Interactive sorption of metal ions and humic acids onto mineral particles

The sorption of metal ions (Pb2+, Zn2+ and Cu2+) and soil humic acids (HA) from aqueous solutions onto mineral particles (sand, calcite and clay) was investigated using a batch equilibrium system. The sorption reactions in tw o-component systems (heavy metals-mineral particles and humic acids-mineral particles), as well as interactions in three-component system (heavy metal s-humic acids-mineral particles) were examined. Results showed that the pre sence of humic acids, dissolved or bound onto mineral surfaces, considerabl y influenced the fixation of heavy metals. The various effects, depending o n mineral type, humic concentration and specific metal-ion, were observed i n three-component system. Sorption of Cu2+-ions on all minerals studied rap idly increased as the concentration of dissolved HA increased. The amount o f pb(2+)-ions sorbed on sand slightly decreased, while on kaolin increased between 15 and 20%. Sorption of Zn2+-ions on all minerals studied decreased at pH 4. At pH 5.5 the sorption of Zn2+-ions onto calcite decreased, while on kaolin and sand increased as a function of the humic acid concentration giving the curve with maximum at c(HA) = 2.5 mmol C L-1. At pH 6.5 sorptio n onto kaolin and sand increased. This effect occurs as a result of the con ditional stability constant of Zn-HA complexes increasing at higher pH whic h in rum promotes the chelation of Zn2+-ions to mineral-bound humic substan ces. The enhanced sorption of metal ions from the aqueous phase in three-co mponent systems is not only the result of mineral sorption of free metals b ut also the result of chelation with HA sorbed on the mineral surface.