Maria Chrysochoou

Phosphate treatment of firing range soils: lead fixation or phosphorus release?

Phosphate treatment of lead (Pb)-contaminated soils relies on the premise that Pb converts to the thermodynamically stable, insoluble mineral class of pyromorphites. Recent research showed that treatment performance is kinetically controlled and strongly dependent on soil pH; this study employed an acidic phosphate (P) form, monobasic calcium phosphate (MCP), to investigate treatment performance of Pb occurring in an alkaline-buffered and an acidic firing range soil. The results of leaching, X-ray powder diffraction (XRPD), and modeling analyses showed that P and Pb dissolution in the alkaline soil and transformation reactions were kinetically controlled, so that: (i) TCLP (toxicity characteristic leaching procedure) and SPLP (synthetic precipitation leaching procedure) results were poor to marginal even at high MCP dosages; (ii) brushite (Ca(HPO(4)).2H(2)O) and cerussite (PbCO(3)) persisted in XRPD patterns; and, (iii) geochemical modeling failed to predict leaching and phase assemblages. In the acidic soil, Pb-P reactions promoted further soil acidification, improved TCLP performance, and generated better agreement with the equilibrium-based model; however, SPLP and modeling results showed that Pb concentrations could not be reduced below 15 microg/L mainly due to the low soil pH. The marginal or inadequate Pb immobilization was observed in both soils despite the elevated MCP dosages, which were well in excess of the pyromorphite stoichiometric ratio (P/Pb = 0.6). Additionally, P leaching concentrations and rates were extremely high (>300 mg/L), under both SPLP and deionized (DI) water extraction conditions, and as predicted by thermodynamic equilibrium. The performance and sustainability of phosphate-based treatment therefore seem questionable.

Investigation of Barium Treatment of Chromite Ore Processing Residue (COPR) 031

Barium addition to chromite ore processing residue (COPR) was investigated in order to address (a) the pronounced heaving phenomena that are associated with mainly the presence of ettringite and (b) hexavalent chromium leaching. Sulfate was added to representative samples of grey-black (GB) and hard-brown (HB) COPR to simulate worst-case conditions of sulfate influx and ettringite formation. Both the X-ray powder diffraction (XRPD) and the modeling results showed that ettringite is a thermodynamically favored reaction in COPR. The subsequent addition of barium lead to the formation of both barite and barium chromate, observed as solid solution between the two phases. Modeling results confirmed that barium sulfate is the more stable species that will dissolve ettringite and that barium chromate will also dissolve COPR chromate phases when sulfate is depleted. The Toxicity Characteristic Leaching Procedure (TCLP) test on GB samples showed that the optimal stoichiometry to maintain Cr and Ba TCLP concentrations below the U.S. Environmental Protection Agency regulatory limit of 5 and 100 ppm, respectively, lies between 1:1 (Ba to sulfate plus chromate ratio) and 1.5:1. The respective optimal stoichiometry for the HB COPR was found to be higher, between 2:1 and 5:1. Considering that COPR is actually a Cr-contaminated cement form, a further area of research is the identification of barium-containing wastes (i.e., heavy-metal sludges, contaminated soils, etc.) that would be suitable for combination with COPR; in this way, an environmentally sustainable yet cost-effective treatment application can be realized.

Remediation of chromite ore processing residue using ferrous sulfate and calcium polysulfide

Batch tests were conducted to assess the potential use of ferrous sulfate and calcium polysulfide for the remediation of chromite ore processing residue (COPR). The remediation process entails addition of ferrous sulfate or calcium polysulfide to chemically reduce hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] in slurry form and pH adjustment to precipitate Cr(III) as chromium hydroxide. The present study investigates the effects of COPR particle size, treatment pH, and chemical dosage on the performance of the treatment. Smaller particle size resulted in increases in alkaline digestion and Toxicity Characteristic Leaching Procedure (TCLP) Cr(VI) concentrations for the untreated samples. The chemical reduction of Cr(VI) with ferrous iron and sulfides was non-stoichiometric. Four times the stoichiometric amount of ferrous iron of two times the stoichiometric amount of polysulfide were needed to meet both the New Jersey Department of Environmental Protection (NJDEP) regulatory limit of 240 mg/kg for Cr(VI) and EPA TCLP regulatory limit of 5 mg/L for chromium [C,r]. pH adjustment was necessary to prevent the formation of ettringite, a swell causing mineral, upon the introduction of sulfate to the COPR material via ferrous sulfate or calcium polysulfide. The slow hydration of some COPR minerals caused the pH of the treated COPR to creep upward during the curing period. However, when sufficient acid was added, the pH value was controlled at less than 9.27 for a curing period of 1.5 years, which prevented the formation of ettringite.

