Removal of metal ions from aqueous solutions by sorption onto untreated low-rank coal (oxihumolite)

Naturally occurring kind of weathered and oxidized young brown coal called oxihumolite was used for an adsorptive removal of heavy metal cations (Cd2+, Cu2+, Co2+, Ni2+, Pb2+, Zn2+) from waters. A working range for the oxihumolite as a sorbent is in a slightly acidic medium (pH ca. 3.5–4.5), where it exhibits a sufficient stability and sorption capability. Typical sorption capacities estimated from equilibrium measurements (sorption isotherms) ranged from ca. 0.1 to 0.4 mmol g−1. The kinetic dependencies for the metal sorption were measured in a batch arrangement and evaluated using both pseudo-first- and pseudo-second-order kinetic models. The metal sorption was a quite rapid process that may be characterized by half-times ranging from ca. 10 to 80 min. The intraparticle diffusion was identified as the main mechanism controlling the rate of the sorption. Various extraction tests, namely the standardized leaching test with water and the three-step sequential BCR test, were used to examine a leachability of the heavy metals from the loaded sorbents and to assess potential risks of their subsequent liberation into the environment. Certain differences in the sorption and extraction behaviours of the examined metal ions (e.g. a high sorption capacity and low leachability of Pb2+) can be related to the well-known “selectivity sequences” (or stabilities of the metal–humate complexes) as published in literature.