Effects of hydroxyapatite addition on heavy metal volatility during tannery sludge incineration

Transformation behavior of heavy metal during Co-thermal treatment of hazardous waste incineration fly ash and slag/electroplating sludge

In this study, the behavior of heavy metal transformation during the co-thermal treatment of hazardous waste incineration fly ash (HWIFA) and Fe-containing hazardous waste (including hazardous waste incineration bottom slag (HWIBS) and electroplating sludge (ES)) was investigated. The findings demonstrated that such a treatment effectively reduced the static leaching toxicity of Cr and Pb. Moreover, when the treatment temperature exceeded 1000 °C, the co-thermal treated sample exhibited low concentrations of dynamically leached Cr, Pb, and Zn, indicating that these heavy metals were successful detoxified. Thermodynamic analyses and phase transformation results suggested that the formation of spinel and the gradual disappearance of chromium dioxide in the presence of Fe-containing hazardous wastes contributed to the solidification of chromium. Additionally, the efficient detoxification of Pb and Zn was attributed to their volatilization and entry into the liquid phase during the co-thermal treatment process. Therefore, this study sets an excellent example of the co-thermal treatment of hazardous wastes and the control of heavy metal pollution during the treatment process.

Co-incineration effect of sewage sludge and municipal solid waste on the behavior of heavy metals by phosphorus

The effects of sewage sludge phosphorus (P) content on heavy metal behavior during co-incineration of sewage sludge and municipal solid waste (MSW) were evaluated. Thermogravimetric differential thermal analysis revealed that MSW incineration was mainly caused by organic matter and fixed carbon, while sewage sludge incineration was caused by volatile matter. During co-incineration, the peak weight loss at 460 °C shifted to slightly higher temperatures and the sludge ratio increased, indicating that interaction effects during co-incineration delayed pyrolysis and polymer/fixed carbon incineration. The residual heavy metal ratios after mono-incineration of sewage sludge were higher than those after MSW mono-incineration. The Cl content of MSW (0.757%) was much higher than that of sewage sludge (0.068%), which resulted in the conversion of heavy metals into metal chlorides and then volatilized during MSW mono-incineration. A synergistic effect of co-incineration was evident for Cu, but not for lead (Pb) or cadmium (Cd). X-ray absorption fine structure (XAFS) measurement revealed that Cu in MSW ash was in the form of CuO(s), but was Cu₃(PO₄)₂ in sewage sludge and co-incineration ashes. CuO(s) is relatively unstable and may be transformed to CuO(g) or CuCl(s) before volatilizing at high temperature or in the presence of Cl. Phosphorus has the effect of stabilizing Cu in sewage sludge during co-incineration.

Pretreatment with Ochrobactrum immobilizes chromium and copper during sludge pyrolysis

To increase the degree of immobilization of heavy metals subjected to sludge pyrolysis, we investigated the effects of pretreating sludge with Ochrobactrum supplementation on the immobilization of chromium (Cr) and copper (Cu) during sludge pyrolysis. The sequential extraction procedure was used to test the metallic forms of Cr and Cu. The immobilization of Cr and Cu was characterized with X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, etc. Results show that: 1) the addition of Ochrobactrum (1-8%) can accelerate the mineralization process in blank sludge and can accelerate the conversion of the oxidizable forms of Cr and Cu into the residual forms subjected to pyrolysis; 2) pretreatment with Ochrobactrum supplementation can inhibit the volatilization of Cr and Cu during sludge pyrolysis, particularly in the case of a high concentration of Cu. Notably, the pretreatment with Ochrobactrum can reduce 20.38-85.09% of the potential ecological risk of Cr and Cu. The pretreatment with Ochrobactrum contributes to the immobilization of Cr and Cu subjected to sludge pyrolysis and thus can prevent pollution of the environment. The results of this study can be used for harmless disposal of municipal sludge.

Sludge aging stabilizes heavy metals subjected to pyrolysis

In this study, we analyzed the effects of sludge aging pre-treatment on the stabilization mechanisms of heavy metals during sludge pyrolysis. First, the form of copper (Cu) and chromium (Cr) was conducted using the sequential extraction procedure proposed by the European Community Bureau of Reference (BCR). The stabilization mechanisms for the sludge pyrolysis of Cu and Cr were then analyzed using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Results indicate the following: 1) with aging pre-treatment, the improvement in adsorption performance and the formation of newly crystallized materials, like polyhydroxy copper phosphate and chromium phosphate minerals, occur concurrently with the stabilization of heavy metals during pyrolysis; 2) after four weeks of aging, active functional groups like amino and carboxyl groups were significantly sharpened, and caused sustained complexation of the heavy metals. Results suggested that the aging pre-treatment aided the stabilization of heavy metals during sludge pyrolysis. Notably, the aging effect can decrease the potential ecological risk of heavy metals.

Hydroxyapatite reduces potential Cadmium risk by amendment of sludge compost to turf-grass grown soil in a consecutive two-year study

Recycling of sludge compost to soil as conditioner is generally regarded as the best means of disposal. However, concerns regarding heavy metal residues and sludge toxicity have recently received increasing public attention. Cadmium (Cd) is a mobile metal commonly found in sludge; therefore, the risk posed by Cd contaminated sludge should be carefully assessed. In this report, the effects of addition of hydroxyapatite (HAP) with sludge compost amendment on potential Cd risk were investigated. The results of consecutive two years showed that exchangeable Cd content in treatment of sludge compost with 1.5% HAP decreased by 6.0% compared with single sludge compost treatment, and residual Cd increased by 7.6%. Compared with single sludge compost, the incremental rate of exchangeable Cd dropped by 38.3% and the reductive rate of residual Cd increased by 37.7% in response to 1.5% HAP addition, indicating that HAP played a role of decreasing Cd phytoavailability. The HAP reduced the amount of Cd uptaken by turf-grass in both root and leaf. Moreover, HAP remarkably improved the quality of turf grass grown in amended soil, including leaf greenness, green maintainable period and root strength. However, HAP did not attenuate the downward mobility of Cd. Taken these together, these findings indicated that HAP can be used as a potential candidate to control surface Cd risk of sludge compost amended soil rather than that from leachate.

