Lead immobilization using phosphoric acid in a smelter-contaminated urban soil

Transformation of soil lead (Pb) to pyromorphite, a lead phosphate, may be a cost-effective remedial strategy for immobilizing soil Pb and reducing Pb bioavailability. Soil treatment using phosphoric acid (H3PO4) was assessed for its efficacy to reduce Pb solubility and bioaccessibility. Soil containing 4,360 mg of Pb kg(-1), collected from a smelter-contaminated site in Joplin, MO, was reacted with 1,250, 2,500, 5,000, and 10,000 mg of P kg(-1) as H3PO4. The reaction was followed by measurements of Pb bioaccessibility, solubility products, and microprobe analyses. Soluble Pb concentration in the soil decreased with increasing H3PO4 addition. Adding 10,000 mg of P kg(-1) reduced bioaccessible Pb by 60%. The logarithm of bioaccessible Pb decreased as a linear function of increasing H3PO4 addition with an R2 of 0.989. A higher soil/solution ratio was required to extract bioaccessible Pb after the treatment. Microprobe analyses showed that the Pb particles contained P and Cl after the reaction, and the spectra generated by the wavelength-dispersive spectrometer were similar to those of synthetic chloropyromorphite. Lead solubility in the P-treated soil was less than predicted for hydroxypyromorphite [Pbs(PO4)3-OH] and greater than predicted for chloropyromorphite [Pbs(PO4)3Cl]. The P treatment caused approximately 23% redistribution of soil Pb from the clay and silt size fractions to the sand fraction. Soil treatment with H3PO4 resulted in the formation of a compound similar to chloropyromorphite and reduced bioaccessibility of soil Pb, which may have a potential as an in situ technique for Pb-contaminated soil remediation.     

Equilibrium solubility and dissolution rate of the lead phosphate chloropyromorphite

Phosphate addition to lead-contaminated soils can immobilize lead in situ through the formation of lead phosphate minerals such as chloropyromorphite (Pb5(PO4)3Cl). The long-term stability of lead immobilized in lead phosphate precipitates depends on the equilibrium solubility and dissolution rates of the lead phosphate solids. The equilibrium solubility and dissolution kinetics of chloropyromorphite were quantified in a series of batch and flow-through reactors. Both equilibrium solubility and dissolution rates were strongly affected by pH. Synthetic chloropyromorphite was more soluble than was predicted using a widely cited solubility product for pure chloropyromorphite, an observation that is consistent with several other recent studies. A trace amount of a more soluble lead solid, such as lead hydroxide, in chloropyromorphite could significantly increase dissolved Pb at neutral pH. The pH-dependence of the dissolution rate was examined in flow-through experiments. A dissolution rate law with a single rate constant for synthetic chloropyromorphite and a constant reaction order of 0.65 with respect to [H+] has been determined. The slow dissolution rate of chloropyromorphite relative to other lead minerals and its low solubility can result in its long-term stability in soils.     

Localization and chemical speciation of Pb in roots of signal grass (Brachiaria decumbens) and Rhodes grass (Chloris gayana)

Lead (Pb) contamination of soils is of global importance but little is known regarding Pb uptake, localization, or the chemical forms in which Pb is found within plants, or indeed how some plants tolerate elevated Pb in the environment. Two grasses, signal grass (Brachiaria decumbens Stapf) (Pb-resistant) and Rhodes grass (Chloris gayana Kunth)(Pb-sensitive), were grown for 14 d in dilute nutrient solutions before examination of roots using transmission electron microscopy (TEM) to determine the distribution and speciation of Pb in situ. In both grasses, Pb was initially present primarily in the cytoplasm of rhizodermal and cortical cells before being sequestered within vacuoles as the highly insoluble (and presumably nontoxic) chloropyromorphite (Pb5(PO4)3Cl). In signal grass, Pb also accumulated within membranous structures (perhaps the Golgi apparatus), prior to apoplastic sequestration as chloropyromorphite. These findings suggest that the ability of signal grass to sequester insoluble Pb in the cell wall represents an additional and potentially important mechanism of Pb tolerance not possessed by the Pb-sensitive Rhodes grass.     

