Arsenic(III,V) adsorption onto charred dolomite: Charring optimization and batch studies

In this work, the removal of arsenic from aqueous solutions onto thermally processed dolomite is investigated. The dolomite was thermally processed (charred) at temperatures of 600, 700 and 800°C for 1, 2, 4 and 8h. Isotherm experiments were carried out on these samples over a wide pH range. A complete arsenic removal was achieved over the pH range studied when using the 800°C charred dolomite. However, at this temperature, thermal degradation of the dolomite weakens its structure due to the decomposition of the magnesium carbonate, leading to a partial dissolution. For this reason, the dolomitic sorbent chosen for further investigations was the 8h at 700°C material. Isotherm studies indicated that the Langmuir model was successful in describing the process to a better extent than the Freundlich model for the As(V) adsorption on the selected charred dolomite. However, for the As(III) adsorption, the Freundlich model was more successful in describing the process. The maximum adsorption capacities of charred dolomite for arsenite and arsenate ions are 1.846 and 2.157mg/g, respectively. It was found that both the pseudo first- and second-order kinetic models are able to describe the experimental data (R²>0.980). The data suggest the charring process allows dissociation of the dolomite to calcium carbonate and magnesium oxide, which accelerates the process of arsenic oxide and arsenic carbonate precipitation.