Formulation of sludge incineration residue based geopolymer and stabilization performance on potential toxic elements

Using sludge incineration residue (SIR) to prepare SIR-based geopolymer can realize the minimization and reclamation of urban sludge as well as the stabilization of potential toxic elements. This work has researched the critical factors that influence the formulation of SIR-based geopolymer and the stabilization performance on potential toxic elements. Results showed that the addition amount of SIR had significant influence on the compressive strength of geopolymer, while the modules of sodium silicate and the sodium silicate/metakaolin ratio had little influence thereon. When the addition amount of SIR was 10%, the modules of sodium silicate was 1.3 and the sodium silicate/metakaolin ratio was 0.86, the compressive strength of geopolymer reached the maximum of 95.6 MPa. When the temperature increased from 25 °C to 800 °C, the compressive strength showed a tendency of increasing first and then decreasing. SIR-based geopolymer could significantly reduce the leaching of Zn and Cu from 1264.8 mg/kg and 856.3 mg/kg to 50.8 mg/kg and 30.7 mg/kg respectively, leading to a stabilizing efficiency more than 96%. pH influenced the stabilization of geopolymer on the potential toxic elements, the effective diffusion coefficients De of Zn, Cu, Ni, Pb and Cr at pH = 4 were higher than those at the pH of 7, which were probably due to the physic-chemical characteristics and the corresponding changes in the leaching and species distribution of the potential toxic elements.

Biomimetic Synthesis of Hydroxyapatite in Presence of Imidazole-4,5-dicarboxylic Acid Grafted Chitosan for Removing Chromium(VI)

In order to biomimetic synthesize hydroxyapatite similar to natural bone. Hydroxyapatite (HAP) is biomimetic synthesized in simulated body fluid (SBF) by addition of imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS). The effect of molar ratio of chitosan (CS) to imidazole-4,5-dicarboxylic acid (IDA) on preparation of HAP was investigated. The structure, size, and crystal phase of the obtained hydroxyapatite were observed by Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. The results show that the molar ratio of CS to IDA is 1 : 3, the temperature is 37.0°C, the aging time is 48 h, the synthesized nanorod-like hydroxyapatite with diameter 20–30 nm, and length ranging from 75 to 120 nm presents excellent phase, which disperses well and is similar to the natural bone of HAP. The obtained HAP can be used to remove chromium(VI) by the orthogonal experiments, and the results indicated that the removal rate can reach 95.66% under the optimum conditions. These results suggest that the morphology of the obtained HAP is more affected by the material ratio of chitosan to imidazole-4,5-dicarboxylic acid than its structure, and the obtained HAP can effectively remove Cr(VI), which provides a novel method for biomimetic synthesis of other biomaterials and application in the water purification.

Risk assessment of heavy metals from combustion of pelletized municipal sewage sludge

Fly ash and slag are important by-products obtained from combustion of municipal sewage sludge (MSS) after pelletization. The quantitative environmental impact assessment of heavy metals in fly ash and slag, compared to MSS, were performed in accordance with bioavailability and eco-toxicity, geo-accumulation index (GAI), risk assessment code (RAC), and potential ecological risk index (PERI). The results demonstrated that not only direct but also long-term bioavailability and eco-toxicity of heavy metals in fly ash and slag decreased except direct bioavailability and eco-toxicity of Pb in fly ash. The GAI demonstrated that combustion significantly weakened (P < 0.05) the pollution levels of heavy metals. PERI indicated that all risks attributed to heavy metals were significantly lowered (P < 0.05) from 777.07 (very high risk) in MSS to 288.72 (moderate risk) and 64.55 (low risk) in fly ash and slag, respectively. In terms of the RAC, seven heavy metals had low even no risk to the environments after combustion besides As in slag. The environmental risk of heavy metals in fly ash and slag was decreased compared with MSS. However, the results of PERI showed that fly ash had a moderate risk.

Chemical speciation, mobility and phyto-accessibility of heavy metals in fly ash and slag from combustion of pelletized municipal sewage sludge

Combustion of pelletized municipal sewage sludge (MSS) can generate pestilent byproducts: fly ash and slag. Comparisons of heavy metal sequential extraction results among MSS, fly ash and slag showed that after combustion, the bioavailable heavy metal fractions (acid soluble/exchangeable, reducible and oxidizable fractions) were mostly transformed into the very stable heavy metal fractions (residual fractions). On the other hand, the results of toxicity characteristic leaching procedure (TCLP), diethylenetriamine pentaacetic acid and HCl extraction (phyto-accessibility assessment) demonstrated that the mobility and toxicity of heavy metals were greatly reduced. The direct and long-term bioavailability and eco-toxicity of heavy metals in fly ash and slag were relieved, which implied that combustion of pelletized MSS could be a promising and completely safe disposal technology for MSS treatment.

Incineration of tannery sludge under oxic and anoxic conditions: study of chromium speciation

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.