Simultaneous immobilization of lead and atrazine in contaminated soils using dairy-manure biochar

Biochar produced from waste biomass is increasingly being recognized as a green, cost-effective amendment for environmental remediation. This work was to determine the ability of biochar to immobilize heavy metal Pb and organic pesticide atrazine in contaminated soils. Biochar prepared from dairy manure was incubated with contaminated soils at rates of 0, 2.5, and 5.0% by weight for 210 d. A commercial activated carbon (AC) was included as a comparison. The AC was effective in immobilizing atrazine, but was ineffective for Pb. However, biochar was effective in immobilizing both atrazine and Pb and the effectiveness was enhanced with increasing incubation time and biochar rates. After 210 d, soils treated with the highest rate of 5.0% biochar showed more than 57% and 66% reduction in Pb and atrazine concentrations in 0.01 M CaCl(2) extraction, respectively. Lead and atrazine concentrations in the toxicity characteristic leaching procedure solutions were reduced by 70-89% and 53-77%, respectively. Uptake of Pb and atrazine by earthworms (Eisenia fetida) was reduced by up to 79% and 73%. Phosphorus originally contained in biochar reacted with soil Pb to form insoluble hydroxypyromorphite Pb(5)(PO(4))(3)(OH), as determined by X-ray diffraction, which was presumably responsible for soil Pb immobilization, whereas atrazine stabilization may result from its adsorption by biochar demonstrated by the significant exponential decrease of extractable atrazine with increasing organic C in biochar (r(2) > 0.97, p < 0.05). The results highlighted the potential of dairy-manure biochar as a unique amendment for immobilization of both heavy metal and organic contaminants in cocontaminated soils.     

Enhanced accumulation of phosphate by Lolium multiflorum cultivars grown in phosphate-enriched medium

Agricultural and animal husbandry practices combined with soil composition have caused phosphate overloading of farmlands in different parts of the U.S. and Europe. Movement of soluble phosphates (Pi) from phosphorus enriched soils results in degradation of natural aquatic systems, triggering serious environmental problems. Remediation of such sites using plants that tolerate and accumulate high concentrations of Pi in their aerial parts may be an attractive remediation technology. In the present study, Pi transport and accumulation potential of Marshall and Gulf ryegrass (Lolium multiflorum cultivars) were determined using a solution culture of seedlings. Ryegrass seedlings accumulated phosphorus (P) in excess of 2% of dry weight in their aerial parts when supplied with 5 g/L KH2PO4 in medium. Phosphorus accumulation was positively correlated with the concentration of phosphate (0-5 g/L KH2PO4) in medium. Plants grew well on medium containing 5 g/L KH2PO4, but concentrations above 5 g/L caused symptoms of toxicity. Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed high P accumulation in different cell types of grass roots and shoots. Phosphate starvation and replenishment experiments point to the unique ability of these grasses to concentrate phosphate in the above-ground parts. It is hypothesized that the unique ability of these ryegrass cultivars may be due to the presence of efficient phosphate transport and sequestration mechanisms.     

Assessment of a sequential extraction procedure for perturbed lead-contaminated samples with and without phosphorus amendments

