can cause adverse health effects. Lead availability and mobility depend on its physicochemical forms in soils. For this reason, understanding Pb availability and ...
Effects of Incubation and Phosphate Rock on Lead Extractability and Speciation in Contaminated Soils Lena Q. Ma,* Angela L. Choate, and Gade N. Rao ABSTRACT Lead distribution in 11 contaminated soils among five physical size fractions was studied. Lead was present in elevated concentrations in these soils, ranging from 198 to 12 523 mg kg"1. Although Pb in these soils was randomly distributed among the five size fractions studied, it was slightly concentrated in the silt and clay (< 53 |xm) fraction with some exceptions. Several single chemical extractants were used to evaluate Pb availability in the contaminated soils. The effects of incubation and phosphate rock on Pb availability were also investigated. Significant amounts of Pb were extracted with all the extractants. The extractability increased in the order: H2O < Ca(NO3)2 < HO Ac < EDTA-NH4OAc. The percentage of Pb extracted generally decreased with increasing incubation time as the soils gradually lost moisture during incubation. Net extractable Pb reduction with incubation time was not correlated with soil pH, organic matter, or total Fe and Mn contents in soils. Reduced Pb extractability with incubation may be attributed to precipitation/adsorption of Pb, rendering Pb less available. Addition of phosphate rock reduced Ca(NO3)2 and HO Ac extractable Pb from Pb-contaminated soils, but had little effect on EDTA-NH4OAc extractable Pb. Net extractable Pb reduction by phosphate rock was not correlated with soil pH, organic matter, or other metals such as Zn, Cu, Ni. and Cd in soils, but it was highly correlated with total Pb (r = 0.7). Our results indicate that physicalchemical speciation of Pb-contaminated soils can be a useful tool in evaluating Pb availability.
T BAD is A HEAVY METAL and toxic to humans and aniI -i mals, especially to young children (Paulozzi et al., 1995; Wixson and Davies, 1994). Its extensive use and Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 326110290. Received 23 July 1996. -"Corresponding author (qma@gnv. ifas.ufl.edu). Published in J. Environ. Qual. 26:801-807 (1997).
widespread disposal in the environment have resulted in numerous Pb-contaminated soils (Turjoman and Fuller, 1987). Since soil serves as a medium for plant growth and groundwater recharge, elevated Pb levels in soils can cause adverse health effects. Lead availability and mobility depend on its physicochemical forms in soils. For this reason, understanding Pb availability and mobility in contaminated soils is important for evaluating the potential environmental and health effects of Pb, Total elemental content of a soil usually provides little information on the processes and dynamics of the availability and mobility of an element. An estimation of element availability is more useful, since it is related to mobility and uptake by plants and extractability by chemical treatments (McBride, 1994). Lead in soils usually is present in different forms: (i) water soluble; (ii) exchangeable; (iii) absorbed, chelated, or complexed and precipitated; (iv) secondary clay minerals and metal oxides of low solubility; and (v) primary minerals (Hani and Gupta, 1984). In general, the water-soluble metals represent the most mobile, and potentially most dangerous fraction in the soil. The exchangeable fraction consists of those metals retained by electrostatic forces that are solubilized by an exchange reaction with excess positive ions present. Finally, heavy metals chemically adsorbed or complexed by organic and inorganic materials can be extracted by the complexing agent EDTA in accordance with the stability constant of the complexes. The combination of the first three listed chemical forms in soils represents the forms of heavy metals of greatest Abbreviations: EDTA, ethylenediaminetetraacetic acid; DPTA, diethylenetriaminepentaacetic acid; OC, Occidental Chemical Corp.
