Solidification/Stabilization Of Cement-Based Materials On Chromium Contaminated Soil

Chromium and its compounds are widely used in industrial production, resulting in a large amount of chromium pollution. Chromium contaminated soil, one of the three kinds of sources of chromium, contains hexavalent chromium and trivalent chromium. The toxicity of chromium is mainly from hexavalent chromium, which is recognized as the one of the three carcinogenic metal and is harmful to human health. It is a major issue for environmental protection to deal with chromium pollution.In this paper, Solidification/stabilization (S/S) is used to deal with chromium contaminated soil. Through macro-performance of Cr(Ⅵ) and total Cr leaching concentration and unconfined compressive strength, cementitious materials such as Portland cement clinker, assistant cementitious materials such as slag, steel slag, and copper slag, and reductant are used to study S/S on chromium contaminated soil. Combining with micro-analysis of XRD, XPS, SEM, DTA, etc, mechanisms of S/S were discussed. The conclusions are obtained:①Portland cement clinker and high alumina clinker have favorite S/S effect on chromium contaminated soil. The unconfined compressive strength and the total Cr and Cr(Ⅵ) leaching concentration of the solidified are improved and decreased respectively with increasing of dosage of clinker. Portland cement clinker has better S/S effect than high alumina clinker.②Singly doping assistant cementitous materials, the total Cr and Cr(Ⅵ) leaching concentration of the solidified are decreased. Compared to singly doping cementitious materials, certain slag, steel slag or copper slag is used to replace the Portland cement clinker. At the same time, strength is decreased, but the range is a little. The total Cr and Cr(Ⅵ) leaching concentration keep the same as singly doping Portland cement clinker at low dosage. When increasing dosage, the stabilization effect of steel slag is first. Copper slag is better than slag. Thereby certain replacement is a ideal means to reduce the cost.③Some reductant such as sulfite, sulfide have poor effect on reducing Cr(Ⅵ) leaching concentration. However ferrous salts still significantly reduce the Cr(Ⅵ) leaching concentration. But it is rebind with time.④Mixing of ferrous salt and cementitious materials, strength is increased and The total Cr and Cr(Ⅵ) leaching concentration are decreased, and it’s no rebind with time. For example, mixing with 0.0075mol FeC12 and 30% clinker, strength of the solidified are 16.2MPa and 17,8MPa respectively at 7d and 28d. At 7d the total Cr and Cr(Ⅵ) leaching concentration are 0.41mg/l and 0.46mg/l respectively. At 28 the total Cr and Cr(Ⅵ) leaching concentration are 0.45mg/l and 0.46mg/l respectively. From 7d to 28d,the Cr(Ⅵ) leaching concentration are increased by 0.04mg/l and the Cr(Ⅲ) leaching concentration are decreased by 0.02mg/l. Mixing of ferrous salt and cementitious materials, the strength is the same as singly doping cementitious materials. The total Cr and Cr(Ⅵ) leaching concentration are only less than 1.5mg/l and 0.5mg/l and it is no rebound with time.⑤The role of S/S mechanism depends mainly on two aspects:One is gelling, that is the clinker hydration products parcel the soil particles and the pore adsorb the Cr ions to prevent the exudation of Cr ions. The other is deoxidizing. There are reducing substances in ferrous salt or assistant cementitious materials, which reduce Cr(VI) toCr(Ⅲ)to reduce the Cr(Ⅵ) leaching concentrationThis study shows that mixing dosage of cementitious materials, supplementary cementitious materials and reductants play a important role in S/S of the chromium contaminated soil. It is a potential means to resource utilization.