Sequential extraction procedures are used to determine the solid-phase association in which elements of interest exist in soil and sediment matrixes. Foundational work by Tessier et al. (Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51, 844-851) has found widespread acceptance and has been employed as an operational definition for metal speciation in solid matrixes. However, a major obstacle confronting sequential extraction procedures is species alteration of extracted metals before, during, and after separation of solids from solution. If this occurs, the results obtained from sequential extraction do not provide an accurate account of metal speciation within the matrix because the metal forms are altered from their field state. Many researchers dismiss this drawback since several sorption and precipitation processes are believed to occur at time scales much longer than any particular extraction step. This assumption may not be valid. The objectives of this study were to investigate the potential formation of pyromorphite (Pb5(PO4)3Cl) during the sequential extraction steps of Pb-spiked samples with and without calcium phosphate amendments and to examine the differences in the operationally defined distribution of Pb in samples with and without the presence of P. The systems that were examined in the absence of phosphate behaved, for the most part, adequately according to the operational definitions of the extraction procedure. However, when the samples were amended with phosphate, results were drastically changed with a significant shift of extractable Pb to the residual phase. This redistribution was due to pyromorphite formation during the extraction procedure as confirmed by X-ray diffraction and X-ray absorption (XAS) spectroscopies. These results indicate that sequential extraction methods may not be suitable for Pb speciation in perturbed environmental systems (i.e., fertilized agricultural soils or amended contaminated soils) and that rigorous interpretation should be avoided, if not supported by methods to definitively prove metal speciation (e.g., XAS).     

AM真菌和牛粪对铅污染土壤的修复效应

铅污染会对环境和人体健康产生较大风险,如何修复污染土壤、降低农产品中的铅残留值得深入研究。在温室盆栽条件下研究了接种丛枝菌根(AM)真菌、施加牛粪及复合处理对铅污染土壤的修复效应和机理。结果表明,在所有铅污染水平下,接种AM 真菌、施加牛粪和复合处理均显著促进烟草生长,改善烟草磷营养。复合处理在所有铅污染水平下均降低烟草铅残留量,而施加牛粪仅在轻度和中度铅污染水平下、接种AM 真菌仅在重度污染水平下作用显著。土壤pH 升高,铅有效性降低。可见,AM 真菌和有机肥对铅污染地区优质烟叶生产及铅污染土壤的植物修复具有一定的应用潜力。     

Lead phosphate minerals: solubility and dissolution by model and natural ligands

Due to their relatively low solubility, lead-phosphate minerals may control Pb solution levels at a low value in natural environments. We reportthe solubility of Pb from two lead-orthophosphate mineral suspensions (beta-Pb9(PO4)6 and PbHPO4) after aging for 3 years. Lead (Pb2+) activity in the aged suspensions was compared to the activity calculated using the Ksp values of various Pb-PO4 minerals reported in the literature. We also determine the time-dependent dissolution of the aged lead-phosphate minerals by organic and inorganic ligands containing S-functional groups (cysteine, methionine, and thiosulfate) and by a soil extracted humic acid. We find the activity of Pb2+ in the aged lead-phosphate suspensions to be 1-2 orders of magnitude higher than predicted by the Ksp values reported in the literature. Disagreement between measured and Ksp-calculated activities has been reported in other investigations of Pb-PO4 minerals; we compiled some of the data and present them together with our results. Furthermore, the time-dependent dissolution experiments indicate that, in most cases, lead phosphates are partly dissolved in the presence of soluble ligands, i.e., model sulfides and humic acid. The soil-extracted humic acid enhanced the dissolution of Pb from the high pH (7.2) lead-phosphate (beta-Pb9(PO4)6) mineral while suppressing Pb dissolution from the low pH (3.8) lead-phosphate (PbHPO4) mineral. While the low molecularweight sulfur-containing ligands enhanced Pb dissolution, their effect was less pronounced. We conclude that (i) nonequilibrium conditions prevail in the mineral suspensions even after 3 years of aging; and (ii) soluble ligands present in soils, sediments, and natural waters can potentially dissolve Pb from lead-phosphate minerals; such ligands, then, may enhance the biological availability and mobility of Pb in the environment.     

