Adsorption experiments

Batch adsorption studies for the adsorption of As(III) and As(V) on PNHM/Fe₃O₄-40 were carried out to evaluate data related to equilibrium, kinetics and isotherm parameters at room temperature (300 ± 3 K) at neutral pH (~7). For this purpose, 100 ml of the arsenic solution of desired concentrations were initially taken in 250 ml capacity of polyethylene bottles (Tarson Co. Ltd., India) followed by subsequent addition of requisite amount of adsorbent dosages. After that, the test bottles were kept into BOD incubator shaker for shaking it for different time intervals at 180 ± 10 r.p.m. for all the experimental studies. The solution was filtered using 0.22 µm filter paper, and the filtrate was analyzed to calculate percentages removal of arsenic, if any, as follows⁴⁸:

where, C₀ and C_t refers initial concentration (t = 0) and concentration at any given time (t = t), respectively. The amount of arsenic adsorbed on PNHM/Fe₃O₄-40 (q_t, mg g⁻¹) has been calculated according to the following equation:

where m and V correspond to the mass (g) of the adsorbent and the volume of arsenic solutions (L) used in each experiment. The amount of As(III) and As(V) adsorbed on PNHM/Fe₃O₄-40 under equilibrium (q_e, mg g⁻¹) was calculated using the following relationship:

where C_e indicates the arsenic concentration at equilibrium (mg L⁻¹). The effect of variation of PNHM/Fe₃O₄-40 dose (0.1-6 g L⁻¹ for As(III) and 0.1–2 g L⁻¹ for As(V)) on the removal of arsenic was conducted at a fixed initial arsenic concentration (1000 µg L⁻¹) and contact time (240 min). The kinetic study (5–420 min) was performed at fixed initial arsenic concentration and the adsorbent dose of 1000 µg L⁻¹ and 1 g L⁻¹, respectively. Further, the isotherm study was conducted at varying the initial concentration of arsenic (100–1000000 µg L⁻¹) while keeping the fixed adsorbent dose (1 g L⁻¹). The kinetics and isotherms modeling was conducted by non-linear least square method, as the various model equation is representing by non-linear relationship and linearization of those equations which is eventually associate with bias⁷⁰. The effect on arsenic removed was also studied by carrying out experiments at different initial solution pH (3–11) at a fixed dose (1 g L⁻¹) of the adsorbent. At the same time, all the experiments were duplicated to eliminate the experimental error and were taken the mean value as a final result.