Variation in the Stability of Immobilized Metals Compounds Following the In Situ Remediation of Contaminated Soils. L. J. Cajuste 1, L. Cajuste Jr. 2, P. Hernandez-R. 1, J. Cruz-D. 1, and A. Ruiz-B. 1. (1) Edafologia, Colegio de Postgraduados,, Montecillo, Chapingo, 56230, Mexico, (2) Geography, University of Arizona, Harvill Bldg Box 2, Tucson, AZ 85721

ABSTRACT In situ remediation of metals-contaminated soils by using chemical amendments to these sites has been given recently increasing attention. Although the resulting metal compounds are stable under favorable environmental conditions, there is some concern about the dissolution of these compounds in the soil due to variation in some soil properties. A batch and pot experiment was conducted to investigate the behavior of the immobilized metals compounds Cd, Zn, and Pb, due to some changes brought about by variation in soil pH, redox conditions of the soil environment, following the cleaning up of the soil by the in situ remediation technique. Surface soil samples (calciorthids) from three metals-contaminated sites (Atitalaquia I, Atitalaquia II and Tezantepec) previously treated with raw (PR-1), acidulated phosphate rock at 25% (PR-25) and 50% (PR-50) solely or combined with composted biosolids) were equilibrated with 0.5 M MgCl2 or 0.5 M NH2 OH – HCl solution at four pH levels. Soil extracts following centrifugation were saved for final pH determination and metals analysis. Results show that in most soil treatments, amount of metals extracted by both conventional and reducing solutions were significantly influenced by solution pH and metal species. In most PR treatments of SI soil, release of metals from immobilized compounds was less effective at solution pH 6.0, however with the inclusion of biosolids in the treatment effect of solution pH 5 was more apparent in the immobilization of metal compounds. Variations in the effect of soil treatment on metal immobilization as reflected through the calculated E values of plant metal uptake, the percentage of total amount of immobilized metal, are also discussed.

INTRODUCTION Farmers from the Mezquital Valley in Mexico (Figure 1) have used for a long period of time, heavy metals-contaminated sewage water as an agricultural practice in croplands with rainfall deficit, which despite their high content in plant essential nutrients can cause toxic elements to accumulate in soils (Cajuste et al, 1991; Juste and Mench 1992; Gaskin et al, 2003; Brown et al, 2004). As a result of many chemical processes, these elements may become bio-available to the crops or can reach the groundwater causing the contamination of the food chain.

Figure 1. Location of the Study Area.

Chemical immobilization of heavy metal pollutants in agricultural soils is becoming a well known option to researchers and farmers in the region. However, the long term efficiency of such an alternative to reclaim contaminated soils depends on the stability of the process. Farmers in the area in addition, practice flood-irrigation in their parcels, which in turn may alter the pH and redox conditions of soils and thus, cause possible dissolution of heavy metal precipitates.

OBJECTIVES The objective of this investigation was to assess the influence of varying acidic and redox conditions on the stability of immobilized compounds and the effectiveness of the overall immobilization process in three heavy metals- contaminated soils from the Mexican Valley of Mezquital.

MATERIALS and METHODS Three surface soil samples Atitalaquia-1 (S1), Atitalaquia-2 (S2) and Tezantepec (S3) (0- 10 cm depth) collected from the Mexican Valley of Mezquital were used in this study; the land was irrigated with wastewater containing a relatively high content of industrial by- products as contaminants, such as heavy metals collected from the metropolitan area of Mexico City. Data on metal content of the wastewater have been reported elsewhere (Cajuste et al, 1991).The soils (typic calciorthids) were air dried, ground and sieved (<2mm), and their physical and chemical characterization was carried out (Table 1). Soil samples were incubated at the laboratory, at field capacity after being treated with phosphate rock (PR) at 4 levels as mentioned in a previous paper (Cajuste et al, 2005) and also with two levels of composted biosolids (C: 0.0 and 5 Mg/2 x 10 6 kg soil), originating an unbalanced experimental design consisted of a 4 x 2 factorial with two missing treatments. Briefly, the resulting six treatments were as follows:Table 1 1)a control, unamended soil (PR-0) 2)1% total P (Phosphate rock without acidulation (PR-1) 3)1% total P acidulated at 50% (PR-50) 4)Composted biosolids (C) 5 Mg/2 x 106 kg soil 5)Phosphate rock acidulated at 50% + Comp. Biosolids (PR-50 + C) 6)Phosphate rock acidulated at 25% + Comp. Biosolids (PR-25 + C)

S O I L S Parameters S1S2S3Pr-0 Clay (g kg -1 ) Silt (g kg -1 ) pH O.M (g kg -1 ) Metal concentrations Total Pb(mg kg -1 ) Total Cd(mg kg -1 ) Total Cu(mg kg -1 ) Total Ni(mg kg -1 ) Total Zn(mg kg -1 ) CaCO 3 equiv.(g kg -1 ) Total P(P )(g kg -1 ) Table 1. Characteristics of the soil and phosphate rock samples used in laboratory and greenhouse studies.

