Arsenic adsorption from ground water using non-toxic waste as adsorbents

Groundwater sources contaminated by arsenic pose a great threat to the environment and the living organism including human. Exposure to arsenic at low concentration for a long period or high concentration for a short period, would create a lot of problem on the health and environment. Treatment of water containing soluble arsenic ions is a must and if possible, followed by recovery or secure disposal of it. Various conventional techniques are being applied to treat arsenic contaminated water which are either expensive or less efficient. Adsorption is the most cost effective, simple and easy way to treat water containing heavy metals. The energy requirement to carry out adsorption process is minimal and could be more economic if we are able to use low cost adsorbents. In an effort to treat arsenic contaminated water in an economic way, this study attempts to find locally available and non-toxic adsorbent so that arsenic could be removed via the process of adsorption. Egg shell (ES) and banana peel (BP) were selected and their arsenic removal capacity were investigated.Some physical and chemical properties of ES and BP were studied using techniques such as FTIR to find the functional group present in the adsorbents, BET surface area analysis, SEM to observe the surface morphology of the adsorbents and EDX to know the chemical composition of the adsorbents. The results of FTIR analysis showed the involvement of calcium carbonate of ES and hydroxyl group of BP for arsenic adsorption. BET surface area analysis revealed that the surface area of ES and BP was 0.77 m2/g and 0. 4m2/g. Lower surface area in both adsorbents was due to the flat surface as seen in the SEM images. EDX analysis showed increment or decrement in weight percentage of element in adsorbents before and after adsorption. This suggests the involvement of the ions during adsorption process and confirmed the adsorption process being held.Adsorption experiments were carried out to optimize the influencing parameters such as adsorbent dose, pH, agitation speed, contact time and initial concentration on both the arsenite [As(III)] and arsenate [As(V)] forms of arsenic. Arsenic removal capacity of ES is greater than the arsenic removal capacity of BP under same experimental conditions. As(V) removal capacity of adsorbents is greater than As(III) removal capacity even in short period of time. As(III) and As(V) adsorption were more sensitive to pH of the solution. As(III) removal was maximum at pH 9 and As(V) removal was maximum at pH 6. Moreover, pHzpc of BP was found at pH 7.06 and of ES was found at 8.03. At optimum conditions, As(III) and As(V) removal efficiency of ES is 68.54 % and 72.01 %, respectively. Likewise As(III) and As(V) removal efficiency of BP is 63.77 % and 67.12 %, respectively. Effect of competing ion such as phosphate and nitrate were also studied. These ions have a great influence on arsenic removal efficiency. As the concentration of these ions increases, arsenic removal efficiency of the adsorbents decreases. Modification of adsorbents were done in order to improve the removal efficiency. Carbonization at 700℃ slightly increased the removal efficiency but acid modification was not favourable. Carbonized banana peel (CBP) and carbonized egg shell (CES) along with BP and ES were further used for experiments at pH 7 to model the adsorption data using different isotherm and kinetic equation. The best isotherm and kinetic equation were chosen with the help of some error analysis tools. As(III) uptake by all adsorbent and As(V) uptake by CBP are best fitted to Langmuir isotherm indicating formation of monolayer during adsorption process. But, As(V) adsorption on BP, ES and CES followed the Freundlich isotherm model indicating multilayer adsorption. Adsorption of As(III) and As(V) on BP, CBP and CES followed pseudo-second order kinetic equation and adsorption of As(III) and As(V) on ES followed Elovich kinetic equation indicating that the adsorption process is controlled by chemisorption process. In addition, adsorption rate on ES decreases exponentially with an increase in quantity of adsorbed ions. Comparison of evaluating suitable isotherm and kinetic model based on R2 and error values revealed that R2 values may not be true representative of the model. Overall result of the study concluded that BP and ES are promising alternate to the toxic and expensive adsorbents for the removal of arsenic from groundwater.