Arsenic Removal by α, β and γ Cyclodextrin Polymers from Contaminated Water

α, β and γ cyclodextrin polymers (CDP) has been successfully tested for arsenic (As) removal from As contaminated water. Removal of As (III) was studied through batch adsorption experiment at pH 3.6 to 9.0 with different time ranges from 0.5 to 24 h. α, β and γ−CDP worked well at pH 6.8 but highest As removal (86.9%) was achieved from γ-CDP at pH 5.0 from the 100 μg L As solution after 24 hours incubation. Considering the time, α-CDP has reached the maximum 86% As removal occurred at pH 6.8 within 30 minutes. But at highly acidic (pH 3.6) and basic medium (pH 9.0), all of these CDP did not performed well for As adsorption. α and β-CDP has the capability to reduce As is less than 20 μg L at pH 6.8 and only γ-CDP has capable to achieve this limit at pH 5.0, which below the limit of As standard in Bangladesh (50 μg L). So, according to pH of water, this adsorbent might be used in different regions as and where is suitable.


Introduction
Arsenic (As) is a metalloid which is very toxic pollutant that adversely affects the health of millions of people at very low level [1]. The occurrence of As in the environment is mainly from minerals and geogenic sources, human activities such as mining, burning of fossil fuels, medicine, electronics, use of As-containing chemicals in agriculture and as wood preservatives [2]. Arsenic is a known carcinogen and mutagen and is detrimental to the immune system [3]. So, As in water has generally been a global concern. 18% of the wells of Bangladesh having As more than 50 µgL -1 causes worst As poisoning in the world [4]. Approximately 59 districts from the total 64 are currently affected at various extents of As in Bangladesh [5]. Hence, there is a pressing demand for more innovation to maintain As at the safe levels in water. There are a number of As removal methods, which include coagulation followed by precipitation, membrane separation, ion exchange, phytoremediation and adsorption, etc.
[6─9]. Adsorption is the most demanding one which has favorable economics, low installation and maintenance cost. Arsenic sorption is commonly studied in terms of sorption capacity, ionic strength, and the effects of pH etc. [10]. Cyclodextrins are useful molecular chelating agents. They possess a cage-like supramolecular structure (figure. 1), which is the same as the structures formed from cryptands, calixarenes, cyclophanes, spherands and crown ethers [11]. Removal of heavy metals by α, β and γ cyclodextrin polymers (CDP) have been used extensively in the last few years' modification with epicholorohydrin, nano zero valent iron, chitosan and others [12─14]. Sikder et al., [13] found that epichlorohydrin cross-linked α, β, and γ-CDP can remove 99% cadmium ions from solution. But very few researches held on As removal using these polymers in home and abroad. So the current study has been designed using unmodified α, β, and γ-CDP for removal of most toxic and carcinogenic As from the water environment.

Materials and Methods
α, β, and γ-CDP are composed of six, seven and eight α−(1,4) linked glycosyl units, respectively (figure. 1). The main properties of those cyclodextrins are given in Table 1 [11]. The adsorption of As (III) by α, β, and γ−CDP was examined using a batch equilibrium technique in aqueous solutions at pH 3.6, 5.0 6.8 and 9.0 at room temperature (25°C). In general, 30 mL of buffer solutions (100 mM phosphate buffer, pH 6.8 or 400 mM acetate buffer, pH 5.0 or 3.6) were prepared, and 100 mg of each adsorbent, along with 100 µg L -1 of As (from sodium arsenite [NaAsO 2 ]), were added into each buffer solution at different pH. The solution was then gently shaken with a constantly rotating stirrer from 0 to 24 hours (h) to attain equilibrium. The stirring period of 30 min to 24 h was chosen to determine the effect of contact time for the sorption of As (III). The adsorbents were removed by centrifugation at 2500 rpm for 5 min after the reaction. The obtained supernatant was filtered by 0.45 µm pore size millipore filters to remove the insoluble materials. The concentrations of As ions were measured using an atomic absorption spectrophotometer (AAS-Shimadzu AA 7000). All reagents were of analytical reagentgrade chemicals. Three replications were done throughout the study and statistical analysis was also done for different sorption data. The accumulated amount of metal ions onto CDP (q t ) and percent (%) removal was calculated by the following numerical formula 1 and 2 [14]: where, q t (mg/g) is the total adsorbed amount in time t (min), Ci and C are the initial and equilibrium concentrations (mg/L) of metal ion in the solution, V is the volume (mL), and W is the weight (g) of the adsorbent.

