Competitive adsorption of humic acid and arsenate on nanoscale iron–manganese binary oxide-loaded zeolite in groundwater

Abstract

Arsenic contamination in groundwater is widespread around the world. The identification and implementation of inexpensive and effective sorbent for arsenic removal are of great importance, and this topic has been intensively studied recently. The sorbent efficiency can be affected by groundwater containing natural organic matter. In this study, a new nanoscale iron–manganese binary oxide-loaded zeolite (NIMZ) sorbent was prepared to evaluate its efficiency for arsenate removal in the presence of natural organic matter. This new sorbent has shown an excellent capacity for arsenate removal, which was investigated by considering the interaction of humic acid (HA) that was before, after and simultaneously adsorbed on NIMZ under different pH values. Results of kinetic studies showed that the removal of arsenate on NIMZ was not affected by the presence of HA at the tested levels up to 10mg/L. Affinity of arsenate, at the application level of 2mg/L, on NIMZ surface was found to be much stronger than HA in terms of adsorption removal. For the pre-equilibrium study of HA adsorption onto NIMZ, the added arsenate could replace HA which was released into the solution, while arsenate was adsorbed on NIMZ. On the contrary, in the study of NIMZ pre-equilibrated with arsenate adsorption, the added HA hardly affected the adsorbed arsenate. In the test of simultaneous adsorption of arsenate and HA on NIMZ, the competition of arsenate on adsorption sites of NIMZ was much stronger than HA. These three cases indicated that NIMZ adsorbent was effective for arsenate removal even when the natural organic matter was present. Zeta potential measurements showed that arsenate and HA can be adsorbed on NIMZ despite the electrostatic repulsion. Arsenate removal in the presence of HA on NIMZ mainly induced by ligand exchange from FTIR analyses of their surfaces, and phenolic OH and COOH groups of HA were responsible for ligand exchange with NIMZ.