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environmental interest. They are more available to plants and have higher mobility, and thus are also more toxic to humans and animals than the last two forms (Lake et al., 1984). The geochemical behavior of Pb indicates that phosphate, when present in sufficient amounts, reduces Pb solubility, possibly Pb availability (Nriagu, 1974; Ruby et al., 1994). Thus, phosphate minerals have potential to immobilize Pb in contaminated soils and their application to soils mayalter the distribution of Pb between various physicochemical forms and hence reduce its availability. Therefore, in situ Pb immobilization may provide an economical alternative to reach cleanup goals while protecting humanhealth and the environment. Ma et al. (1993, 1994a,b, 1995) have shown that phosphate minerals such as hydroxyapatite and phosphate rock effectively immobilized Pb from aqueous solutions and soils. Selective extractants can be used to evaluate the effectiveness of in situ Pb immobilization using P to remediate Pb-contaminated soils. Various chemical extraction schemes (single and sequential) have been developed to assess bioavailable Pb and to evaluate the importance of Pb mobilization from contaminatedsoils (Fiedler et al., 1994; Pickering, 1986). Most chemical tests are designed to extract the quantity of an element in a soil that statistically correlates to its bioavailability, i.e., extract a portion or all of the first three forms (Hani and Gupta, 1984). Extraction using mild acids or chelates has traditionally been used to measure available metal for crop uptake (Hani and Gupta, 1984). For example, acetic acid has been successfully used to estimate the exchangeable fraction of heavy metals such as Co, Zn, Ni, and Pb (Berrow and Mitchell, 1980). Ellis and Alloway (1983) found acetic acid-extractable Cd, Pb, and Ni to be useful in determining plant-available concentrations in sludgeamendedsoils. Chelates such as ethylenediaminetetraacetic acid (EDTA)and diethylenetriaminepentaacetic acid (DPTA) have been used to extract metals from soils (Erickson et al., 1991; Hughes and Noble, 1991; Misra et al., 1990). Others have also established the relationships between metal plant uptake and the fraction of total soil metal extracted by particular reagents such as EDTA,DTPA, acetic acid, ammoniumacetate, and various inorganic salts (Forstner, 1985; Adamsand Sanders, 1985; Hani and Gupta, 1985; Sauerbeck and Styparek, 1985). Metal concentrations in water or neutral salt extracts were used for simulating the availability of heavy metals to plants (Sauerbeck and Rietz, 1982; Herms and Brummer,1980; Mitchell et al., 1978). Another commonlyused fractionation procedure is separation based on particle size (Queralt and Plana, 1992; Harrison and Wilson, 1982). Finer soil particles have larger relative surface areas available for metal adsorption than the coarser particles; thus, metals usually tend to accumulatein the smaller grain size fractions in uncontaminated soils (Berrow and Mitchell, 1991; Kabata-Pendias, 1993). However, metal enrichment in finer particles maynot occur in all contaminated soils, partially because metals that originated from natural and anthropogenic processes mayhave different proper-
ties. In addition, pedogenic processes may have less impact on metals from anthropogenic than natural sources (Mogolionet al., 1995; Queralt and Plana, 1992). In the current study, water, Ca(NO3)2(neutral salt), HOAc (weak acid), and EDTA-NH4OAc(chelate) were selected as single extractants to evaluate Pb extractability and availability based on thei:v wide uses in the literature (Berrow and Mitchell, 1980, 1991; Lakanen and Ervio, 1971; Jenne and Luoma, 1977). Wehave determined the total Pb in five size frac.tions in each of the soils. This information in conjunction with the extractable Pb using single chemical extraction should be useful for determining Pb availability in soils (Petruzzelli et al., 1989; Clevenger, 1990). The objectives of this study were to (i) determine Pb distribution as a function of size fraction in 11 contaminated soils, (ii) evaluate Pb availability in Pb-contaminated soils using various extracting reagents, and (iii) examinethe effects of incubation and phosphate rock on Pb availability. MATERIALS
AND METHODS
Materials Lead-contaminatedsoils were collected from various locations and their properties and total concentrationsof Pb, Fe, Mn,Zn, Cu, Ni, and Cd are listed in Table 1. These soils reflect various sources of Pb contamination, which can be classified into twobroadcategories:agriculturalactivities, such as application of pesticide PbHAsO4; and industrial activities, such as smelting, battery breaking, and burningof wastematerials. Phosphaterock from Occidental ChemicalCorp. (OC), shownpreviouslyto be effective in immobilizingPb in contaminated soils (Maet al., 1995),wasused in this study. Experimental Procedures Particle-SizeFractionation Soil sampleswere fractionated into five size fractions via physical sieving: lmmto 500 p,m (coarse sand), 500 to 250 i~m (mediumsand), 250 to 106 ~m(fine sand), 106 to 53 (very find sand), and 1 mgL-1 and graphite furnace was used to measurePb concentrations 500 Ixm) in three of six soils contaminated via smelter activities (PT, CW,and EF2), whereas Pb concentrations increased with de-
30000
Soil size fractions i mm.5OOi.t m
~ 10000-
~1 500-250gm [~ 250-106gm ~] 106-53gm N
