Experimental and kinetic modeling of As(V) and As(III) adsorption on treated laterite using synthetic and contaminated groundwater: Effects of phosphate, silicate and carbonate ions

An efficient porous adsorbent for arsenic is developed from abundantly available laterite using an optimized acid followed by base treatment methodology. XRD, HRTEM, surface area analyzer and FTIR are used to characterize the treated laterite (TL). Arsenic adsorption capacity of TL is evaluated under varying process conditions in batch mode using synthetic solution of single arsenic species, mixture of both As(III) and As(V) species and real arsenic contaminated groundwater (CGW). The effects of competitive ions like phosphate, silicate, carbonate, etc., on arsenic adsorption using TL are explored in details. Langmuir isotherm is found to be a better fit of the experimental isotherm data of arsenic/TL system. The Langmuir maximum adsorption capacity and constant related to adsorption energy for As(V) and As(III) on TL are found to be 21.6 ± 0.8 mg/g, 20.6 ± 0.5 L/mg and 9.4 ± 0.4 mg/g, 5.0 ± 0.1 L/mg, respectively. The competitive adsorption kinetics of individual arsenic species on TL using mixed arsenic spiked water and CGW are studied by speciation technique. Shrinking core model is applied to match the predicted bulk concentration profile of arsenic with experimental data. The model is applied to evaluate the effective pore diffusivity (De) and external mass transfer coefficient (Kf) of arsenic for arsenic/TL system and these values are in the range of 1.3–1.6 × 10−9 m2/s and 0.076–11.25 × 10−4 m/s, respectively. Fixed bed column runs using 6.5 cm TL bed (empty bed contact time: 2.92 min) is capable to produce ∼3000 bed volume (96.7 L, 1 bed volume = 32.23 mL) of water for effluent concentration at <10 μg/L (initial arsenic concentration in CGW: 385 ± 25 μg/L). The study reveals that TL is porous and highly efficient adsorbent for both arsenic species.