Solidified structure and leaching properties of metallurgical wastewater treatment sludge after solidification/stabilization process

Potential health risk assessment of contaminants in soil-like material recovered from landfill mining

Landfill mining is an innovative technique to clear dumpsites and recover valuables from legacy waste. Bio-earth, referred here as soil-like material, constitutes a major portion of the legacy waste fraction. The characterisation of Soil Like Material from Ariyamangalam Dumpyard and estimation of pollution indices by comparing with the background soil helps in the identification of the contamination level. The potential health risk associated with the contaminants in Soil Like Material is highlighted. A statistical correlation was also done for various parameters of Soil Like Material to identify the indicator element through the Spearman rank correlation method. The degree of contamination based on eight heavy metals taken into account shows a value of 48.23, which is in the range of very high contamination. The major pollution is induced by the lead present in the Soil Like Material compared to the background soil, followed by chromium and zinc. Lead shows the maximum contamination factor of 19, pollution index of 23.3, geo-accumulation index of 4.2 and enrichment factor of 19. The hazard index is in the order of Cr > Pb > Cu > Ni > Cd > Zn > Hg > As for the heavy metals. Among all heavy metals, the cumulative cancer risk is more due to nickel (0.06 and 0.006 for children and adults, respectively) which is harmful to human health. Hexavalent chromium was found to be an indicator element by significantly correlating with four other parameters. The results of the health risk assessment (HRA) and pollution indices add value to the application of Soil Like Material fractions both onsite and offsite.

Mechanical, Leaching, and Microstructure Properties of Mine Waste Rock Reinforced and Stabilised with Waste Oyster Shell for Road Subgrade Use

Two waste materials, oyster shell (NCOS; non-calcined oyster shell as coarse aggregate and COSP; calcined oyster shell powder as total and partial cement replacement) are used to reinforce and stabilise poorly graded and heavy metal-contaminated mine waste rock (MWR) for pavement subgrade use. Mechanical, leaching, and microstructural tests and analysis were performed on reinforced and stabilised samples to evaluate the effectiveness of the reinforcement and stabilisation of the MWR. Experimental results revealed NCOS and COSP improved the mechanical, leaching, and microstructural properties of the stabilised composite, with a 5% cement–15% COSP–15% NCOS mix being optimal when compared to the control mixes of cement only and no- NCOS. Higher COSP contents beyond 10% reduced the heavy metal contents significantly, but with relatively lower unconfined compressive strengths. Microstructural test results revealed the formation of calcium silicate hydrate (CSH), calcium aluminium silicate hydrate (CASH), ettringite, and calcite as the stabilisation products. Heavy metal complexes in both the cement-only and cement–NCOS–COSP mixes were also found. It is concluded that NCOS reinforced and improved the grading of poorly graded MWR, and that COSP stabilised and immobilised heavy metals present in MWR, thereby improving strength and other engineering properties for subgrade use.

Stabilization of heavy metals in sewage sludge by attapulgite

The mobility and bioavailability of heavy metals in sewage sludge are the major risks to utilize for agricultural application. In this study, the chemical speciation of Cu, Ni, Cr, and Zn in the Lanzhou municipal sewage sludge were investigated with the addition of a natural attapulgite. The influences of attapulgite amendment in sewage sludge on heavy metals stabilization were evaluated by investigating the leaching procedure and sequential chemical extraction experiments. The sequential extraction procedure described by European Community Bureau of Reference was used in sludge to determine the distribution of heavy metal species. The addition of attapulgite accelerated more reducible speciation of all metals transformed into residual speciation evidently. It reduced the leaching content of metals significantly and decreased the ecotoxicity accounted for the germination index values climbed rapidly with increased attapulgite addition.Implications: This research developed a method to stabilize heavy metals in municipal sewage sludge with clay. The activated attapulgite improved the treatment of sewage sludge containing heavy metals, and reduced the environmental risk of heavy metals.

Transformation of hazardous lead into aluminosilicate ceramics: structure evolution and lead leaching

This study investigated crystallization mechanisms for the formation of lead aluminosilicate by sintering lead stabilization with kaolin-based precursors. PbAl₂Si₂O₈ was found to be the only stable lead aluminosilicate in low-PbO system and demonstrates its highly intrinsic resistance to acid attack in leaching test. A three-stage PbAl₂Si₂O₈ formation mechanism was supported by the results of the changing temperature in the system. Amorphization of sintered products was observed in both PbO/kaolinite and PbO/mullite systems at 600-700°C. When the temperature was increased to 750-900°C, the crystallochemical formation of lead aluminosilicates (i.e., Pb₄Al₄Si₃O₁₆, Pb₆Al₆Si₂O₂₁, and PbAl₂Si₂O₈) was observed. Pb₄Al₄Si₃O₁₆ and Pb₆Al₆Si₂O₂₁ were found to be the intermediate phases at 700-900°C. Finally, PbAl₂Si₂O₈ was found to be the only crystallite phase to host Pb at above 950°C. A maximum of 80% and 96.7% Pb can be incorporated into PbAl₂Si₂O₈ in PbO/kaolinite and PbO/mullite systems, respectively, but the final products exhibited different microstructures. To reduce environmental hazard of lead, this strategy demonstrated a preferred mechanism of immobilizing lead into PbAl₂Si₂O₈ structure via kaolin-based precursors.

Advanced treatment of sewage by membrane bioreactor associate with genetically engineered autotrophic nitrifying bacteria

This study introduced a process of MBR combing with genetically engineered bacteria of expressing nirs and ppk genes (GEB-Nirs/PPK) for advanced treatment of sewage in scenic area. An industrial scale application was staged anaerobic digestion, aerobic digestion. Over more than 150 days of continuous operation, TMP in this system was less than 0.18 bar. With a membrane flux of 6.48 m³/h, TMP remained below 0.2 bar and effluent remained above 70 m³ during continuous operation. Average COD and BOD removals averaged 94.2% and 93.6%, and were obtained with average effluent COD and BOD below 10.4 mg/L and 4.2 mg/L, respectively. The TN and TP removals averaged 98.8% and 94.3%, and never higher than 3.2 mg/L and 0.2 mg/L, respectively, in the processing system. In conclusion, these results indicate that the process of MBR associate with genetically engineered autotrophic nitrifying bacteria is of high-efficiency for advanced treatment of sewage.

Interrelationships among geotechnical and leaching properties of a cement-stabilized contaminated soil

Relationships among selected performance properties have been established using experimental data from a cement-stabilized mixed contaminated soil. The sandy soil was spiked with 3,000 mg/kg each of Cd, Cu, Pb, Ni and Zn, and 10,000 mg/kg of diesel. It was then treated with 5%, 10%, 15%, and 20% dosages of Portland cement. Different water contents were considered for lower dosage mixes. Selected geotechnical and leaching properties were determined on 28-day old samples. These include unconfined compressive strength (UCS), bulk density, porosity, hydraulic conductivity, leachate pH and granular leachability of contaminants. Interrelationships among these properties were deduced using the most reasonable best fits determined by specialized curve fitting software. Strong quadratic and log-linear relationships exist between hydraulic conductivity and UCS, with increasing binder and water contents, respectively. However, the strength of interrelationships between hydraulic conductivity and porosity, UCS and porosity, and UCS and bulk density varies with binder and water contents. Leachate pH and granular leachability of contaminants are best related to UCS and hydraulic conductivity by a power law and an exponential function, respectively. These results suggest how the accuracy of not-easily-measurable performance properties may be constrained from simpler ones. Comparisons with some published performance properties data support this.