Evaluation of natural goethite on the removal of arsenate and selenite from water.

Elevated arsenic and selenium concentrations in water cause health problems to both humans and wildlife. Natural and anthropogenic activities have caused contamination of these elements in waters worldwide, making the development of efficient cost-effective methods in their removal essential. In this work, removal of arsenate and selenite from water by adsorption onto a natural goethite (α-FeOOH) sample was studied at varying conditions. The data was then compared with other arsenate, selenite/goethite adsorption systems as much of literature shows discrepancies due to varying adsorption conditions. Characterization of the goethite was completed using inductively coupled plasma mass spectrometry, X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and Brunauer-Emmett-Teller surface area analysis. Pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models were applied; including comparisons of different regression methods. Various adsorption isotherm models were applied to determine the best fitting model and to compare adsorption capacitates with other works. Desorption/leaching of arsenate and selenite was studied though the addition of phosphate and hydroxyl ions. Langmuir isotherm modeling resulted in maximum adsorption capacities of 6.204 and 7.740 mg/g for arsenate and selenite adsorption, respectively. The PSO model applied with a non-linear regression resulted in the best kinetic fits for both adsorption and desorption of arsenate and selenite. Adsorption decreased with increasing pH. Phosphate induced desorption resulted in the highest percentage of arsenate and selenite desorbed, while hydroxide induced resulted in the fastest desorption kinetics.