Current World Environment

Introduction

Arsenic compounds are common contaminants in the environment. Because of arsenic toxicity and inducedcarc inogenetic agents(Eblin et al.,2006; Hughes.,2002), higher arsenic concentration in the environment represents serious problems for human health, especially for populations in Bangladesh, Western Bengal, Vietnam,China, Mexico and Chile. The danger of elevated arsenic concentration in waters in these countries was under lined by WHO, which estimated the recommended limit for arsenic concentration in drinking waters up to 10 μg/LArsenic-contaminated drinking water can cause adverse health effects in human beings. Arsenic plays crucial role in making disturbance in   RNA and DNA synthesis, which consequently lead to cancer. Increasing birth of exceptional child, low birth weight, malformed child and dead births were reported due to Arsenic compounds (Jain et al., 2000; Kiping et al.,1997; Ng et al.,2001; Bissen et al.,2003; Penrose et al.,2009; Ng et al.,2003; Burkel et al.,1999; Smedley et al.,2002). The conventional technologies for arsenic removal from waters are based on processes of coagulation, sorption, ion-exchange reactions or methods of reverse osmosis. Materials used in these processes are Fe⁰, Fe (III) oxyhydroxides, Mn (II), Al(III), apatite, silicate sands, carbonates, sulphides, ashor various types of coal(Chmielewská et al.,2008; Daus et al.,2004; DeMarco et al.,2003; Hiller et al.,2007; Lin et al.,2001; Sato et al.,2002; Song et al.,2006). Nowadays, there is a trend to use the alternative and low-cost materials for arsenic removal from the waters in laboratory or medium-scale experiments, too. Effectiveness of chemically modified or native biomass in processes of arsenic removal was evaluated and provedby various authors (Abdel-Ghani et al.,2007; Boddu et al.,2008; Cernan sky et al.,2007; Loukidou et al.,2003; Malakootian et al.,2009; Murugesan et al.,2006; Rahaman et al.,2008; Seki et al.,2005).Calcium hydroxyapatite (HAp), Ca₁₀(PO₄)₆(OH)₂, has also been used for the removal of heavy metals from contaminated soils, waste water and fly ashes (Omar et al.,2003; Takeuchi et  al.,1990).Calcium hydroxyapatite (Ca-HAp) is a principal component of hard tissues and has been of interest in industry and medical fields. Its synthetic particles find many applications in bioceramics, chromatographic adsorbents to separate protein and enzyme, catalysts for dehydration and dehydrogenation of alcohols, methane oxidation, and powders for artificial teeth and bones paste germicides (Elliott et al.,1994). These properties relate to various surface characteristics of HAp, e.g., surface functional groups, acidity and alkaline, surface charge, hydrophilicity, and porosity. It has been found that Ca-HAP surface with to P–OH groups acting as sorption sites (Tanaka et al., 2005). The sorption properties of HAp are of great importance for both environmental processes and industrial purposes. Hydroxyapatite is an ideal material for long-term containment of contaminants because of its high sorption capacity for actinides and heavy metals, low water solubility, high stability under reducing and oxidizing conditions, availability, and low cost(Krestou et al.,2004). HAP has been utilized in the stabilization of a wide variety of metals (e.g., Cr, Co, Cu, Cd, Zn, Ni, Pu, Pb, As, Sb, U, and V) by many investigators (Omar et al.,2003; Ramesh et al.,2012; Vega et al.,1999). They have reported the sorption is taking place throughionic exchange reaction, surface complex with phosphate, calcium and hydroxyl groups and/or co-precipitation of new partiallysoluble phases. In this study, the effect of Hap nanoparticl eson removal efficiency of arsenic ions in different conditions investigated.

Materials and Methods

The hydroxyapatite nanoparticles previously have prepared (Montazeri and Biazar., 2011)and characterized by using the different analyses. The pH values of the solution were roughly adjusted from 2 to 12 by adding HNO₃ and Na OH respectively. The pH of the solutions was then accurately noted. Hydroxy apatite nanoparticles with different concentrations were added to each flask and securely capped, immediately. The suspension was then manually agitated. The pH values of the supernatant liquid were noted.Metal salt of (HAsNA₂O₄.H₂O)was used to prepare metal ion(As(V)) solution. Sorption studies were carried out by shaking a series of bottles containing different amounts of H Ap-nano in 50 mL of metal ions solution with different concentrations and pH. Suspensions were exposed to ultra sonic waves (50W,20 min), to disperse nanopaticles. The samples were stirred at room temperature at 250 rpm for 1 h (Equilibrium time), then centrifuged for 5 min and the supernatant liquid was analyzed by an atomic absorption spectrometer(S-series, Thermo Scientific; USA).

Results and Discussion

Effect of pH

The pH is a significant factor for determining the form ofthe metallic species in aqueous media. It influences the adsorption process of metal ions, as it determines the magnitude and sign of the charge on ions(Gupta et al.,2005). The effect of solution pH on the sorption of As(V)ions from the aqueous solution byHap-nanoin different concentrations was investigated in the pH range of 2–12 with the As(V) concentrations of 0/6g/L. The result is shown in Figure 1. It was found that the adsorption capacity of HAp increases with increase inpH in acidic medium. But in alkaline conditions, the removal efficiency remains constant, approximately. The effects of pH on arsenate removal by HAp-nano suggest that electrostatic attraction is not a major mechanism responsible for the arsenate sorption under our experimental conditions; otherwise the apparent arsenate sorption would decrease with increasing pH. The sorbent dissolution can result in a decrease of sorbent mass and an increase of phosphate concentration in water, both of which can inhibit arsenate sorption. Lowering pH can favor the dissolution HAp-nano, and thus suppress arsenate sorption (Sneddon et al.,2005; Valsami-Jones et al.,1998; Mohan  et al.,2007).  

