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Rapid progress has been made recently in the understanding of heavy metal sorption and speciation on sediment and soils. One aspect that was overlooked in the previous studies was the process of pollutant transformation and transportation in hyper-concentrated solid-liquid systems. In this paper, batch experiments on copper sorption in association with loess at high sediment concentrations were conducted. However, some reaction mechanisms were difficult to determine experimentally due to the limitations of speciation extraction methods. In an additional study, the MINTEQA2 chemical equilibrium model was used to calculate the speciation and precipitation of copper sorption by loess to give quantitative predictions and detailed information about the reaction process. The experiments and the modeling simulation were made under the same sorption conditions, with sediment concentrations ranging from 50 to 200 kg/m3 and adsorbates of CuSO4 and Cu(NO3)2, in order to compare their results. The modeling results clearly supported the experimental results, fully explained the mechanisms of the effects of chemical form and sediment concentration on the copper sorption, and strengthened the dominant role of carbonates among the main components of loess in the process of copper sorption. [ABSTRACT FROM AUTHOR]

The Langmuir equation has been widely used for description of the adsorption characteristics in different solid–liquid systems, although it was originally advanced for adsorption of gas molecules on a planar surface. In this paper, the applicability of the Langmuir equation was discussed when it is modified or extended to describe sorption on heterogeneous surfaces such as soils or soil components. The sorption of Cu(II) by loess was investigated experimentally with sediment concentration ranging from 10 to 100 kg/m3, and copper concentration from 64.80 to 7499 mg/l. The measured results were found well fitted with the nonlinear Langmuir isotherm in form, but for the loess with high carbonate content, the Cu precipitation becomes the primary mechanism instead of the original adsorption in the ordinary cases with low carbonate content. Thus, the parameters identified from the same Langmuir form would be essentially different in their physical meanings comparing with those in the conventional Langmuir equation. Using the MINTEQA model, copper precipitation was calculated and the implications of parameters in Langmuir equation were interpreted. Both the `sorption coefficient' and the `sorption maximum' in the Langmuir equation for copper precipitation are very close to those for copper sorption, but the `sorption coefficient' for copper precipitation is greater than that for copper sorption and oppositely for the `sorption maximum', which explains why precipitation dominates the sorption process. [ABSTRACT FROM AUTHOR]