Effects of Fly Ash and Sewage Sludge Amendments on Transport of Metals in Different Soils

Abstract

Utilization of coal combustion by-products as agricultural soil amendments is of considerable importance not only as an avenue to dispose these by-products but also to explore the potential advantages as sources of some plant nutrients and to improve soil physical and chemical properties. Although some studies have shown the benefits of using Fly Ash (FA) as an agricultural soil amendment, this source generally is low in nitrogen (N) and phosphorus (P). Therefore, combining FA with another by-product which contains relatively high concentration of N and P would increase the nutrient value of the former, thereby, increase its beneficial effects on plant growth and/or production. Sewage sludge (SS) generally contains N and P, and is also slightly acidic in reaction, unlike the FA which is very alkaline. Therefore, mixing SS with FA can supplement the plant essential nutrients which are lacking in FA, and can also neutralize FA high alkalinity, counteract to some extent high pH of the FA. Accordingly, the addition of a mixture of FA and SS has greater potential benefits for plant growth as compared to single application of either of the products. Since some of the industrial by-products may contain high concentrations of heavy metals, the potential for increased accumulation of heavy metals in the soil and potential adverse effects on the crop plants are major concerns. Soil solution displacement method has been adapted to evaluate the available forms of metals in soil solution of an Orangeburg loamy sand and a Candler fine sand, with application of either 24.7, 49.4, 98.8 or 148.2 Mg ha-1 of 1: 1 (wt: wt) mixture of FA and SS. Two additional treatments were included with application of 74.1 Mg ha-1 each of FA or SS. The results of this study showed that the total quantities of metals in leachate, in seven successive displacement of 220mL volume each (total of 1,540mL), were generally greater when the amendment was applied to a Candler fine sand compared to an Orangeburg loamy sand, with the exception of Al. Furthermore, the combined quantities of metals displaced with addition of 74.1 Mg ha-1 each of FA or SS were lower than that displaced with addition of 148.2 Mg ha-1 of 1: 1 mixture of FA and SS. Therefore, it appears that the combined addition of FA and SS favored some retention of the metals in the soil compared to addition of each of the amendments separately.