Municipal solid waste compost as a novel sorbent for antimony(V): adsorption and release trials at acidic pH

The ability of two municipal solid waste composts (MSW-Cs) to sorb antimony(V) in acidic conditions (pH 4.5) was investigated. Sorption isotherms and kinetics showed that both MSW-Cs could sorb antimony(V), even if in different amounts (~ 0.18 and 0.24 mmol g^-1 of Sb(V) by MSW-C1 and MSW-C2, respectively). These differences were ascribed to the chemical composition of composts, as well as to the total acidity of their humic substances. The Sb(V) sorption by both MSW-Cs followed a pseudo-second-order kinetic model, while the sorption isotherms data fitted the Freundlich model better than the Langmuir one. The humic acids extracted from composts contributed to 4.26 and 8.24% of Sb(V) sorption by MSW-C1 and MSW-C2 respectively. SEM-EDX spectra of the MSW-C+Sb(V) systems showed a certain association of Ca(II) with Sb(V), while sequential extraction procedures indicated that more than 80% of the Sb(V) sorbed was strongly retained by MSW-Cs. On the other hand, treatment with oxalic acid at pH 4.5 favored the release of more than 98 and 65% of the Sb(V) sorbed by MSW-C1 and MSW-C2 respectively, supporting a possible role of calcium in Sb(V) retention. The results from this study suggest that MSW-Cs could be used as amendments for the in-situ immobilization of Sb(V) in acidic-polluted soils.