A siderite/limestone reactor to remove arsenic and cadmium from wastewaters

A two-column reactor was designed to remove dissolved As and Cd from contaminated water. The reactor functions by equilibrating the targeted water with CO₂ and directing it via saturated flow through a column of crushed siderite. This results in siderite dissolution and an increase in dissolved Fe(II). The feedwater is then directed into the top of a second, aerated column of crushed limestone, where it passes by unsaturated flow. The Fe²⁺ ion oxidizes quickly to Fe³⁺ and precipitates as Fe(III) oxyhydroxide, which is an effective sorbent of AsO₄^3?. The aeration that occurs in the second column also removes dissolved CO₂ from the feedwater. This causes precipitation of Ca and Cd carbonates. Together, the two processes reduce As and Cd concentrations from 1 and 3 mg/l, respectively, to below detection (respectively <0.005 and <0.01 mg/l). A time-limited reduction in Cr concentration also occurred. Much of the As was removed in the first column of the reactor, because Fe(III) oxyhydroxides also formed there. This was due to oxidation of Fe(II) by Cr(VI) and other oxidants present in the input wastewater. Although As is removed in the reactor columns by a sorption mechanism, the sorbent responsible, Fe(III) oxyhydroxide, is continuously produced during the operation of the reactor. Thus, unlike attenuation in a system with a fixed amount of sorbent, breakthrough of the As contaminant should never occur.