In this study, the effect of phosphate on Ni sorption by Mahan soil was investigated. In batch experiments, an initial solution concentration of 1 mM phosphate increased Ni sorption by 15.1%, and initial concentrations of 1 mM Ni increased phosphate sorption by 12.8%. The effect of phosphate on Ni sorption, and vice versa, was dependent on initial concentrations of either solute, with greater effects being observed with greater initial concentrations. Sequential extractions suggest that the increased sorption of either species in the presence of the other is due to a combination of electrostatic interactions and the formation of ternary complexes. A novel Freundlich-type multi-component isotherm (FMC) was developed and described the sorption of Ni both in the absence and presence of phosphate with a single set of parameters. The effect of phosphate on the sorption kinetics of Ni was evaluated using the stirred-flow approach. The presence of phosphate increased the maximum sorbed concentration of Ni by 7.3% and increased the rate of Ni sorption. A novel Freundlich-type multicomponent kinetic model (FKM) was developed and described the stirred-flow data well. The FKM was able to describe the sorption-desorption of both Ni and phosphate applied individually as well as simultaneously. The FMC and FKM offer advantages over previous modeling approaches in that they require far fewer parameters than surface complexation models and have greater descriptive capabilities than traditional empirical models. However, both models are empirical, and their ability to describe the observed data should not be taken to imply specific molecular-level mechanisms.

The immobilization of toxic metals in soils using amendments is a cost-effective remediation technique for contaminated soils. Therefore, this study aimed to assess the efficiency of various amendments to immobilize nickel (Ni) and zinc (Zn) in soil and reduce their phytoavailability. A greenhouse pot experiment was established with a contaminated agricultural floodplain soil. The soil was treated with activated carbon (AC), bentonite (BE), biochar (BI), cement bypass kiln dust (CBD), chitosan (CH), coal fly ash (FA), limestone (LS), nano-hydroxyapatite (HA), organo-clay (OC), sugar beet factory lime (SBFL), and zeolite (Z) with an application rate of 1 % (0.2 % for HA) and cultivated by rapeseed (Brassica napus). After plant harvesting, the soil was analyzed for water-soluble and geochemical fractions of Ni and Zn. Additionally, both metals were analyzed in the aboveground biomass. Application of the amendments increased significantly the biomass production of rapeseed compared to the control (except for OC, HA, and FA). Water-soluble Ni and Zn decreased significantly after adding the amendments (except OC, Z, and CH). The SBFL, CBD, LS, BE, AC, and BI were most effective, resulting in a 58-99 and 56-96 % decrease in water-soluble Ni and Zn, respectively. The addition of SBFL, CBD, and LS leads to the highest decreasing rate of concentrations of Ni in plants (56-68 %) and Zn (40-49 %). The results demonstrate the high potential of CBD, SBFL, LS, BE, AC, and BI for the immobilization of Ni and Zn in contaminated floodplain soils.

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