RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Hypothesis

Oxides, hydrous oxides and hydroxides of aluminium and iron are important in determining the availability of trace and heavy metals in soil systems. The presence of complexing anions is also known to affect the binding of these metals in soils. Since organophosphates, such as inositol hexaphosphate (IP₆), are present in most soil systems they are expected to affect the nature of the interaction between metal ions and metal (hyr)oxides.

Experiments

Both adsorption edge and isotherm experiments were conducted on Cd(II)–gibbsite and Cd(II)–IP₆–gibbsite systems. In addition, solid-state ³¹P MAS NMR measurements were performed on the ternary system. All results were used to develop Extended Constant Capacitance surface complexation models of both the Cd(II)–gibbsite and IP₆–Cd(II)–gibbsite sorption systems.

Findings

The presence of IP₆ significantly increased sorption of Cd(II) to gibbsite below pH 8 especially at higher concentrations of Cd(II) and IP₆. The ³¹P MAS NMR spectra, together with surface complexation modeling, indicated the presence of two outer-sphere ternary complexes with the first, [(SOH₂)₃³⁺(LHCd)⁹⁻]⁶⁻, important at relatively low concentrations, while the second, [SLH₃⁸⁻Cd²⁺]⁶⁻, dominated sorption at higher sorbate concentrations. Thus the presence of organophosphates in soil systems increases sorption and may therefore decrease the availability of trace and heavy metals to plants.