Urban Gardens and Soil Contaminants: Alum-Based Water Treatment Residuals to Attenuate Metal Uptake By Vegetables Grown in Contaminated Soils.

Alum- and ferric alum-based and drinking water treatment residuals (DWTRs) have demonstrated potential for beneficial re-use in the remediation of contaminated lands.  Adsorption and co-precipitation processes arising from interactions between DWTRs and heavy metals/metalloids in contaminated soils result in their immobilisation, limiting absorption by plants and entry into the food chain.  These DWTRs also exhibit a strong affinity and high adsorption capacity for phosphorus, which is counterproductive to plant growth and production through the reduction of phosphorus availability to plants.  The phytoavailability of cadmium and arsenic in contaminated soils amended with alum-based DWTRs was examined in glasshouse pot trials using Chinese cabbage (Brassica pekinensis (Lour.) Rupr.).  The effects of DWTR amendment rate (0- 6% w/w) and phosphorus fertilizer placement on B. pekinensis growth and plant nutrient status were assessed.  Results indicated that DWTR amendment at a rate of 5% (w/w) coupled with banded phosphate fertilizer application minimised cadmium phytoavailability without inducing phosphorus deficiency in B. pekinensis.  Results from this study confirm that soil amendment with DWTRs can attenuate cadmium phytoavailability, whilst concomitant banded application of phosphate fertilizer can overcome the strong sorption of fertilizer phosphate by DWTRs.  The ubiquitous nature of DWTRs and associated issues surrounding waste disposal (e.g. land availability and cost) along with global food security concerns in the face of a growing and increasingly urbanised population provide strong impetus for the use of DWTRs to reclaim contaminated soils.  Further study examining a range of metal/metalloid contaminants in a range of different soils and environments is necessary to determine optimum DWTR application rates under different scenarios; however, there is substantial potential for the productive re-use of DWTRs to remediate contaminated soils, particularly in urban and peri-urban environments.