Environmental behavior of cement-based stabilized foundry sludge products incorporating additives

Stabilization of lead (Pb) and zinc (Zn) in contaminated rice paddy soil using starfish: A preliminary study

Lead (Pb) and zinc (Zn) contaminated rice paddy soil was stabilized using natural (NSF) and calcined starfish (CSF). Contaminated soil was treated with NSF in the range of 0-10 wt% and CSF in the range of 0-5 wt% and cured for 28 days. Toxicity characteristic leaching procedure (TCLP) test was used to evaluate effectiveness of starfish treatment. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analyses were conducted to investigate the mechanism responsible for effective immobilization of Pb and Zn. Experimental results suggest that NSF and CSF treatments effectively immobilize Pb and Zn in treated rice paddy soil. TCLP levels for Pb and Zn were reduced with increasing NSF and CSF dosage. Comparison of the two treatment methods reveals that CSF treatment is more effective than NSF treatment. Leachability of the two metals is reduced approximately 58% for Pb and 51% for Zn, upon 10 wt% NSF treatment. More pronounced leachability reductions, 93% for Pb and 76% for Zn, are achieved upon treatment with 5 wt% CSF. Sequential extraction results reveal that NSF and CSF treatments of contaminated soil generated decrease in exchangeable/weak acid Pb and Zn soluble fractions, and increase of residual Pb and Zn fractions. Results for the SEM-EDX sample treated with 5 wt% CSF indicate that effective Pb and Zn immobilization is most probably associated with calcium silicate hydrates (CSHs) and calcium aluminum hydrates (CAHs).

Environmental assessment and management of metal-rich wastes generated in acid mine drainage passive remediation systems

As acid mine drainage (AMD) remediation is increasingly faced by governments and mining industries worldwide, the generation of metal-rich solid residues from the treatments plants is concomitantly raising. A proper environmental management of these metal-rich wastes requires a detailed characterization of the metal mobility as well as an assessment of this new residues stability. The European standard leaching test EN 12457-2, the US EPA TCLP test and the BCR sequential extraction procedure were selected to address the environmental assessment of dispersed alkaline substrate (DAS) residues generated in AMD passive treatment systems. Significant discrepancies were observed in the hazardousness classification of the residues according to the TCLP or EN 12457-2 test. Furthermore, the absence of some important metals (like Fe or Al) in the regulatory limits employed in both leaching tests severely restricts their applicability for metal-rich wastes. The results obtained in the BCR sequential extraction suggest an important influence of the landfill environmental conditions on the metals released from the wastes. To ensure a complete stability of the pollutants in the studied DAS-wastes the contact with water or any other leaching solutions must be avoided and a dry environment needs to be provided in the landfill disposal selected.

Recovery and safer disposal of phosphate coating sludge by solidification/stabilization

Solidification/stabilization (S/S) of automotive phosphate coating sludge (PS) containing potentially toxic heavy metals was studied. The hazardous characteristics of this waste were assessed according to both Turkish and U.S. Environmental Protection Agency (EPA) regulations for hazardous solid waste. Unconfined compressive strength (UCS) and leaching behavior tests of the solidified/stabilized product were performed. Solidification studies were conducted using Portland cement (PC) as the binder. UCS was found to decrease with increasing waste content. It was found that recovery of the waste for construction applications was possible when the waste content of the mortar was 20% and below, but solidification for safe disposal was achieved only when higher waste concentrations were added. Cu, Cr, Ni, Pb and Zn were found to be significantly immobilized by the solidification/stabilization process. Ni and Zn, which were present at particularly high concentrations (2.281 and 135.318 g/kg respectively) in the PS, had highest the retention levels (94.87% and 98.74%, respectively) in the PC mortars. The organic contaminants and heavy metals present in PS were determined to be immobilized by the S/S process in accordance with the BS 6920 standard. Thus, the potential for hazardous PS waste to adversely impact human health and the environment was effectively eliminated by the S/S procedure. We conclude that S/S-treated PS is safe for disposal in landfills, while recovery of S/S-treated PS constituents remains possible.

Stabilization and solidification of Pb in cement matrices

Pb was incorporated to a series of cement matrices, which were submitted to different testes of solidified/stabilized product. The leaching behaviors of aqueous solution were monitored by graphite furnace atomic absorption spectroscopy (GF-AAS). The mechanical strengths were evaluated by unconfined compressive strength (UCS) at 7 and 28 ages. Data are discussed in terms of metal mobility along the cement block monitored by X-ray fluorescence (XRF) spectrometry. Complementary techniques, namely, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), small angle X-ray scattering (SAXS) and X-ray diffraction spectroscopy (XRD) were employed in the characterization of the modified matrices. The Pb incorporated matrices have shown that a long cure time is more suitable for avoiding metal leaching. At pH 8 lower Pb leaching took place both for both short and long cure time. For a longer cure period there is a decreasing in the compressive strength. TGA and DRIFTS analyses show that the resistance fall observed in the UCS tests in the sample with Pb are not caused by hydration excess. XRF analyses show that there is a lower Ca concentration in the matrix in which Pb was added.

