Natural Red Earth as a low cost material for arsenic removal: Kinetics and the effect of competing ions

This study investigates the effect of reaction time and competing ions on As retention on Natural Red Earth (NRE). The initial As [As(III) or As(V)] concentrations were varied between ∼10−5 and ∼10−4 M for competitive adsorption studies while samples were spiked with ∼2.67 μM As for kinetic studies. Batch experiments were performed for solutions with different concentrations of PO43-, NO3- and SO42- (5.26 × 10−5, 8.06 × 10−4, and 2.60 × 10−3 M, respectively) as competing ions for the two systems. One system had controlled conditions (pH 5.5, 0.01 M NaNO3) while the second is uncontrolled (no pH control and no NaNO3). Kinetic data were best described by a pseudo-second order model demonstrating strong interaction between As species and >FeOH and AlOH sites on the NRE surface. The equilibrium solid phase concentrations for As(III) and As(V) were observed as ∼20 and ∼12.5 μg/g, respectively. The time taken to equilibrium was the same (90 min) for both As species. Competitive adsorption isotherm experiments showed a greater effect of PO43- on the reduction of adsorption of both As species than with SO42- and NO3-. Arsenic(III) agreed with the Langmuir equation signifying monolayer formation while As(V) adsorption was in accord with a Fruendlich isotherm indicating multilayer adsorption. FTIR spectra indicated an inner sphere bonding of arsenate and Fe–O sites with PO43- while an outer-sphere weak complexation was observed with NO3-. The substrate appears to show a potential for a similar rate of adsorption under both controlled and uncontrolled conditions indicating its possible use in domestic water filters to remove As from water.

Research highlights
► The effect of reaction time and competing ions on arsenic sorption to Natural Red Earth was examined.
► Arsenate on Fe–O sites with PO43- showed inner-sphere bond formation.
► Weak complexation was observed for NO3-.
► As(III) adsorbed better onto NRE than As(V) with all the competing ions.
► Time taken to the equilibrium was 90 min for both As species.