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Application of P amendments impacted the pH of this sandy soil with relatively low buffer capacity

Impacts of phosphate amendments on lead biogeochemistry at a contaminated site.

ENVIRONMENTAL SCIENCE & TECHNOLOGY, no. 24 (2002): 5296-5304

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摘要

Soil amendments can be used to cost-effectively reduce the bioavailability and mobility of toxic metals in contaminated soils. In this study a field demonstration was conducted at a Pb-contaminated site to evaluate the effectiveness of P-induced Pb immobilization. Phosphate was applied at a 4.0 molar ratio of P to Pb with three treatments...更多

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简介
  • Biogeochemical cycling of metals in the environment depends on their chemical forms. Significant fractions of metals in contaminated soils are often present in the potentially bioavailable fractions (1).
  • Natural lead-phosphate minerals have been identified in contaminated soils (10-13).
  • In light of their intrinsically low solubilities, efforts have been made to form leadphosphates in lead-contaminated soils through P addition.
  • Soluble forms of P (e.g. Na2HPO4 or KH2PO4) can significantly reduce Pb bioavailability (14, 15) via formation of lead-phosphate (4).
  • Phosphate rock [PR, primarily Ca10(PO4)6F2] is shown to effectively immobilize Pb from aqueous solution and leadcontaminated soils (6).
  • Direct evidence for the formation of pyromorphite in Pb contaminated soils upon P addition has been reported using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and extended X-ray absorption fine structure spectroscopy (EXAFS) techniques (7, 9, 17, 18)
重点内容
  • Biogeochemical cycling of metals in the environment depends on their chemical forms
  • In this study a field demonstration was conducted at a Pb-contaminated site to evaluate the effectiveness of P-induced Pb immobilization
  • Phosphate was applied at a 4.0 molar ratio of P to Pb with three treatments: T1, 100% of P from H3PO4; T2, 50% P from H3PO4 + 50% P from Ca(H2PO4)2; and T3, 50% P from H3PO4 + 5% phosphate rock
  • Lead immobilization was attributed to the P-induced formation of chloropyromorphite [Pb10(PO4)6Cl2], which was identified in the surface soil, subsurface soil, and plant rhizosphere soil
  • Application of P amendments impacted the pH of this sandy soil with relatively low buffer capacity (>89% sand, Table 1)
  • On day 50, residual Pb increased by 20%, 22% and 11% for T1, T2, and T3, respectively, not enough for lead phosphate to be detected by XRD (Figure 2)
  • Application of Ca(H2PO4)2 or phosphate rock combined with H3PO4 maintained a soil pH slightly higher than H3PO4 alone
方法
  • The demonstration site is located in northwest of Jacksonville, Florida.
  • It is a nearly-level area of ∼4100 m2 (71.4 m × 57.3 m) with surface runoff in a west to southwesterly direction.
  • The site map, soil characteristics, pollutants of concern, and contamination distribution were reported in Cao et al (21).
  • Cerussite was the predominant form of Pb at the site.
  • The hotspot is centered in the north-central part of the site.
  • For other metals (Cu and Zn), their concentrations remained 10-100 times higher compared to their normal concentrations found in Florida soils (21)
结果
  • Satisfactory precision and accuracy were required to be within (20% and between 85 and 120%, respectively.
  • Phosphate treatments increased residual Pb by 19-48%, 22-50%, and 11-55% for T1, T2, and T3, respectively, implying that all P treatments were able to modify the partitioning of Pb from the nonresidual/potentially available phase to the residual/ unavailable phase.
  • Shoot tissue Pb contents decreased significantly (p < 0.05) after P amendment with reduction of 3164%, 20-29%, and 44-71% for T1, T2, and T3, respectively, relative to the control T0
结论
  • Soil pH was reduced in all P-treated plots due to addition of H3PO4 (Figure 1).
  • Soil pH in the T3 plot decreased to its lowest point on day 7 after the treatment, gradually recovering to stabilize on day 130.
  • For the T1 and T2 treatments, soil pH reached its lowest point on day 50, and stabilized on day 220 and 330, respectively.
  • Soil pHs in all P-treated plots were close to those in the control soil one year after P application, though still slightly lower.
表格
  • Table1: Selected Physical and Chemical Properties of the Soil and Phosphate Rocka
  • Table2: Equilibrium Reactions for Lead Minerals and Complexes at 25 °C
  • Table3: Pb Concentration in the St. Augustine Grass (Stenotaphrum secundatum) Grown in the Control and P-Treated Plots
Download tables as Excel
基金
  • This research was supported in part by the Florida Institute of Phosphate Research (Contract No 97-01-148R)
  • We also thank Wayne Acree, Karen Kelley, and Eric Fodran for their help in electron microscopy analysis. hypothesis was supported by TEM evidence of roots grown in T2 treated plot that showed Pb deposited on root cell walls of roots (Figure 8)
研究对象与分析
samples: 20
Metals (Pb, Cu, Zn, etc.) were determined by using inductively coupled plasma spectrometry (Thermo Jarrell Ash ICAP 61-E, Franklin, MA) and by using graphite furnace atomic absorption spectrometry (Perkin-Elmer SIMMA 6000, Perkin-Elmer Corp., Norwalk, CT). Elemental analysis followed EPA approved QA/QC plan with a blank, a duplicate, a spike and a SRM analysis every 20 samples. Quality control samples included Standard Reference Materials of soil (SRM 2709 San Joaquin Soil, 2710 Montana Soil) and plant (1547 Peach Leaves) (U.S Department of Commerce National Institute of Standards and Technology, Gaithersburg, MD 20899)

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