An Investigation of the Heaving Mechanism Related to Chromite Ore Processing Residue

Significant heaving has been observed over time at chromite ore processing residue (COPR) deposition sites in Maryland and New Jersey. Confined swell tests were employed in order to investigate the geochemical mechanisms that lead to the manifestation of heave in COPR. Ettringite, a known heave culprit in cement and soil-related literature, was identified in numerous samples across the sites and was therefore considered as the primary heaving mechanism in COPR. In addition, other possible mechanisms, such as brownmillerite hydration to hydrogarnets, carbonation reactions, calcium aluminum chromium oxide hydrate (CAC) formation, and change of hydration state were also investigated. The confined swell tests were conducted under wet/dry cycles. Sulfate, carbonate, and chromate solutions were introduced to the samples during wet cycles in order to validate the different heaving hypotheses associated with phase transformations. The test results showed that swell development occurred only in COPR upon 0.7 mole/l sulfate addition. Furthermore, x-ray diffraction analyses confirmed the formation of ettringite in the COPR sample following the addition of sulfate. Conversely, carbonation reactions led to no height change or even to consolidation, while the formation of hydrogarnets and CACs could not be established. It was, therefore, demonstrated that ettringite formation is a thermodynamically powerful reaction and also a viable expansion and failure mechanism in COPR.

DREDGED MATERIAL STABILIZATION: THE ROLE OF MELLOWING ON CURED PROPERTIES

This study presents the results of a treatability study for DM stabilization using fifteen combinations of stabilizing agents (lime, Cement Kiln Dust (CKD), fly ash). Two paste studies were conducted, both employing a mellowing period of up to 5 days under open and closed conditions, respectively. To illustrate the impacts of the different mellowing conditions on the characteristics of the stabilized DM, select physicochemical, geotechnical and mineralogical differences of the two sample sets are presented. Mellowing under closed conditions yielded non-compactable material for 10 of 15 blends, as the moisture content was too high. Conversely, open mellowing yielded compactable material for all blends as early as 2 days. The mineralogical analyses showed that cement reactions proceeded more slowly under closed conditions. Compaction curves generated for two blends under open and closed conditions yielded contradictory results in terms of blend performance, the higher densities being obtained under open conditions. Consequently, since open mellowing more closely resembles field conditions, it is recommended as a procedure prior to curing in future S/S paste studies.

Current Knowledge on Heaving Mechanisms of Chromite Ore Processing Residue

Chromite Ore Processing Residue (COPR) has in certain cases been associated with heaving phenomena. To understand the COPR heave mechanism, the site owner sponsored research that included extensive field investigation, site characterization and laboratory programs. Based on the results, this paper presents the current knowledge on COPR heave mechanisms. Among the different heaving mechanisms considered, ettringite formation had previously been reported as an expansion mechanism in cements and lime-treated soils. While site investigation revealed significant, localized presence of ettringite, it could not be correlated to the observed heave features. However, observations from test-trench activities and heave phenomena in ferrous sulfate treated COPR, confirmed that ettringite induced expansion constitutes a viable COPR heave mechanism. From other potential mechanisms that were evaluated, brownmillerite hydration to hydrogarnets remains the most viable. Although brownmillerite hydration proceeds extremely slowly at ambient temperature, hydrogarnet-rich material has been found in discrete layers in trenches at different sites, and has lately been associated with a major heave feature at SA-7. Accordingly, the conditions for rapid brownmillerite hydration and its associated potential for heave manifestation remain open to further investigation.

Nano-Zero-Valent Iron: An Emerging Technology for Contaminated Site Remediation

Soil contamination is by definition a very complex and expensive problem to solve, because a three-phase medium is contaminated by chemicals, commonly as mixtures, which not only interact together but also with each phase of the soil concurrently. Although pump-and-treat, monitored natural attenuation, and vertical engineered barriers are the most frequently applied groundwater treatment technologies for contaminated sites, new remediation technologies have appeared as emerging trends. The most promising among them is nanoremediation, a technology based on the injection of nanoparticles (NPs) into contaminated aquifers for transformation and detoxification of pollutants. The most widely NPs among them is nanoscale zero-valent iron (nZVI), which may be used for organic or inorganic contaminants such as heavy metals and especially hexavalent chromium (Cr(VI)). The present study aims at reviewing the current knowledge on Cr(VI) remediation by nZVI, focusing on the mechanisms by which Cr(VI) is treated by nZVI, the fate and transport of these NPs in the subsurface, the modifications of nZVI, and the removal efficiency that have been published. Finally, the potential toxicity created after the injection of nZVI particles in aquifers is also discussed.

Reductive treatment of Chromite Ore Processing Residue (COPR): lessons from a field study

A bench- and pilot-scale treatability study conducted on Chromite Ore Processing Residue (COPR) from a deposition site in New Jersey showed that regulatory compliance to 20 (current) or 240 (up to 2006) mg/kg Cr(VI) was not possible using several common reductants (zero valent iron, pyrite, ferrous sulfate) due to the slow release of Cr(VI) from the binding minerals and the competing oxidation of iron and ferrous by oxygen. Initial success as evidenced by alkaline digestion was an artifact of the method, which facilitated Cr(VI) reduction during the test itself. X-ray Absorption Near-Edge Structure (XANES) analyses showed that the actual Cr(VI) concentrations in the solid were significantly higher (>1,000 mg/kg) in both ferrous sulfate and calcium polysulfide treated samples. Calcium polysulfide decreased Cr(VI) to non-detectable values (<2 mg/kg) for long monitoring times (up to 1½ years), as measured by alkaline digestion. The apparent success of calcium polysulfide is attributed to its slow oxidation, so that sulfide is available during analysis to reduce Cr(VI) that slowly diffuses out of the COPR matrix. The comparison of the in situ sulfide oxidation and Cr(VI) release rates can facilitate the design of a successful reductive treatment.