Acid washing is a common method for soil remediation, but is not always efficient for chromium-contaminated soil. Both soil particle size and the forms of chromium existing in the soil affect the efficiency of soil washing. Laboratory batch and column dissolution experiments were conducted to determine the efficiencies of citric and hydrochloric acids as agents to extract chromium from soils contaminated with chromium. The effects of soil particle size and chromium fractionation on Cr leaching were also investigated. About 90% of chromium in the studied soil existed either in residual form or bound to iron and manganese oxides, and Cr fraction distributions were similar for all soil particle sizes. Almost all exchangeable and carbonate-bound chromium was removed by washing once with 0.5 M HCl, whereas organic chromium was more effectively removed by washing with citric acid rather than with HCl solution of the same concentration. For chromium fractions that were either bound to Fe-Mn oxides or existed as residual forms, the efficiencies of acid washing were usually 20% or less, except for 0.5 M HCl solution, which had much higher efficiencies. Separation of the soil sample by particle size before the separate washing of the soil fractions had little improvement on the chromium removal.

Current application of acid washing is to practice fractionation of soil based on particle size prior to washing. However, there is little published information concerning the relationship between the cost-effectiveness of this practice and soil characteristics; this shortage contributes to less than adequate application of washing in most remediation jobs in Taiwan. This research collected soil samples from a copper-contaminated agricultural field in central Taiwan to study the cleansing efficiency of using acid washing combined with particle sieving for remediating the contaminated soil. The soil texture is a sandy loam. The sandy portion constitutes more than 50% of the total soil mass; and its copper content is below the regulated limitation. Therefore, after separation by sieving, this saves more than 50% of the cost of acid washing. For the sandy portion, the hydrochloric acid solution is more effective but for the silt and clay portions, a mixture of hydrochloric acid and citric acid is most effective to remove copper. This study contributes information valuable to advance the application of acid washing to remediate contaminated soil.

Air-drying and oven-drying are pretreatment processes often used before testing and analyzing various soil characteristics in the laboratory. This study selected three kinds of soil, including red soil, entisol, and alluvial soil, and examined the variation of the Cr(VI) content and Cr bond forms in these soils during air-drying and oven-drying. The results show that when the soil is air-dried in natural environment, the Cr(VI) content decreases with air-drying time. On day 10 of air-drying, the Cr(VI) content in these soils is 22.8∼47.9 % of the initial value. When the soil is oven-dried, the Cr(VI) concentration decays faster; on day 8, the Cr(VI) is no longer detected in these soil samples. When the Cr(VI)-contaminated soil is treated by air-drying and oven-drying, the Cr bond form converts into a more stable form. After oven-drying, the Cr mainly exists in Fe-Mn oxide form, organic bond form, and residual form. The air-drying and oven-drying pretreatment processes of soil reduce the Cr(VI) content and stabilize the Cr bond form. If the laboratory analytic results are applied to risk analysis or remediation strategy planning for chromium-contaminated soil, the toxicity, bioavailability, and mobility of Cr in soil may be underrated.

Improper treatment and disposal of industrial wastewater and solid wastes result in serious heavy metal contamination of soil. A commonly used method for remediation of the soil contaminated by heavy metals is acid washing. This method is simple in principle, easy to operate and efficient in achieving the removal of heavy metals.The experience gained on soil remediation in Taiwan reveals that the soil particle distribution influences the heavy metal removal efficiency to a large extent. Hence, in this research, soil samples were collected from the contaminated sites and used in the investigation on the distribution of heavy metal Cd in soils of various particle sizes and the efficiencies of acid washing to remove the Cd from the soil of various particle distributions. The results will be used for future engineering implementation of the acid washing technique.The research results indicate that soils containing particles with sizes below 0.150 mm will hold increasing quantities of Cd and organic matter at decreasing particles sizes. As far as the acid washing efficiency is concerned, smaller particle diameters and higher organic matter contents result in decreasing efficiencies.