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

摘要

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 ～2.67μM 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, 0.01M NaNO_3) while the second is uncontrolled (no pH control and no NaNO_3). 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 ～12.5μg/g, respectively. The time taken to equilibrium was the same (90min) 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 SO_42- 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.