Effects of aging and pH on dissolution kinetics and stability of chloropyromorphite

The objectives of this research were to understand the effect of aging time on chloropyromorphite stability by dissolution, to examine physical and chemical alterations of the pyromorphite samples, and to model the kinetic data collected from the dissolution experiments. The results of this investigation indicate that chloropyromorphite formation is kinetically rapid and that its dissolution in acid is thermodynamically stable, ideal conditions for Pb immobilization that has emerged as a potential remediation strategy. In terms of aging prior to dissolution, X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) spectroscopies were unable to distinguish fundamental differences in progressively aged samples; however, high-resolution thermogravimetric analysis (HRTGA) did demonstrate thatthe thermostability of the chloropyromorphite material increased with increasing residence time. The stirred-flow and batch dissolution studies suggest that the aging process ceased within 24 h and that the dissolution rate of the 1-day aged sample was not significantly different than the 1-year aged specimen. The amount of Pb released peaked at 21% (1-h sample, stirred-flow, pH 2.0) and was as low as 0.17% (1-year sample, batch method, pH 6.0). Postdissolution analyses of chloropyromorphite with XAFS, XRD, and HRTGA revealed no detectable chemical alterations of the pyromorphite samples signifying only release of dissolved Pb to solution and no formation of secondary products during dissolution.     

Quantifying the contribution of different sorption mechanisms for 2,4-dichlorophenoxyacetic acid sorption by several variable-charge soils

Previous research with phenolic, carboxylic, and urea type organic acids demonstrated that hydrophilic sorption was primarily due to anion exchange, which was linearly correlated to chemical acidity (pKa) and the soil anion exchange capacity. However, for dichlorophenoxyacetic acid (2,4-D), sorption by a kaolinitic soil was much higher than expected relative to all other organic acid-soil data. The enhanced sorption was hypothesized to involve calcium bridging of 2,4-D to hydrophilic domains. In this study, the mechanisms contributing to 2,4-D sorption by variable-charged soils were probed and quantified by measuring sorption from CaCl2, KCl, CaSO4, KH2PO4, and Ca(H2PO4)2 solutions. Linear sorption coefficients estimated for 2,4-D sorption from the different electrolytes decreased as follows: CaCl2 > KCl > CaSO4 > Ca(H2PO4)2 approximately equal to KH2PO4. Differences in 2,4-D sorption from CaCl2 and phosphate solutions were attributed to sorption by hydrophilic domains, which ranged between 46 and 94% across soils. Differences in 2,4-D sorption from CaCl2 and KCl were attributed specifically to Ca-bridging between 2,4-D's carboxyl group and the silanol edge on kaolinite and quartz and ranged from negligible to 40% depending on the soil mineral type. Differences in sorption from CaCl2 and CaSO4 was attributed to anion exchange, which ranged from 16 to 91%, followed the trends with pKa developed previously for other organic acids, and correlated well to the soil anion to cation exchange capacity ratio (AEC/CEC). The sum of anion exchange and Ca-bridging contributions agreed well with the sorption estimated to be from hydrophilic domains. All other sorption was attributed to hydrophobic processes, which correlated well to a linear free-energy relationship between pH-dependent organic carbon-normalized sorption coefficients and pH-dependent octanol-water partition coefficients developed for several other organic acids.     

Linking solid phase speciation of Pb sequestered to birnessite to oral Pb bioaccessibility: implications for soil remediation

Lead (Pb) sorption onto oxide surfaces in soils may strongly influence the risk posed from incidental ingestion of lead-contaminated soils. In this study, Pb was sorbed to a model soil mineral, birnessite, and was placed in a simulated gastrointestinal tract (in vitro) to simulate the possible effects of ingestion of a soil contaminated with Pb. The changes in Pb speciation were determined using extended X-ray absorption fine structure and X-ray absorption near edge spectroscopy. Birnessite has a very high affinity for Pb with a sorption maximum of 0.59 mol Pb kg(-1) (approximately 12% Pb sorbed by mass) in which there was no detectable bioaccessible Pb (< 0.002%). Surface speciation of the birnessite Pb was determined to be a triple corner sharing complex in the birnessite interlayer. Lead sorbed to Mn oxide in contaminated media will have a very low (approximately equal to 0) Pb bioaccessibility and present little risk associated with incidental ingestion of soil. These results suggest that birnessite, and other Mn oxides would be powerful remediation tools for Pb-contaminated media because of their high affinity for Pb.     