Leaching behavior of pollutants in ferrochrome arc furnace dust and its stabilization/solidification using ferrous sulphate and Portland cement

In this study, dissolution properties under different conditions and pollution potential by toxicity characteristic leaching procedure (TCLP) of arc furnace dust generated in the production of ferrochrome were examined and some stabilization/solidification (S/S) techniques were applied to the dust depending on contaminants determined. Dissolution properties and pollution potentials of all the materials stabilized/solidified were also studied under the similar conditions. It was determined that the metallic components concentrations dissolved from the ferrochrome arc furnace dust (FAFD) except for chromium and zinc were below the detection limits. The chromium concentration dissolved from the FAFD by TCLP was found to be 9.8 mg/l. Portland cement (PC), PC-FeSO(4) and PC-sand-FeSO(4) mixtures for S/S of the FAFD were tested. Although metal ions in the cationic form were stabilized when the PC was only used, Cr(VI) in the sample was not changed depending on PC amount and remained in the soluble chromate form. The stabilization efficiency of Cr(VI) increased by the increasing amounts of PC and FeSO(4). The best S/S of the FAFD was accomplished when the 5 stoichiometric amounts of FeSO(4), 30% PC and 16% sand mixture were used. TCLP leaching results of the samples obtained under the optimum conditions were below the EPA landfilling limits.

Evaluation of lead desorption from cement matrices

Desorption of lead immobilized into cement and concrete matrices was evaluated in the pH range between 4 and 8 within a period of 2 h to 15 days. Lead-desorbed content in aqueous milieu was monitored by Graphite Furnace Atomic Absorption Spectroscopy, while matrices were directly analyzed by X-ray photoelectronic spectroscopy or Rutherford Backscattering spectrometry. For low pH, higher Pb desorption was observed, about 5-7 times higher than that measured in the case of concrete matrices. The inclusion of polyvinyl alcohol as a matrix modifier was evaluated through a factorial design. Polymer inclusion yielded a reduction in Pb desorption, but its effect cannot be justified in terms of the ability of this polymer to chelate the metal. The resulting modified matrices did not present toxicity to Daphnia magna.

Leaching behavior and immobilization of heavy metals in solidified/stabilized products

Solidification/stabilization (S/S) of hazardous sludge from steel processing plant has been studied. Mechanical strength and leaching behavior test of solidified/stabilized product was performed. Mechanical strength decreases with increase in waste content. Pb, Zn, Cu, Fe and Mn could be considerably immobilized by the solidification/stabilization process. The elements least immobilized were Na, K, and Cl. Leaching of heavy metals in the S/S matrix can be considered as pH dependent and corresponding metal hydroxide solubility controlled process. Geochemical modeling was performed for the prediction of speciation. On the basis of test results, mobility and mechanism of leaching was assessed. Dominant leaching mechanism was surface wash off in the initial stages followed by diffusion for Pb, Zn, Cu, Fe and Mn. Diffusion coefficient was above 11.5 indicating low mobility in the cement matrix.

Stabilization and solidification of waste phosphate sludge using portland cement and fly ash as cement substitute

Stabilization and solidification of the waste phosphate sludge (WPS) using Portland cement (PC) and fly ash (FA) were studied in the present work. The WPS content in the cement mortars varied from 5% to 15%. Setting times were measured, and unconfined compressive strengths (UCS) were determined for the mortars cured in water for 3, 7, 28, 56, and 90 days. Zinc and nickel leaching of the solidified products were measured according to the Toxicity Characteristic Leaching Procedure. Setting times were extended as the WPS content in the paste samples increased. The UCS values of the mortar containing 5% WPS solidified by using 95% PC were similar to the reference sample. Use of 10% FA as cement substitute increased the UCS values by 10% at the end of curing period of 56 days. The WPS contained initially 130.2 mg L(-1) of zinc and 22.7 mg L(-1) of nickel. The zinc and nickel leached from the 5% WPS solidified by using 95% PC were measured as 3.8 mg L(-1) and 0.4 mg L(-1), respectively. These metal concentrations were below the limits given by the U.S. Environmental Protection Agency for landfilling the solidified wastes.