Availability of toxic metals in saline-sodic soils treated with sewage sludges and biosolids

Abstract

An alternative to valorize sludge generated by wastewater treatment is to use it as soil improver. Sludge may contain organic matter and minerals that are beneficial to soil. However, it could also have pathogens, metals and toxic organic compounds. Therefore, it is important to treat the sludge appropriately, to transform it into a chemically and biologically safe product, biosolids. In this work, we studied the availability of the most abundant elements in a saline-sodic soil, which was treated in different ways involving the addition of sludge and biosolids. The Tessier fractionation technique was used for this purpose. Regardless of the treatment, the largest proportion of Na (95%), K (86%) and Ca (57%) was found in fraction 2 (F2), corresponding to metals bound to carbonates; Mg (85%) and Pb (59%) were found in F1, as exchangeable metals; while 46% of Cr was found in the F5, combined with minerals, and only 20% in F1. These results indicate that Mg and Pb are the most available metals and represent the greatest risk, because of their concentration and toxicity, respectively. Na, K, Ca and Cr (in F1) might be available if the soil pH decreased. Acidification would be recommended only if it is combined with a washing treatment to remove excess of Cr and Na.