No abstract

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.

The transformation of lead (Pb) in contaminated soils to pyromorphite, by the addition of phosphate minerals, may be an economic in-situ immobilization strategy that results in a reduction of bioavailable Pb. To test this hypothesis, we conducted two sets of soil-solution experiments under constant (i.e., fixed) or dynamic (i.e., variable) pH conditions, as a function of time. In both sets of experiments, Pb-contaminated soil was reacted with synthetic hydroxyapatite in order to determine the transformation rate of soil Pb to pyromorphite and the soluble Pb level during the reaction period. In the constant pH system, the soluble Pb concentration decreased with the addition of apatite at pH 4 and above. However, the transformation was pH-dependent and incomplete at relatively high pH (g6). The solubility of cerrusite (PbCO 3 ), the major Pb mineral in this soil, still exhibited a strong influence on the solubility of soil Pb. In the dynamic pH experiments, which simulated gastric pH conditions (i.e., pH variation from 2 to 7 within 25 or 45 min), both cerrusite and added apatite were dissolved at low pH values (pH 2 and pH 3), and chloropyromorphite was rapidly precipitated from dissolved Pb and PO 4 when the suspension pH was increased. Complete transformation of soil Pb to chloropyromorphite occurred in the pH dynamic experiments within 25 min, indicating rapid reaction kinetics of the formation of chloropyromorphite. Chloropyromorphite solubility controls the soluble Pb concentration during the entire duration of the pH dynamic experiments. This study demonstrates the importance of considering specific site conditions, such as pH, when considering evaluation of soil Pb bioavailability and in-situ immobilization of Pb in Pbcontaminated soils using phosphate amendment. Furthermore, this study demonstrates that the kinetics of conversion of soil Pb to chloropyromorphite in the presence of apatite is fast enough to occur during ingestion and that gastric pH conditions would favor the formation of chloropyromorphite, thus rendering ingested soil Pb nonbioavailable.

Effects of biomass types (bark mulch versus sugar beet pulp) and carbonization processing conditions (temperature, residence time, and phase of reaction medium) on the chemical characteristics of hydrochars were examined by elemental analysis, solid-state ¹³C NMR, and chemical and biochemical oxygen demand measurements. Bark hydrochars were more aromatic than sugar beet hydrochars produced under the same processing conditions. The presence of lignin in bark led to a much lower biochemical oxygen demand (BOD) of bark than sugar beet and increasing trends of BOD after carbonization. Compared with those prepared at 200 °C, 250 °C hydrochars were more aromatic and depleted of carbohydrates. Longer residence time (20 versus 3 h) at 250 °C resulted in the enrichment of nonprotonated aromatic carbons. Both bark and sugar beet pulp underwent deeper carbonization during water hydrothermal carbonization than during steam hydrothermal carbonization (200 °C, 3 h) in terms of more abundant aromatic C but less carbohydrate C in water hydrochars.