Waste Phosphogypsum Impact on the Mobility of Plant Macro- and Micronutrients in Soil Amended with Sewage Sludge

Mobility of primary plant macronutrients - nitrogen (N), phosphorus (P) and potassium (K), secondary macronutrient - magnesium (Mg), as well as micronutrients Zn and Cu, was studied and compared in different sewage sludge, clay soil and cement dust mixtures amended with waste phosphogypsum, as a source of soluble PO43-, at dosages of 0.05, 0.1, 0.2 and 0.4g per g of solids. Weak extraction solution approaching the average concentration of many soil solutions - 0.01 M CaCl2 was used as universal extraction solvent suitable for evaluation of soil fertility as well as soil pollution. Electrical conductivity and pH was measured in centrifugates. Phosphogypsum additive reduced electrical conductivity in 0.01 M CaCl2 extracts of sewage sludge and its mixture with soil, while the conductivity of strongly alkaline soil extracts was not affected. Acidifying effect of phosphogypsum on the studied mixtures was negligible due to their high buffering capacity. Phosphate, originating from phosphogypsum, immobilising effect on Zn and Cu was different. Zn treatment efficiency increased together with the increasing pH condition of the system. Meanwhile, Cu leaching from soil-sludge mixtures with 5-40% phosphogypsum additive was reduced by a factor ranging from 37 to 1105. Copper immobilization process was substantially assisted by Cu ion adsorption on soil colloids. Phosphogypsum additive reduced the amount of N-NO3- in sewage sludge from 4 (5% additive) up to 8 times (40% additive). Phosphogypsum also reduced concentration of 0.01 M CaCl2 extractable K+ in almost all the studied mixtures, but the main reason of such a decrease was low initial concentration of potassium in the additive applied. Meanwhile, addition of phosphogypsum significantly increased concentration of P-PO43- in all the studied mixtures with neutral pH. Furthermore, waste phosphogypsum is suitable for soil amendment not only as a source of calcium and mobile sulphur, but also as soil additive, other than lime, containing magnesium.