Effect of Arsenic concentration

For study the effect of solution arsenate amount on the sorption of As(V)ions from the aqueous solution byHAp-nanoin different concentrations was investigated in the range of (0/1,0/2, 0/4,0/6g/L in pH:8).The result is shown in figure2. increase arsenate absorption occurred with in creasingar senaterate. Arsenate adsorption, indifferent concentrations of arsenate in water, investigated in nano-hydroxyapatite, showed a similar increase approximately,As a result, absorption levels for these different amounts of arsenate are significant.  

Effect of contact time

The time-dependent behavior of As(V) dsorption was measured by varying the contact time between adsorbate and adsorbent in the range of 5–120 min. The percentage adsorption of As(V) with different contact time is shown in Fig. 3. From Fig. 3, it can be observed that the rate of removal of As(V) ions was higher at the initial stage, due to the availability of more active sites on the surface of HA and became slower at the later stages of contact time, due to the decreased or lesser number of active sites (Kannan and Karrupasamy 1998). It is apparent from Fig. 3 that until 1 h, the percentage removal of As(V) from aqueous solution increases rapidly and reaches up to 85 %. A further increase in contact time has a negligible effect on the percentage removal. Therefore, a 1 h shaking time was considered as equilibrium time for maximum adsorption. The decrease in rate of removal of As(V) with time may also be due to aggregation of As(V) around the HA particles. This aggregation may hinder the migration of adsorbate, as the adsorption sites become filled up, and also resistance to diffusion of As(V)molecules in the adsorbents increases(Mittal et al. 2010).  

Effect of mass of adsorbent on As(V) removal

The effect of HA dosage on As(V) removal was analyzed by varying the dosage of HA and the result is shown in Fig. 4. It was observed that the removal efficiency increases with the increase in HA dosage. This reveals that the instantaneous and equilibrium sorption capacities of As(V) are functions of the HA dosage  

Adsorption isotherms

Equilibrium isotherm is described by a sorption isotherm,characterized by certain constants whose values express the surface properties and affinity of the sorbent sorption equilibrium is established when the concentration of sorbentin the bulk solution is in dynamic balance with that at the sorbent interface (Oladoja et al.,2008). The adsorption isotherm study is carried out on well-known isotherms such as Langmuir Langmuir.,1915).

Where b is the constant that increases with increasing molecular size, q_max is the amount adsorbed to form a complete mono layer on the surface (mg/g), X is weight of substance adsorbed (mg), M is weight of adsorbent (g), and Ce is the concentration remaining in solution (mg/L). The essential features of the Langmuir isotherm may be expressed in terms of equilibrium parameter RL, which is a dimension less constant referred to as separation factor or equilibrium parameter (Weber and Chakkravorti, 1974).

The value of RL indicates the type of the isotherm to be either unfavorable (RL[1]), linear (RL = 1), favorable (0\RL\1) or irreversible (RL = 0). The Freundlich isotherm is expressed as (Freundlich, 1906).

Where Kf and n are the constants depending on temperature An isotherm plot for sorption of As (V) by HAp-nano is shown in Figure 5 and 6 The diagram indicates that the Freundlich isotherm isfavorable for removal of As (V) by HAp-nano. The value of R also indicates that Langmuir isotherm is favorable. It can be concluded that Freundlich isotherm is the best fit langmuir isotherms. The adsorption capacity of HAp-nanofor As (V) adsorption is compared with other adsorbents(Table 3). The value of As (V) uptake by HAp-nano found in this work is significantly higher than that of other adsorbents.

Three types of reactions may control As(V) immobilizationby HAp-nano: surface adsorption, cation substitution orprecipitation. The first mechanism is the adsorption of As(V) ions on the HAp-nano surfaces and following ion exchange reaction between As(V) ions adsorbed and Ca ions of HAp-nano(Suzuki et al.,1984). This ion exchange reaction mechanism (Ma et al.,1994)is expressed as:

Showed that HAp-nano dissolution and hydroxy pyromorphite (HP) precipitation were the main mechanisms for As(V) immobilization by HAp-nanoin theabsence of other metals. These chemical reactions can bedescribed as follows:

Information about the sorption mechanisms have been inferred by the values of molar ratios (Qs) of cations bound by HAp-nanoto Ca desorbed from HAp-nano (Aklil  et al.,2004). Figure 7 presents the effect of calcium concentration on arsenate removal by HAp-nano. For all sorbents tested, increasing calcium level appeared to assist arsenate sorption. When calcium concentration increased from 0 to 2.5 mM, the arsenate removal efficiency increased from 2.4% to 5.4% by using HAp-nano. The calcium effects on arsenate sorption to HAp-nanoare to be due to two reasons. First, according to increasing calcium concentration in water can inhibit HAp-nano, which can inhibit arsenate sorption to the sorbents.  Second, Ca²⁺ in water can complex with phosphate on HAp-nanosurface, resulting in an increase of sorption sites and subsequently an increase of arsenate sorption(Czerniczyniec et al.,2007; Sneddon et al.,2005). SEM image of absorb As(V) by nanoparticles of  HAp-nano shown in figure8.

Conclusion

The result shows that hydroxy apatite nano particle (HAp-nano)is a powerful adsorbent for removing As(V) from aqueous solution. The optimum dose of HAp-nano for As(V)removal is found to be 0.2 g/L with the removal efficiency of 88 %. Freundlich isotherm had best fit than Langmuir, for experimental data. The adsorption capacity of HAp-nanowas found to be 526 mg/gHAp-nano dissolution and hydroxy pyromorphite precipitation were the main mechanisms for As(V) immobilization byHAp-nano.

Acknowledgment

The authors gratefully acknowledge financial assistance of Iran nanotechnology initiative council.

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