Subsurface transport of heavy metals mediated by biosolid colloids in waste-amended soils

Land application of agricultural and industrial solid waste products (biosolids) is being widely promoted as a cost-effective nutrient management alternative with dramatic increases worldwide in recent years (Linden, 1995). Although many biosolid materials are considered excellent sources of plant macronutrients, some contain high levels of heavy metals in labile forms that may pose a significant threat to soil and groundwater quality (McBride et al., 1997, Scancar et al, 2001). There is a general perception that many hydrophobic contaminants, such as heavy metals, readily sorb to the soil matrix and are considered to be virtually immobile, posing little danger to groundwater supplies (McCarthy and Zachara, 1989). However, the risk may be greater than anticipated considering that a significant metal load may be associated with dispersed biosolid colloid particles, which may be mobilized through soil macropores to lower soil depths and greater distances (Kretzschmar et al., 1995; Gove et al., 2001). The high organic carbon content and increased surface reactivity of mobile biosolid colloids may out-compete the soil matrix in heavy metal sorption affinity, thus facilitating metal transport (Karathanasis, 1999; Karathanasis and Ming, 2002). This colloid-mediated metal transport may explain the negative metal mass balances found by some researchers trying to account for the losses of biosolid-applied heavy metals in soils (McGrath and Lane, 1989; Baveye et al., 1999). Organically enriched and organic-coated colloids, such as biosolid colloids, have been shown to enhance colloid stability by acting as steric stabilizers, neutralizing positive edge sites on mineral colloids (Kaplan et al., 1993). While mineral colloids tend to destabilize and flocculate more readily, organic colloids or organically coated mineral particles show considerably higher stability and mobility through electrostatic and steric repulsion forces even under high ionic strength conditions (Ryan and Gschwend, 1994). Organic mobile colloids have also been implicated as the primary vector for transporting contaminants in some subsurface envi ronments, where they may be transported at a greater velocity than conservative tracers due to size exclusion effects (Kretzschmar et al., 1995, Karathanasis and Ming, 2002). Studies with undisturbed soil columns have demonstrated rapid elution of Cd, Zn, Cu, and Pb, in the presence of organics due to preferential flow processes (Camobreco et al., 1996). In other field and laboratory studies involving biosolid applications, soluble and colloidal organic constituents exhibited considerable ability to facilitate metal transport through soil porous media (del Castillo et al., 1993; Persicani, 1995). Li and Shuman (1997) reported enhanced mobility of Cd, Zn, and Pb associated with a poultry litter extract as a result of induced solubilization or the formation of soluble metal-organic complexes derived from the poultry litter. Lime-stabilized biosolid colloids have also been shown to significantly enhance heavy metal mobility in the soil, either by chemisorption or coprecipitation mechanisms onto the colloids or by the formation of soluble metal-organic complexes, particularly at high pH ranges (Karathanasis and Ming, 2002). Soil Cd mobility evaluations in field and laboratory experiments following biosolid applications suggested at least two-fold increases, mainly in the form of soluble organic complexes, with the main sorption pools being the organic and Fe-oxide fractions (Lamy et al., 1993; Hettiarachchi et al., 2003). These patterns emphasize the alarming consequences of colloid-enhanced metal transport and the ramifications for predicting the real risks of groundwater pollution by heavy metals associated with applied biosolid amendments. The objectives of this study were: (i) to assess the stability and mobility of three biosolid colloid suspensions through selected undisturbed soil monoliths, (ii) to evaluate the mobility of Cu, Zn, Pb, Cd, Cr and Mo in association with the transported biosolid colloids, and (iii) to evaluate colloid and soil properties enhancing or inhibiting metal transport.