Changes in Soil Properties after Soil Washing of Metal-contaminated Soil near the former Janghang Smelter

Abstract

Objectives : Changes in soil properties after washing of metal-contaminated soil near the former Janghang Smelter were investigated in this study. Contaminated input soils and remediated output soils were sampled from three different soil washing plants and analyzed for soil physical and chemical properties. Soil quality was evaluated by the soil fertilization guideline suggested by the Korea Rural Development Administration (KRDA). This study revealed the necessity of soil quality management for the remediated soil as an ecosystem member.Methods : Three soil washing plants (1OU, 2OU, 3OU) were commonly divided into the five steps: 1) the particle separation (crushing and grinding etc.) → 2) soil particle classification (big stone, fine soil, minimal fine soil) → 3) chemical washing (fine soil) → 4) neutralization of washed soil (lime) → 5) return-back to the original position. The separating minimum particle diameters of the 1OU, 2OU, and 3OU washing processes were 5 µm, 20 µm, and 10 µm, respectively, and the chemical washing solutions used were respectively 0.1 M H2SO4, 0.5 M H2SO4/0.5 M H3PO4, and 0.1 N NaOH-Na2CO3 (alkali reduction). Soils were collected before and after washing, air-dried, sieved with < 2 mm and analyzed for soil texture, bulk density, aggregate stability (AS), water holding capacity (WHC), pH, electrical conductivity (EC), organic matter content (OM), total nitrogen (TN), available phosphate (AvP), cation exchange capacity (CEC), exchangeable cations (potassium, calcium, magnesium, sodium).Results and Discussion : Sandy soil showed a big change in soil texture before and after soil washing, while there was no change in soil texture for fine soil. Sandy soil showed an increase in bulk density, a decrease in WHC, and a decrease in AS. The pH of remediated soil was affected by the type of washing chemical. The acidic washing processes (1OU, 2OU) resulted in low pH soils, while an alkali reduction process (3OU) showed high pH soil. The soil OM, TN, AvP and CEC decreased after soil washing. In the case of silty paddy soil, OM and TN were significantly reduced by washing. The most important change in soil property after washing was EC. After soil washing, the soil electrical conductivity increased sharply in all OUs : 1OU 0.51 → 6.21 ds/m, 2OU 1.09 → 3.73 ds/m, 3OU 0.99 → 9.30 ds/m. The EC values of the contaminated soil before washing were all less than 2 ds/m, which is an appropriate agricultural level. However, EC was significantly increased after washing, implying a strong salty soil level. The soil quality evaluation results before and after washing showed that the soil quality of heavy-metal contaminated soil was apparently degraded by washing. The number of soil property in the optimal range before washing (contaminated soil) was 10, but the number decreased to 5 after washing (remediated soil).Conclusions : Soil quality may be significantly changed after soil washing. The most noticeable change was the significant increase in the EC of soil and the soil health should be restored first to recycle the remediated soil. The important causes of changes in the soil quality were the separation of fine soil particles containing relatively high heavy metals from the bulk soil, soil disturbance by chemical washing solution and addition of high salts such as coagulants and pH adjust. Soil management schemes considering soil health should be soon prepared to restore the remediated soil back as an ecosystem member.