Sludge Solution Research Articles

Cadmium, copper, and zinc adsorption by a forest soil in the presence of sludge leachate

Samples from three genetic horizons of an acidic forest soil were equilibrated with solutions containing Cd, Cu, and Zn in the presence and absence of a municipal sewage sludge leachate. Copper adsorption was greater than Cd and Zn in all three horizons, while Cd and Zn adsorption was quite similar. Relative to a NaN03 background solution, sludge leachate reduced Cu adsorption in all horizons; however, in the presence of leachate Zn adsorption increased in the B2 and C horizons, while Cd adsorption decreased in the Al and was unaffected in the B2 and C horizons. Distribution coefficients generally increased as solution concentration and adsorption increased. In all horizons additions of Cd and Zn were able to reduce apparent surface charge. Copper not only reduced surface charge in the Al horizon, but caused a charge reversal in the 132 and C horizons. Adsorption data were best fit by linear or Freundlich equations. Differences in adsorption between sludge leachate and NaN03 solutions could not be explained simply by differences in metal activities as calculated using the GEOCHEM program. Competition from cations and organics present m the sludge leachate appears to contribute to lower metal adsorption.

Residual Cadmium Forms in Acid‐extracted Anaerobically Digested Sewage Sludge

AbstractThe effects of phosphorus and lime additions after acid extraction on residual Cd solubility and chemical forms in an anaerobically digested sewage sludge were investigated. High Cd content (11 mmol Cd kg−1) anaerobically digested sewage sludge was aerated and then acidified to pH 2 to solubilize Cd. After 18 h of acidification, the sludge was dewatered and the supernatant and solids separated. Seventy or more percent of the Cd was removed from the solids. Similar amounts of Ni, Mn and Zn were also removed, but Cu removal was only 26% and that of Pb was < 5%. The acid extracted sludge was neutralized to pH 5.9 with Ca(OH)2. Before liming, the sludge was amended with rock phosphate (RP) or monocalcium phosphate (MCP). The RP was insoluble while MCP dissolved, providing a high level of phosphate ligand for Cd precipitation or coprecipitation. Solution Cd in the limed MCP sludge was 12% of that in the limed RP sludge or in the limed sludge without P addition. Estimated sludge solution solubility products for major Fe, Al and Ca phosphates showed that several of these minerals could have precipitated with P addition, especially with MCP, and Cd may have coprecipitated with these solid phases. Cadmium phosphate may also have been formed in the MCP sludge. Chemical fractionation indicated that 50% of the Cd in the aerated unextracted sludge existed as inorganic precipitates with another 40% Na4P2O7 extractable (organically bound). Acidification solubilized 98% of the inorganic Cd and 86% of the organically bound Cd. Seventy‐nine percent of the Cd remaining in the dewatered acidified sludge was in the KNO3 extractable (exchangeable) fraction. Liming redistributed the Cd with 13 to 19% as inorganic precipitates, 70 to 85% organically bound and < 3% in the exchangeable fraction. Phosphate addition had no significant effect on Cd fractionation.

Effect of Sewage Sludge on Trace Element Mobility in Soils

AbstractAdsorption of Be, F, B, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Tc, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Pb, Bi, and P was measured in a sandy soil and a sandy loam soil at concentration levels closely corresponding to those to be expected for field conditions. Mobilities of the elements in the soils were calculated from the adsorption data. Of the trace elements studied, F, B, and Tc were found to be very mobile in both the sandy and sandy loam soils. Manganese, Sr, and Sb were highly mobile only in the sandy soil, and Mo in the sandy loam soil. In general, sludge solutions appeared to increase the mobility of elements in a soil. This is due to a combination of complexation by dissolved organic compounds, high background concentrations, and high ionic strengths of the soil solutions. The relative effects of these factors vary strongly among elements.Equations were derived predicting the rates of accumulation in soils and accompanying increases in the soil solutions of trace elements added with sewage sludge.When adsorption was related to soil organic matter content, for many trace elements the strength of adsorption was found to depend only on pH, increasing with increasing pH.

Concentration of radioactive ferric hydroxide sludge and recycle solutions by combined evaporation

Concentration of radioactive ferric hydroxide sludge and recycle solutions by combined evaporation