The effect of humic acid (HA), on the adsorption and transport of arsenic (As) onto and within a model iron oxide-based adsorbent, iron oxide-coated diatomite (IOCD), is investigated. Experimental results indicate that the adsorption of both As and HA is highly pH-dependent. As uptake was suppressed by HA, with the level of suppression increasing with HA concentration. The suppression is attributed to the partial coverage of the adsorption sites, as confirmed by elemental analysis. Adsorption energy analysis indicates that for As(III), the main interaction with IOCD is physical adsorption, whereas for As(V), it is more likely ion exchange. The presence of HA may alter the adsorption energy and interaction of As with the adsorbent, particularly at higher HA concentrations. Kinetic results indicate that HA did not affect the diffusional transport of As in systems with both As and HA. However, for IOCD preloaded with HA, the adsorption kinetics of As was significantly slower, although the As uptake was similar to the conditions of co-sorption with HA. The slower kinetics and similar equilibrium uptake of As in the HA-preloaded IOCD system may be attributed to the partial blockage of the intraparticular pores within IOCD, which slowed down the diffusion of As.
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