Immobilization of hexavalent chromium in cement mortar: leaching properties and microstructures

Does Current Knowledge Give a Variety of Possibilities for the Stabilization/Solidification of Soil Contaminated with Heavy Metals?-A Review

Stabilization/solidification of contaminated soil is a process that allows simultaneous strengthening of the soil structure, disposal of contamination and recycling of industrial waste, implemented as substitutes for Portland cement or additives to improve the properties of the final product obtained. Extremely intensive development of studies pertaining to the S/S process prompted the authors to systematize the binders used and the corresponding methods of binding the contamination, and to perform an analysis of the effectiveness expressed in geomechanical properties and leachability. The study pays close attention to the types of additives and binders of waste origin, as well as the ecological and economic benefits of their use. The methods of preparing and caring for the specimens were reviewed, in addition to the methods of testing the effectiveness of the S/S process, including the influence of aging factors on long-term properties. The results of the analyses carried out are presented in the form of diagrams and charts, facilitating individual evaluation of the various solutions for the stabilization/solidification of soils contaminated with heavy metals.

Combined Treatment of Cr(VI)-Contaminated Soils by Reduction, Adsorption, and Solidification

Remediation of Cr(VI)-contaminated soil usually includes reducing Cr(VI) to Cr(III) with sub-sequent solidification. In this paper, a treatment technique that combines reduction, adsorption, and solidification was proposed. By introducing an adsorbent into the reduction process, the remediation effectiveness was improved and the amount of reducing and solidified agent was decreased. Synthetic precipitation leaching procedure (SPLP), unconfined compressive strength (UCS) test, and scanning electron microscope (SEM) analysis were carried out to evaluate the remediation effect under different agent combinations and different agent-adding procedures. The results of SPLP showed that the reduction/adsorption/solidification treatment significantly reduced the leachability of Cr. UCS increased with increasing dosage of cement and CaS5, and decreased with an increasing dosage of vermiculite. The best agent dosage was CaS5 of 2 times molar stoichiometric ratio of Cr(VI), 15% of vermiculite, and 20% of cement. Orthogonal test showed that for soil with low Cr(VI) content, CaS5 dosage was the most important factor that affected the leachability of Cr. Cement and vermiculite have greater impact in limiting the leachability of Cr when Cr(VI) content in soil increased.

Propositure of maximum permissible limits for environmental assessment and classification of materials stabilized by solidification incorporated with oil waste

In the present work, oil sludge was subjected to the process of stabilization by solidification, the objective was to indicate maximum permissible limits of chemical demand for oxygen and oils and greases to evaluate the resulting material. A factorial design was used with the addition of three repetitions at the central point, to evaluate the performance of different percentage of residue and different curing times. The factors adopted were the percentage of oil sludge and the curing time. This material was evaluated using contaminants immobilization tests. From the leached and solubilized liquid, the concentration of the contaminants was determined and the environmental assessment was also carried out through the analysis of chemical demand for oxygen and oils and greases. One of the contributions of this work was to propose the maximum allowable limit for chemical oxygen demand, which is 1,000 (one thousand) milligrams per litre and, for oil and grease content, 100 (one hundred) milligrams per litre, both for the leaching test. For the solubilization test, 10 (ten) milligrams per litre were proposed for chemical oxygen demand and 1 (one) milligrams per litre for the oil and grease content. It was observed that the best results are obtained, when lower values of percentage of oil sludge were used and longer cure time. This work allows to affirm that the proposal of limits of environmental evaluation contributes to assure an adequate disposition and use of the cement matrix, that is the final product of the oily residue stabilized by solidification.

Stabilization of lead contaminated soil with traditional and alternative binders

The application of an innovative solidification/stabilization (S/S) process was investigated for the remediation of Pb contaminated soil. The performance of Pb stabilization was evaluated by comparing the use of calcium aluminate cement (CAC) and an alkali activated metakaolin binder vs the Ordinary Portland Cement (OPC). The phase composition of the stabilized products was investigated by XRD and correlated to the internal microstructure obtained by SEM-EDX imaging. Leaching tests were performed to ascertain the effectiveness of the proposed binders in the S/S of the contaminated soil, and Pb release was evaluated for each binding system. The overall results proved that multiple mechanisms are involved in Pb retention and that key parameters regulating the stabilization performance are strongly dependent on the type of applied binder system. Pb was found to be associated to C-S-H in the case of OPC, whereas ettringite played a key role in the retention of this contaminant using the CAC binder. The use of a NaOH activated metakaolin resulted in almost total retention of Pb, despite a lack of solidification, highlighting the importance of pH in the regulation of the leaching behavior.