Arsenic Adsorption Behavior of α, β and γ-CDPs at
Acidic Medium Arsenic adsorption efficiency (i.e., % removal) of α, β and γ−CDP at acidic medium (pH 3.6 and 5.0) were given in Table 2. At pH 5.0, As adsorption was gradually increased by γ−CDP and highest amount of As (86.9%) was adsorbed after 24 h shaken in a stirrer (Table 2). Arsenic adsorption was fluctuated at the same acidic medium in case of α-CDP and β−CDP at different time intervals ranges from 42.5−58.9 and 63.3−76.2%, respectively. The lowest As adsorption occurred at acidic condition (pH 3.6) for all of the adsorbents ranges from 2.5−14.5, 7−17.5 and 6.5−19.9%, respectively, for α, β and γ−cyclodextrin polymers ( Table 2). All of the CDPs adsorbed As at increasing trends with increasing stirring time. There were only 13, 23 and 41 µg L -1 As present in the solution after treated with γ, β and α−cyclodextrin polymers, respectively at pH 5.0 (figure 2a).

Arsenic Adsorption Efficiency of α, β and γ-CDPs at
Neutral Water Environment α, β and γ−cyclodextrin polymers worked well at neutral water environment (pH 6.8) and maximum 86% of As adsorbed by α−CDP within 30 minutes incubation from 100 µg L -1 As concentration (Table 3). Though β−CDP adsorbed 83.2% As after 8 h stirring with the As tainted solution but γ−CDP removed almost similar amounts of As throughout the experimental period (Table 3). There were only 14 µg L -1 As present in the solution from 100 µg L -1 As (initial concentration) after treated with α−CDP for 30 minutes and gradually desorbed with increasing time though β and γ−CDPs showed fluctuated behaviors for As adsorption at pH 6.8 (figure 2b).

Discussion
The present study showed the maximum removal efficiency of As (86.9%) happened by γ−CDP at pH 5.0. This is might be due to presence of high cavity volume of γ−CDP (Table 1) and have high chance of interaction with As. On the other hand, all of the CDP adsorbents performed very poor As adsorption at highly acidic (pH 3.6) or basic (pH 9.0) condition (Table 2 and 3). But average performance for As adsorption by α, β and γ−CDP was good at neutral condition (pH 6.8). So, the As adsorption behavior was totally dependent on pH and α, β and γ−CDP removed As in the order of pH 6.8 ˃5.0˃3.6. Chen et al. [17] and Ferna´ndez et al. [18] also pointed out that pH is the key controlling parameter in every adsorption process. The current findings is consistent with Sikder et al. [14] who found that the adsorption capacity of β−CDP increased with increasing the pH (3.0 to 9.0) in case of an initial concentration of 100 mg L -1 Cu 2+ . Ferna´ndez et al. [18] showed that desorption of solutes in basic medium was governed by diffusion, but at pH 1.2 the mechanism was more complex by using β−CDP. Sikder et al. [12] also experimented that these adsorbent have identical adsorption potential for both Cd 2+ and Cr 6+ and removal efficiency was 99% for cadmium and 40% for chromium at pH 6.5−7.5 and 3.6−6.5, respectively.
The uptake of As was fast at the early stages of the contact period and slow near the equilibrium at neutral condition (Table 3). This may be explained from the fact that repulsive forces between the solute molecules on the solid and bulk phases made it difficult to fill the surface sites with the solute [19]. The solution reached equilibrium at pH 6.8, and the sorption capacities significantly dropped at pH 3.6 ( Table 2). It can be described as an insignificant competitive adsorption of hydrogen ions [20] with As.

Conclusions
α, β and γ−CDP worked well for As adsorption at pH 6.8 but not good at highly acidic or basic water environment. So, these adsorbent might be suitable to treat As in drinking water as pH of most of the underground and surface water are at neutral condition. Additionally due to negligible cytotoxic effects of CDPs, it will be an attractive adsorbent for environmental remediation. Detailed investigations are absolutely necessary to treat contaminated water, to understand the regeneration, and to reuse of these adsorbent in field level.