Effect of clay minerals on extractability of heavy metals and sewage sludge mineralization in soil

Abstract

An incubation experiment lasting 111 d was carried out to study the effect of the addition of three clay minerals (Na-bentonite, Ca-bentonite, and zeolite) to soil derived from sewage sludge on water-extractable and exchangeable forms of four heavy metals (Zn, Cd, Cu, and Ni), as well as on soil organic matter mineralization, microbial biomass C and the release of inorganic N. The addition of clay minerals led to a significant decrease in water-extractable and exchangeable forms of heavy metals. The extent of decrease ranged from 14 to 75% for the water-extractable heavy metals and from 12 to 42% for the exchangeable form over the incubation time, as compared with untreated soil. The reduction in extractability of heavy metals was greater due to the addition of Na-bentonite and Ca-bentonite than that due to the addition of zeolite. Addition of clay minerals did not affect any of the following microbiological parameters in the soil: microbial biomass C, organic C (C_org) mineralization, and metabolic quotient (qCO₂), and release of inorganic N during the first 3 weeks of incubation. However, as the incubation period increased, these parameters were significantly increased by the addition of clay minerals, especially by the addition of Na-bentonite and Ca-bentonite. This result is explained by a strong reduction in extractability of heavy metals after the addition of Na-bentonite and Ca-bentonite.

Keywords:

• Clay minerals
• Heavy metals
• Organic matter mineralization
• Microbial biomass C
• Metabolic quotient
• Inorganic N

Acknowledgements

The authors would like to thank S. Köhler for determination of inorganic N, K. Ruf for determination of total N, Dr A. Lehmann for soil information, Dr M. Zarei and D. Frobel for analysis of clay mineralogy, and A. Hardie for linguistic help. We are very thankful to Fa. IKO Minerals GmbH for the use of their bentonites and zeolite. This work was partly funded by the Egyptian government in the scope of a fellowship for A. R. A. Usman.