Phosphate sorption isotherms for some Sierra Leone soils

Phosphate sorption isotherms were obtained for three acid highly weathered soils of Sierra Leone, by equilibrating air dry samples for 24 h with 0.01 _M KH₂PO₄ solution at 25 °C. The Langmuir and Temkin equations were used to evaluate the sorption capacities for phosphate. Phosphate sorption maxima of 2439, 2137 and 1429 mg/kg soil and x/log c of 774, 718 and 610 were calculated; where x=phosphate sorbed (mg/kg soil) and c=equilibrium phosphate concentration (μmol dm^?3). Qualitative relationships between these sorption parameters and other soil variables are shown.     

Implications of the use of As-rich groundwater for agricultural purposes and the effects of soil amendments on as solubility

An agricultural site in Segovia province (Spain) contains high levels of arsenic (As) of geological origin in its groundwater, which is used intensively for irrigation. Crops, irrigation waters, and soils were analyzed to evaluate the occurrence of As in this area and its potential impact on the food chain. High As mobility was found in the agricultural soils, related to the application of As in the irrigation waters (14.8-280 μg As L(-1)) and the general alkaline and sandy character of these soils, which imposes a low capacity for As sorption and therefore enhances plant uptake. The use of amendments can also affect the solubility of As in these soils. Evidence for this was evaluated based on a study of the effect of organic (compost) and inorganic (iron oxides-rich rolling mill scale and phosphate fertilizer) amendments. Arsenic solubility in soil and plant uptake were high, but not significantly affected by organic matter or phosphate addition, while As immobilization was associated with addition of iron oxides with the rolling mill scale, although this did not result in a decrease of As uptake by the tested plants.     

Cadmium and lead ion capture with Three dimensionally ordered macroporous hydroxyapatite

The capability of three dimensionally ordered macroporous (3DOM) hydroxyapatite, Ca10(PO4)6(OH)2 (HAp), to capture cadmium and lead ions from their respective salt solutions was studied as a function of temperature. Synthesis of 3DOM material was achieved by colloidal crystal templating of polystyrene spheres (1 microm diameter) using calcium nitrate (Ca(NO3)2) and orthophosphoric acid (H3PO4) as precursors. The macroporous product consisted primarily of HAp (>80% depending on the sintering temperature) together with amorphous calcium phosphate. The sorption ability of 3DOM material to Cd/Pb ion was benchmarked against HAp powder prepared via the same route without the template. On the basis of quantitative X-ray diffraction (XRD) and analytical transmission electron microscopy(ATEM) 3DOM HAp demonstrated a higher uptake of cadmium, viz. x = 0.71 in Ca10-xCdx(PO4)6(OH)2 than nonporous HAp (x = 0.42). The incorporation of Cd was homogeneous in the 3DOM HAp crystals (as compared to the powder) leading to a decrease in lattice parameters as Cd2+ has a smaller ionic radius compared to Ca2+. A preference for Cd to enter the Ca" tunnel site of HAp was consistent with this being the readily exchangeable site. The lead-bearing solution acted to collapse the macropores through the rapid crystallization of pyromorphite (Pb10(P04)6(OH)2) via a dissolution-precipitation mechanism, possibly promoted by the amorphous component, that overwhelmed HAp ion exchange. The rapid crystallochemical incorporation of Cd and fixation of Pb by 3DOM HAp demonstrates the potential of thin-walled porous structures for the treatment of contaminated waters.     

