Study of sorption of heavy metals on organic and inorganic fractions of biosolids-amended soils

Abstract

Much research has demonstrated that the phytoavailability of heavy metals (HM) are generally greatest in the first year of biosolids applications. It then gradually decreases with time after applications are terminated. However, some studies with biosolids-amended soils have revealed that the reduced metal phytoavailability is still maintained even though organic matter (OM), which is responsible for metal sorption, has disappeared or decreased to background soil OM levels. This indicates that inorganic materials applied to soils in biosolids applications and those already .in the soil are also possibly involved in HM retention. This paper describes the sorption of four metals, cadmium (Cd) and copper (Cu), nickel (Ni) and zinc (Zn) by soils sampled from the Bromley sewage treatment farm in Christchurch, New Zealand. In an attempt to examine the relative importance of organic and inorganic adsorptive phases, sorption was determined for intact soils, and for samples treated to remove organic matter. Five different soils were used, sampled from sites that had received differing amounts of biosolids over a period of 25 years.

A preliminary study was undertaken to assess the potential of three different methods for removing organic matter from soil. These were (i) low temperature ashing (LTA), an oxidation using radio frequency at low temperature, (ii) wet digestions using 3% NaOCl (pH 8), and (iii) 30% H₂O₂ (pH 2). Of these three, H₂O₂ oxidation produced the greatest removal of total carbon and the least effect on soil inorganic constituents. The five intact soils (ISs) were treated with H₂O₂ to remove organic matter to produce soil inorganic fractions (SIFs). Cadmium, Cu, Ni and Zn sorption isotherms were determined for both the intact soils and their respective SIFs in a background electrolyte of 0.01 M Ca(NO₃)₂ solution at three different pH values (pH 4.5,5.5, and 6.5). Sorption isotherms for both ISs and SIFs were determined by equilibration with a range of Cd, Cu, Ni or Zn concentrations (nitrate salts). The results indicated that sorption isotherms at low metal concentrations used were essentially linear, and thus isotherm gradients provided a measure of the Cd, Cu, Ni or Zn distribution coefficients (Kd).

An initial examination of the ISs suggested that, in spite of the substantial additions of biosolids over the past 25 years, there was little effect on soil contents of inorganic constituents likely to provide sorption sites for heavy metals, e.g. aluminium, iron or manganese oxides. This tended to be supported by the sorption isotherm data for SIFs, which indicate that removing soil organic matter significantly resulted in much lower Kd values at three pH values examined. To determine the percentage of sorption associated with the inorganic soil fraction (Pᵢ), the following equation is used:

[INSERT EQUATION HERE]

where KdI = slope of isotherm for SIF, and Kd = slope of isotherm for original soil.

The results suggested that the affinity of the soil inorganic phase to retain Cd, Cu, Ni and Zn was extremely small in relation to the affinity of the soil organic fraction. For Cd, Cu, Ni and Zn, Pi values varied between 1.3 -18%, 0.2 - 9.6%,0.4 -7.8%, and 0.4 - 20%, respectively. Thus it would appear that the inorganic components in the Christchurch biosolids have had little effect on the long-term metal sorption capacity of soil receiving biosolids applications.