Speciation and phytoavailability of cadmium in selected surface soils of South Australia

A modified sequential extraction scheme was developed for partitioning the particulate-bound cadmium (Cd) into 9 fractions: exchangeable, carbonate-bo und/specifically adsorbed, metal-fulvic acid-complex-bound, metal-humic aci d-complex-bound, easily reducible metal oxide-bound, organic-bound, amorpho us mineral colloid-bound, crystalline Fe oxide-bound, and detrital (bound t o mineral lattices). Results on 11 surface soils showed that Cd in these so ils was predominantly present in detrital form, bound to the mineral lattic es, accounting for 15.8-61.9%, with an average of 33.4%, of the total Cd in the soils. The average relative abundance of Cd bound to the different par ticulate forms in the soils is in the order: detrital (0.077 mg/kg) > speci fically adsorbed/carbonate-bound (0.066 mg/kg) > organic-bound (0.033 mg/ k g) > metal-fulvic acid-complex-bound (0.031 mg/ kg) > easily reducible meta l oxide-bound (0.019 mg/ kg) > exchangeable (0.013 mg/ kg) > metal-humic ac id-complex-bound (0.011 mg/ kg) > crystalline Fe oxide-bound (0.001 mg/ kg) amorphous mineral colloid-bound (0.001 mg/ kg). The phytoavailable Cd content was determined as Cd concentration in the sho ot and leaf of durum wheat plants grown on the soils in a greenhouse study. Statistical treatment of the data showed that the exchangeable Cd (r = 0.7 35, P = 0.01) and the metal-fulvic acid-complex-bound Cd (r = 0.824, P = 0. 002) correlated significantly with the plant-available Cd, compared with ot her species. The exchangeable and fulvic acid fraction of the metal-organic -complex-bound Cd contents, together, could explain 91.5% of the variation in plant-available Cd, determined as Cd concentration in leaf and stem of t he durum wheat plants (r = 0.956, P = 0.0001). The significance of metal-fu lvic acid complexes on Cd phytoavailability has not been reported so far an d needs in-depth research in explaining the toxicity and food chain contami nation of Cd in the environment.