Phosphorus speciation in manure and manure-amended soils using XANES spectroscopy

Previous studies suggested an increase in the proportion of calcium phosphates (CaP) of the total phosphorus (P) pool in soils with a long-term poultry manure application history versus those with no or limited application histories. To understand and predict long-term P accumulation and release dynamics in these highly amended soils, it is important to understand what specific P species are being formed. We assessed forms of CaP formed in poultry manure and originally acidic soil in response to different lengths of mostly poultry manure applications using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Phosphorus K-edge XANES spectra of poultry manure showed no evidences of crystalline P minerals but dominance of soluble CaP species and free and weakly bound phosphates (aqueous phosphate and phosphate adsorbed on soil minerals). Phosphate in an unamended neighboring forest soil (pH 4.3) was mainly associated with iron (Fe) compounds such as strengite and Fe-oxides. Soils with a short-term manure history contained both Fe-associated phosphates and soluble CaP species such as dibasic calcium phosphate (DCP) and amorphous calcium phosphate (ACP). Long-term manure application resulted in a dominance of CaP forms confirming our earlier results obtained with sequential extractions, and a transformation from soluble to more stable CaP species such as beta-tricalcium calcium phosphate (TCP). Even after long-term manure application (> 25 yr and total P in soil up to 13,307 mg kg(-1)), however, none of the manure-amended soils showed the presence of crystalline CaP. With a reduction or elimination of poultry manure application to naturally acidic soils, the pH of the soil is likely to decrease, thereby increasing the solubility of Ca-bonded inorganic P minerals. Maintaining a high pH is therefore an important strategy to minimize P leaching in these soils.     

Phosphate uptake by lettuces and carrots from different soil depths in the field

In order to investigate the depths in field soils from which plants derive their phosphate nutrition, uptake of ³²P from depths of 6, 12, 24 and 36 inches was studied, using a shallow-rooted crop (lettuce) and a deeper-rooted crop (carrot). a soil at two different fertility levels was used, the higher level of fertility having been attained by heavy applications of farmyard manure over a period of twelve years. The exchangeable pool of phosphate in the high-fertility soil, at the depths investigated, was about twice that in the low-fertility soil. Total phosphate up take by lettuces on the two soils was almost proportional to the size of the exchangeable pool, but the uptake by carrots on the high-fertility soil was much less than twice that on the low-fertility soil. Phosphate uptake, computed as the product of measured radioactive phosphate and the size of the exchangeable phosphate pool at the depths investigated, shows that lettuces derived almost all their requirements of phosphate from the uppermost foot of soil. Carrots obtained most of their phosphate from this layer, but also obtained appreciable amounts from depths of 24 and 36 inches. Some implications of these findings are discussed.     

基于烟叶Cd消减率和修复边际效率评价Cd钝化剂修复效果

我国农田镉污染问题日益严重,低成本、高效、绿色环保的土壤镉钝化技术研发是我国目前农田重金属污染治理需要解决的关键问题。以湖南镉（Cd）重度污染水稻土为对象,以云烟87和K326两个烤烟品种为试材,通过盆栽试验,以土壤Cd有效态含量降低率、烟叶Cd消减率和修复边际效率为指标,对4种不同钝化剂的修复效果进行了评价。结果表明,土壤钝化剂的添加显著降低了烟叶对Cd的吸收（p<0.05）。与对照相比,土壤Cd有效态含量降低率为17.69%~35.65%,烟叶Cd消减率为25.08%~60.75%,4种钝化剂修复边际效率为2.84%~21.98%;随着钝化剂施用浓度的增加,土壤Cd有效态含量降低率和烟叶Cd消减率均显著增加,但烟叶Cd修复边际效率呈明显递减趋势,1%施用浓度比2%和4%施用浓度分别高出69.42%和152.73%;钝化剂对K326烟叶的Cd修复边际效率比云烟87高13.21%。综合土壤Cd有效态含量降低率、烟叶Cd消减率和修复边际效率3个评价指标,不同钝化剂的修复效果顺序为生物炭钝化剂FB > 羟基磷钝化剂FP > 岩基钝化剂FA > 黏土矿物钝化剂FS。     

Thermodynamic stabilization of hydrous ferric oxide by adsorption of phosphate and arsenate

