engleski

A geochemical model of arsenic sorption on clay mineral functional sites

The Method The mobility of arsenic species in the environment is largely controlled by solid phase sorption reactions.. Quantitative evaluations of the solid phase/water partitioning of many toxic metals, including arsenic, are best accomplished by a surface complexation approach. Equilibrium-based thermodynamic modelling is currently one of the most appropriate methods to evaluate the competitive geochemical processes that affect the transport and toxicity of arsenic, including predictions regarding arsenic persistance and mobility in the environment. Computation The USGS numerical computer code PHREEQC version 1.6 was used for all simulations. The code was used to simulate arsenic surface complexation from a small watershed with naturally high levels of arsenic on clay mineral components (kaolinite and montmorillonite) of a stream sediment. Surface complexation mass-action coefficients were obtained from the literature in the generalized two-layer model form, or from linear free energy relationships. The WATEQ4F thermodynamic database formed the core to which surface complexation parameters were added. The code was used for the calculation of saturation indices, sensitivity analysis of parameters such as Eh, pH and temperature, modeling the mixing of stream water of different compositions. The surface complexation routine was used in the generalized two-layer model and competition between arsenic and other ions for sorbing phases for a finite number of sites was allowed. Both mineral assemblages were allowed to come to equilibrium by simulating the flushing of many pore volumes through the sediment-mineral surface assemblage. The model output was evaluated using the ratio "R" of modeled vs. expected arsenic concentrations. Small differences in the conceptual model and data acquisition techniques can have a large effect on the simulation error. The error of neglecting competition by common compounds such as bicarbonate or silicic acid can equal or exceed the bias resulting from inappropriate choice of mineral phases.

Arsenic ; clay minerals ; sorption ; geochemical model

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