Removal of chromium from a highly contaminated soil/slag matrix by washing at low pH

Removal of chromium by soil/slag washing with low pH sulfuric acid solutions and temperature to 95°C is described. The extraction parameters are acid concentration, contact time, temperature, solvent/slag ratio, and acid type. The effect of two-stage extraction is also discussed. Soil/slag washing with sulfuric acid concentrations of 2% w:v (weight: volume) at 75:1 v:w solvent/slag ratios yielded chromium extraction efficiencies of 95%, but 50% of the matrix was dissolved. Acid washing of soil/slag originally containing 21,000 ppm resulted in residual chromium concentrations of 1,000 ppm or less. Residual chromium is postulated to consist of immobilized chromium fixed to the soil/slag matrix, whereas all surface adsorbed and free chromium is removed. A study on kinetics revealed that extraction at 95°C is completed within five minutes. The effect of temperature is such that chromium removal is significantly improved when extraction is carried out at 95°C rather than at 20°C. At a 75:1 v:w solvent/slag ratio, peak extraction is achieved at a lower concentration than at 25:1 and matrix weight loss is greater. In addition, at 75:1, 95% chromium extraction is achievable, but at 25:1, the maximum chromium removal levels off at 80%. No significant difference is detected in the extractive capabilities of the different mineral acids tested: sulfuric, hydrochloric, and nitric acids. Sulfuric acid is a suitable choice as extractant because of its lower cost and it has other advantages, such as its reduced corrosivity.