Effect of Immobilizing Substances and Salinity on Heavy Metals Availability to Wheat Grown on Sewage Sludge-Contaminated Soil

Abstract

The objective of the investigation was to evaluate the effect of immobilizing substances and NaCl salinity on the availability of heavy metals: Zn, Cd, Cu, Ni, and Pb to wheat (Triticum aestivum L.). In greenhouse pot experiment, a sewage sludge amended soil was treated with the following immobilizing substances: three clay minerals (Na-bentonite, Ca-bentonite and zeolite), iron oxides (goethite and hematite), and phosphate fertilizers (superphosphate and Novaphos). The pots were planted with wheat and were irrigated either with deionized or saline water containing 1600 mg L −1 NaCl. Wheat was harvested two times for shoot metal concentrations and biomass measurements. Metal species in soil solution were estimated using the software MINEQL+.The addition of metal immobilizing substances to the soil significantly decreased metal availability to wheat. The largest reduction in metal bioavailability was found for bentonites. The irrigation with saline water (1600 mg L −1 NaCl) resulted in a significant increase in metal chloride species (MCl + and MCl 0 2 ). The highest metal complexation with Cl occurred for Cd, which was about 53% of its total soil solution concentration. The total concentration of Cd (Cd T ) in soil solution increased by 1.6-2.8-fold due to saline water. The NaCl salinity caused a significant increase in uptake and shoot concentration of Cd for two harvests and small but significant increase in shoot Pb concentration for the second harvest. It was concluded that the use of bentonites is the most promising for the reduction of heavy metal availability to plants. Saline water containing 1600 mg L −1 NaCl increased the availability of Cd and Pb to wheat and decreased the efficiency of bentonites to immobilize soluble Cd.

Growth and Physiological Responses of Two Rice Varieties to Applied Lead in Normal and Salt-Affected Soils

“…Sari and Din (2012) recently recognized that higher heavy metal accumulation under saline conditions was related to the maintenance of cell osmosis mechanisms by plants. Usman et al (2005) found that increased concentration of Pb in wheat shoot in the presence of salinity were the result of increased Pb mobility and availability due to the formation of Pb-Cl complexes. Under saline conditions, the Cl − more considerably increased the mobility of Pb than SO 4 2− due to the formation of soluble ion pairs like PbCl + (Acosta et al, 2011).…”

“…Acosta et al (2011) reported that the soil salinity resulted in enhanced heavy metal mobilization. This can be due to the reasons that heavy metals complex formation occurred with highly mobile inorganic ligands such as Cl − in soil solution (Usman et al, 2005). The subsequent association has a lesser positive charge than the free metal ions, and may be uncharged or carry a net negative charge, thereby has higher metal mobility (McLean and Bledsoe, 1992).…”

“…Under saline conditions, the Cl − more considerably increased the mobility of Pb than SO 4 2− due to the formation of soluble ion pairs like PbCl + (Acosta et al, 2011). The Cd and Pb speciation in soil solution reported to be altered, added Na + compete with other cations for sorption sites, extra Cl − form complex with Cd and Pb, and ionic strength increased which affects activity coefficients along with the degree of preference of divalent over monovalent cations by exchange complex (Khoshgoftar et al, 2004;Usman et al, 2005). The metalschloro complexes as dominant species resulted in increased diffusion of Cd and Pb via soil solution to plant roots or greater uptake of Cd and Pb if metal-chloride complexes were transported through the root membrane (Usman et al, 2005).…”

“…Generally, soil properties such as soil pH, redox status and salinity, SOM, CaCO 3 contents and texture are considered important factors in describing bioavailability of heavy metals including Pb in soil (Connell, 2005). In particular, anions have been recognized to greatly reduce sorption and enhanced mobility of Cd and Pb in soils by complexation of Cl − with these metals (Usman et al, 2005;Ghallab and Usman, 2007). Salinity change the bioavailability of heavy metals such as Cd and Pb in soil, irrespective of soil pH (Weggler et al, 2004) and it is found to be one of the key features in translocating metals from roots to foliar parts of the plants (Fitzgerald et al, 2003;Manousaki et al, 2009).…”

“…The Cd and Pb speciation in soil solution reported to be altered, added Na + compete with other cations for sorption sites, extra Cl − form complex with Cd and Pb, and ionic strength increased which affects activity coefficients along with the degree of preference of divalent over monovalent cations by exchange complex (Khoshgoftar et al, 2004;Usman et al, 2005). The metalschloro complexes as dominant species resulted in increased diffusion of Cd and Pb via soil solution to plant roots or greater uptake of Cd and Pb if metal-chloride complexes were transported through the root membrane (Usman et al, 2005). Khamis et al (2014) also described that with increasing soluble forms of Pb in solution, plant roots were able to take up more amounts of Pb; consequently toxic accumulation of Pb cause inhibitory impacts on plant metabolic activities.…”