Abstract
Chemical assessment of soil pollution with heavy metals was made by analyzing the changes in pH, ionic strength, cationic concentration and chemical species in the soil solution. Saturated pastes of the unpolluted soils were made by adding solutions containing Cu or Cd and the final Cu or Cd concentrations were in the range of 0 to 400 mg/kg. After equilibrating for 24 hours at $25^{\circ}C$, the soil solution was extracted from the saturated pastes by the vacuum extraction method and analyzed for pH, electrical conductivity, Cu, Cd, cations and inorganic ligands. Chemical species in soil solution were calculated by the GEOCHEM-PC program employing the input variables of pH, ionic strength(${\mu}$), molar concentrations of cations and ligands. Increasing Cu or Cd additions lowered pH of the soil solution but increased concentrations of Ca, Mg and K resulting in increases of ${\mu}$ of the soil solution. Effects of Cu on lowering pH and increasing ${\mu}$ were greater than those of Cd. Concentrations of Cu or Cd in soil solution were relatively very low as compared to those of additions, but increased linearly with increasing additions representing that concentrations of Cu were higher than those of Cd. At 400 mg/kg additions, concentrations of Cu were in the range of 0.51 to 11.70 mg/L but those of Cd were 34.4 to 88.5 mg/L. Major species of Ca, Mg and K were free ions and these species were equivalent to greater than 95 molar % of the existing respective molar concentrations. These cationic species were not changed by Cu or Cd additions. Major species of Cu in lower pH soils such as SiCL and SL were free $Cu^{2+}$ (>95 molar %), but those in LS having a higher pH were free $Cu^{2-}$ and Cu-hydroxide complex. At 100 mg Cu/kg treatment, $Cu^{2+}$ and Cu-hydroxide complex were equivalent to 73 and 22.4 molar %, respectively. These respective percentages were decreased and increased correspondingly with increasing Cu treatments. Major species of Cd in soil solution were free $Cd^{2+}$ and Cd-chloride complex, representing 79 to 85 molar % for $Cd^{2+}$ and 13 to 20% for Cd-chloride complex at 10 mg Cd/kg treatment. With increasing Cd additions to 400 mg/kg, $Cd^{2+}$ species decreased to $40{\sim}47%$ but Cd-chloride complexes increased to $53{\sim}60$ molar %. These results demonstrated that soil contamination with heavy metals caused an adverse effect on the plant nutritional aspects of soil solution by lowering pH, increasing cations temporarily, and increasing free metal concentrations and species enough to be phytotoxic.
중금속에 의한 토양오염을 화학적으로 평가하기 위한 일환으로 토양을 중금속으로 오염시킨 후 토양용액을 추출하여 pH, 이온강도, 이온농도, 화합물형태 변화를 분석했다. 토양이 Cu와 Cd에 의해 오염될수록 토양용액의 이온강도는 증가했다. 이런 변화는 Cd보다 Cu에 의해 더 컸다. 토양용액 중의 Cu와 Cd의 농도는 가한 양(量)에 비해 소량으로 존재했으나 처리량에 비례하여 금속의 농도는 증가했다. 토양용액 중의 Cd의 농도는 Cu보다 훨씬 높았다. 토양용액 중의 Ca, Mg, K의 농도는 중금속에 의해 증가되었으나 주로 free ion상태로 존재했으며 화합물형태는 Cu나 Cd의 처리에 의해 변화되지 않았다. 토양용액 중 중금속이온의 화합물의 변화 경향은 Cu와 Cd이 달랐다. Cu의 경우 토성과 pH에 의해 영향을 받았는데 pH가 낮은 SiCL 및 SL 토양에서는 free $Cu^{2+}$가 95 molar % 이상으로 주된 화합물형태이었고 pH가 높은 LS토양의 경우 free $Cu^{2+}$과 Cu-hydroxide이 주된 화합물형태로 Cu처리량에 따라 이들 화학종의 molar %는 변화되었다. Cd의 경우 free $Cd^{2+}$과 Cd-chloride 화합물이 주된 화합물형태이었고 Cd처리량이 증가함에 따라 $Cd^{2+}$는 감소한 반면 Cd-chloride 화합물 형성은 증가했다. 중금속에 의한 토양용액의 pH감소, 중금속의 흡착에 따른 양이온의 일시적인 증가, 중금속의 화합물 변화들은 식물영양적 측면에서 볼 때 식물에 불리한 변화로 간주되며, 중금속의 생물유효성과 독성을 평가하는 자료로 활용될 수 있을 것이다.
(Dept. of Agric. Chem., Kangwon National Univ.)
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