Hydrous ferric oxide (HFO) is an X-ray amorphous compound with a high affinity for anions under strongly or mildly acidic conditions. Because of the usually small particle size of HFO, the adsorption capacity is high and adsorption may significantly impact the thermodynamic properties of such materials. Here we show that adsorption of phosphate and arsenate stabilizes HFO by experimental determination of enthalpies of formation (by acid-solution calorimetry) and estimates of standard entropies for six phosphate- or arsenate-enriched HFO samples. At pH values lower than ～5, the phosphate-doped HFO is not only less soluble than ferrihydrite (anion-free HFO) but also crystalline FeOOH polymorphs feroxyhyte and lepidocrocite. The arsenate-doped HFO is also stabilized with respect to the ferrihydrite. Phosphate availability in soils can be controlled by the phosphate-enriched HFO which is many orders of magnitude less soluble than apatite or crystalline Fe(III) phosphates, for example strengite (FePO(4)·2H(2)O). Thermodynamic dissolution models for scorodite (FeAsO(4)·2H(2)O) and As-enriched HFO show that under mildly acidic or circumneutral conditions, scorodite dissolves, As-HFO precipitates, and a substantial amount of As(V) is released into the aqueous solution (at pH 7, log m(As) ～ -2.5). The data presented in this paper can be used to model the equilibrium concentration of Fe(III), P(V), or As(V) in soil solutions or in natural or anthropogenic sediments polluted by arsenic.     

In vivo-in vitro and XANES spectroscopy assessments of lead bioavailability in contaminated periurban soils

Lead (Pb) bioaccessibility was assessed using 2 in vitro methods in 12 Pb-contaminated soils and compared to relative Pb bioavailability using an in vivo mouse model. In vitro Pb bioaccessibility, determined using the intestinal phase of the Solubility Bioaccessibility Research Consortium (SBRC) assay, strongly correlated with in vivo relative Pb bioavailability (R(2) = 0.88) following adjustment of Pb dissolution in the intestinal phase with the solubility of Pb acetate at pH 6.5 (i.e., relative Pb bioaccessibility). A strong correlation (R(2) = 0.78) was also observed for the relative bioaccessibility leaching procedure (RBALP), although the method overpredicted in vivo relative Pb bioavailability for soils where values were <40%. Statistical analysis of fit results from X-ray absorption near-edge structure (XANES) data for selected soils (n = 3) showed that Pb was strongly associated with Fe oxyhydroxide minerals or the soil organic fraction prior to in vitro analysis. XANES analysis of Pb speciation during the in vitro procedure demonstrated that Pb associated with Fe minerals and the organic fraction was predominantly solubilized in the gastric phase. However, during the intestinal phase of the in vitro procedure, Pb was strongly associated with formation of ferrihydrite which precipitated due to the pH (6.5) of the SBRC intestinal phase. Soils where Fe dissolution was limited had markedly higher concentrations of Pb in solution and hence exhibited greater relative bioavailability in the mouse model. This data suggests that coexistence of Fe in the intestinal phase plays an important role in reducing Pb bioaccessibility and relative bioavailability.     

Interaction between calcium and phosphate adsorption on goethite

Quantitatively, little is known about the ion interaction processes that are responsible for the binding of phosphate in soil, water, and sediment, which determine the bioavailability and mobility of phosphate. Studies have shown that metal hydroxides are often responsible for the binding of PO4 in soils and sediments, but the binding behavior of PO4 in these systems often differs significantly from adsorption studies on metal hydroxides in laboratory. The interaction between PO4 and Ca adsorption was studied on goethite because Ca can influence the PO4 adsorption equilibria. Since adsorption interactions are very difficult to discriminate from precipitation reactions, conditions were chosen to prevent precipitation of Ca-PO4 solids. Adsorption experiments of PO4 and Ca, individually and in combination, show a strong interaction between adsorbed Ca and PO4 on goethite for conditions below the saturation index of apatite. It is shown that it is possible to predict the adsorption and interaction of PO4 and Ca on electrostatic arguments using the model parameter values derived from the single-ion systems and without invoking ternary complex formation or precipitation. The model enables the prediction of the Ca-PO4 interaction for environmentally relevant calcium and phosphate concentrations.