subtropics suffering from high As contents in ground and drinking water and producing rice as main agricultural crop at the same time (e.g., Vietnam and Burkina Faso) Acknowledgements This investigation was performed at the Guest Chair within the project “Exceed – Excellence Center for Development Cooperation – Sustainable Water Management in Developing Countries” at the Technische Universitaet Braunschweig, Prof Pehlivan being the visiting professor, and Ms Hien and Mr Ouedraogo being the international exchange staff members The Exceed Project is granted by the German Federal Ministry for Economic Cooperation and Development (BMZ) and German Academic Exchange Service (DAAD), and their financial support we gratefully acknowledge References Table The relationship of desorption and pH values Leaching agent pH Desorption (%) HCl (30 %) NaOH (1 M) 1.5 12 14 12.3 50.3 70.2 85.6 90.6 [1] P Ravenscroft, H Brammer, K.S Richards, Arsenic Pollution: A Global Synthesis, Wiley-Blackwell, U.K., 2009 [2] H Brammer, P Ravenscroft, Arsenic in groundwater: a threat to sustainable agriculture in South and South-east Asia, Environment International 35 (2009) 647–654 [3] B Casentini, M Pettine, Effects of desferrioxamine-B on the release of arsenic from volcanic rocks, Applied Geochemistry 25 (2010) 1688–1698 [4] V.K Sharma, M Sohn, Aquatic arsenic: toxicity, speciation, transformations, and remediation, Environment International 35 (2009) 743–759 E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517 [5] Y Jia, L Xu, Z Fang, Observation of surface precipitation of arsenate 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Effect of pH on As(V) removal Iron oxides have been considered already as effective materials for removal of As(V) in water streams and sediments [21] The adsorption process of As(V) on these materials