Cadmium speciation assessment in salinised environmental conditions

Abstract

Increased topsoil salinity and trace elements content due to inappropriate land management practices (fertigation, soil amendments application) represent some of the most widespread threats to food safety and security in modern agriculture. Phytodeposition, as the most important entry pathway for biotoxic and nonessential trace element cadmium (Cd) into the human foodstuffs, corresponds positively to rhizosphere salinity. By using computational chemical speciation approach (Visual MINTEQ), the biogeochemistry of Cd-contaminated (1µM) solution was evaluated in terms of different levels (low-high) of some of the most abundant naturally-occurring salt minerals (Na + 15-150, Cl -12-90 and SO 4 2-1.5-30 mM) in a wide pH (3.5-9.5) range and low presence of dissolved organic carbon (1 mg Doc/L). According to the modelling results, the concentration of free Cd 2+ predominated in most of pH ranges tested under low salinity, whereas concentrations of Cd-Cl-and Cd-SO4-complexed pool prevailed in medium to high salinity environments . The NICA-Donnan modelling confirmed the importance of Cd-organic complexes only under higher pH values (>8.0) and low salinity conditions. These results confirm that as a consequence of diminished dissolved organic pool due to excessive salinity (e.g. naturally salt-affected soils, water used in irrigated agriculture) Cd biogeochemistry in the rhizosphere can be affected in a way that would enhance Cd mobility and thus phytoextraction by irrigated food crops.

Keywords

• rhizosphere salinity
• trace elements
• geochemical modelling