Heavy metal accumulation and mobility in a soil profile depend on the organic waste type applied

Purpose

While organic waste amendments can initially improve soil physicochemical properties, including nutritional benefits to plants and increased microorganism activity, long-term application of excessive amounts of organic wastes can cause accumulation of heavy metals (HMs). Thus, the current study examined the accumulation of HMs in agricultural soil profiles following organic waste application.

Materials and methods

Three common organic sludge, including municipal sewage sludge (MSS), industrial sewage sludge (ISS), and leather sludge (LS), were applied annually to an agricultural soil under field conditions over 7 years (1994–2000) at a rate of 25 and 50 t ha^?1 year^?1. Subsequently, when organic sludge amendments were ceased, the experimental plots were cultivated without any treatments for another 12 years (2001–2012) and the changes in HM concentrations along the soil depth profile were monitored together with soil pH, dissolved organic carbon (DOC), and dehydrogenase activity (DHA).

Results and discussion

Significant increases in Cu, Pb, and Zn concentrations were observed down to a depth of 80 cm in soils treated with ISS and LS, where sludge application also increased the levels of Cd, Cr, Pb, and Zn and their movement down the soil profile. However, with the exception of Cu, no significant changes in HM concentrations were observed following treatment with MSS. At a depth of 80 cm, soils which had received 25 and 50 t ha^?1 LS showed, respectively, 4 and 14 times higher Cr levels than the control soil.

Conclusions

Organic sludge induced changes in soil pH and soil DOC concentration which were the key factors influencing HM movement and accumulation following organic sludge treatment.

     

Application of soil amendments to contaminated soils for heavy metal immobilization and improved soil quality—a critical review

ABSTRACT

Today, soil metal pollution has become a significant environmental issue of great public concern. This is because soil is both a major sink for heavy metal(loid)s (HMs) released into the environment, by both pedogenic and anthropogenic activities; and also a major source of food chain contamination mainly through plant uptake and animal transfer. In addition, HM contamination of soil leads to negative impacts on soil characteristics and function by disturbing both soil biological and physiochemical properties (e.g. extreme soil pH, poor soil structure and soil fertility and lack of soil microbial activity). This eventually leads to decreased crop production. Various soil remediation techniques have been successfully employed to reduce the risks associated with HMs efflux into soil. Among these, the use of low-cost and environmentally safe inorganic and organic amendments for the in-situ immobilization of HMs has become increasingly popular. Immobilization agents have successfully reduced the availability of metal ions through a variety of adsorption, complexation, precipitation, and redox reactions. Soil amendments can also be a source of nutrients and thus can also act as a soil conditioner, improving the soil’s physiochemical properties and fertility, resulting in enhanced plant establishment in metal contaminated soils. This article critically reviews the use of immobilizing agents in HM contaminated agricultural and mining soils paying particular attention to metal immobilization chemistry and the effects of soil amendments on common soil quality parameters.