Redox and pH Variation In each of the six treatments, two levels of redox (oxidized and reduced) conditions and four levels of acidity (pH 4, 5, 6, and 7) were combined to produce 8 experimental units per amended treatment. The units with oxidized condition were incubated with a 0.5 M MgCl 2 solution, whereas the units with reduced condition were incubated with a 0.5 M NH 2 OH – HCl solution. To obtained the four acidic levels for each redox condition group, the units were treated with a Mg(OH) 2 buffering solution to adjust the pH of the soil solution. Metal fractionation. To assess the effect of soil treatment on contaminant extractability, a sequential extraction of metal was made following the procedure of Basta and Gradwohl (2000), slightly modified to obtain the water soluble metal fraction. In this method, triplicate (1g) soil were extracted sequentially with 20 ml of deionized water (E1), 0.5M Ca(NO 3 ) 2 (E2), 1.0M NaOAc (E3), 0.1M NaEDTA (E4), and 4M HNO 3 (E5). These extractions could be associated with the water soluble, exchangeable, carbonate, and oxide-bound, organic-bound, and residual fractions respectively. The solution from each extraction was centrifuged and analyzed for Pb, Cd, Cu, Ni and Zn, using an atomic absorption spectrophotometer Perkin-Elmer In a greenhouse experiment, maize (Zea mays L.) seeds were sown in pots of 500 g of soil. Treatments of phosphate rock alone or in combination with composted biosolids were arranged in an unbalanced 4 x 2 factorial design (Figure 2), as already mentioned, and with four replicates. Plant and soil samples are still being analyzed for results.Figure 2

Figure 2. Greenhouse Experiment after a 45-day growing period.

RESULTS and DISCUSSION (PRELIMINARY) Results are preliminary and do not include data analysis from the greenhouse experiment. In this paper we only show our first laboratory results. In particular, the concentrations of metals correspond to the soluble fraction of these. Our discussion therefore is focused particularly on the soluble fraction of all metals.

Metal concentrations The effects of amendment treatments produced considerable variation in metal concentrations for the three soils, and were influenced by pH and redox conditions (Figures 2 and 3). Overall, there was a decrease of metal concentration when applying any of the five amendment treatments. More consistent results were obtained in oxidized environments, for Cu and Cd in all soils, and for Pb only in Tezontepec soil. This was not the case in contrast, with reduced environments; only Zn in Atitalaquia soil presented a consistent result of metal immobilized by amendment treatments (data not shown).

Figure 3. Cu and Pb concentrations under oxidized environment and varying pH, in Tezontepec soil.

Effect of overall treatments Treatments with phosphate rock only as amendment (1 and 5) produced distinctive results compared with treatments which included compost (2, 3, and 4) as an amendment; inasmuch the effects of the earlier treatments showed a lower variability in response (Figures 3 and 4), compared to the latter treatments, whether it was only compost, or compost combined with phosphate rock.

Effect of varying acidity Most metal concentrations were at their lowest magnitude in experimental units within pH 5 (Figures 3 and 4). In particular, release of soluble Cd was less effective in all three soils and especially under reducing environments. This fact is certainly related to the solubility product of the precipitated metal phosphate-salts. As the solubility product of the salt is closer to neutral pH, less metal is immobilized, and when the environment is more acidic, more metal is immobilized.

Figure 4. Cu and Cd concentrations under reduced environment and varying pH, in Atitalaquia-1 soil.

Effect of REDOX environments Oxidized environments showed a higher variability of response (metal concentration), either to pH conditions or to amendment treatment, compared to experimental units with a reducing environment. Lower concentrations of Cu, Zn and Ni where found at pH 5; whereas at pH 7, Pb was best immobilized, under oxidized conditions. Cd presented an erratic behavior under same conditions. In reducing environments, Pb and Cd had their lowest concentrations in all soils at pH 5; a similar behavior was found for Cu and Zn in most soil treatments.

Treatments with compost in oxidizing conditions tend to buffer the variations of metal concentrations in all soils, caused by variations in soil acidity. Soil Cu and Cd are cases of such a behavior. Nevertheless, this buffering capacity remained secluded in reducing conditions where redox potential had a tampering effect on most soil treatments. This observation was more evident with Cu, where there was no significant mobilization of this metal in all soils under such a condition.

In addition, soluble soil Cu concentrations were lower in reduced environments than in oxidized environments; a pattern that was also similar to soluble soil Ni. The rest of the metals (Pb, Zn, and Cd) presented an inverse pattern, where higher concentrations were found in reduced environments, compared to those under oxidizing conditions. The End.