Experimental investigation of synthetic (top) soils for mine rehabilitation: Column leaching study

Linz-Donawitz (LD) slag is one of the major by-products generated in the iron and steel industry. Alternative uses for waste by-products in the iron and steel industry are required due to increasing production, less land for disposal and stringent environmental regulations. Three different synthetic soils (M1 = 30% slag + 70% compost, M2 = 50% slag + 50% compost, and M3 = 70% slag + 30% compost) prepared from LD slag and sewage sludge compost were subjected to continuous column leaching studies for two months. The aim of the leaching study was to understand the variation in pH trends during leaching, mobility of metals and nutrients. Results showed that the pH of M2 and M3 synthetic soil and steel slag leachates remained in alkaline pH ranges. The pH for M1 was circumneutral until the liquid to solid ratio (L/S) = 0.5 and thereafter reached above pH 10. pH generally decreased with increasing percentage of compost. LD slag is a heterogeneous material comprising of 35-40% CaO and 6-8% MgO. Previous studies also suggest that with ageing and in presence of water, steel slag may form new mineral phases. Readily soluble minerals may dissolve in water more quickly however, minerals which are entrapped within complex heterogeneous phase may dissolve slowly. The highly alkaline pH trend during this leaching study may be due to presence of Ca-CO3-OH type ion matrix in leachates. Therefore, when steel slag or synthetic soils are used as an artificial (top) soil, high pH leachates may have some negative ecological impacts.