Upcycling textile sludge into magnesium oxychloride cement: Physical properties, microstructure, and leaching behavior

Textile sludge is a by-product produced during the wastewater treatment process in the textile printing and dyeing industry. Textile sludge is rich in heavy metal elements, which makes it a potential risk to the surrounding environment. This study designs a magnesium oxychloride cement (MOC) components to solidify harmful substances in textile sludge and studies the influence of textile sludge ash (TSA) on the mechanical properties and microstructure of MOC samples. The results indicated that adding 5 %-20 % TSA is beneficial for increasing the compressive strength of air-cured MOC paste and improving its water resistance. Meanwhile, the MOC sample shows volume expansion in 168 h, which is related to the further hydration of residual MgO. Incorporating 10 %-20 % TSA substantially increased the volume expansion ratio of the mixture compared to plain MOC sample. In addition, the porosity of TSA-modified MOC after water curing did not change significantly compared to the sample before water curing, while the pore structure of plain MOC after water curing significantly coarsened. This is mainly because TSA reacts with MOC and generates Mg-Al-Cl-Si-H and Mg-Cl-Si-H gels, consequently improving the water stability of MOC sample. At the nanoscale, the 3/5-phase crystal and unreacted MgO content in the 15 % TSA-modified MOC sample is relatively reduced by 7.79 % and 25 %, respectively, compared to the plain sample, but the 13 % gel phase is detected. In addition, the MOC component can effectively solidify heavy metal elements in textile sludge. For the leachate of 20 % TSA-modified MOC paste, the Ni element is not detected, and its solidifying effect on heavy elements such as Zn and Mn exceeded 99 %.

Converting ash into reusable slag at lower carbon footprint: Vitrification of incineration bottom ash in MSW-fueled demonstration-scale slagging gasifier

Globally waste incineration is becoming the predominant treatment method of solid waste. The largest fraction of solid residue of this process is incineration bottom ash (IBA) requiring further treatment before applications such as in the construction industry become feasible. In this study, vitrification of IBA was conducted in a demonstration-scale high-temperature slagging gasification plant fueled with MSW and biomass charcoal as a green auxiliary fuel. High IBA co-feeding rates of up to 491 kg/h (equivalent to 107% of MSW feeding rate) were achieved during the trials. A highly leaching-resistant slag immobilizing heavy metals in the glass-like amorphous structure and recyclable iron-rich metal granules were generated in the process. The heavy metal migration into the solid by-product fractions depended on the IBA feeding rates and process conditions such as cold cap temperature, charcoal-to-ash ratio, and gasifier temperature profile. Slaked lime and activated carbon powder were used in a dry flue gas treatment and stack gas emissions were kept well below Singapore's regulatory limits. Steam from the hot flue gas was generated in a boiler to drive a steam turbine. The application of biomass charcoal instead of fossil fuels or electricity lead to a lower carbon footprint compared to alternative vitrification technologies. The overall results reveal promising application of high temperature slagging gasification process for commercial-scale vitrification of IBA.

Heavy metal migration regimes in the production of syngas from solid waste by thermal plasma treatment

In recent years, thermal plasma technology has been widely used in the harmless and resource-efficient treatment of solid waste (SW). This study investigates the migration behaviors of heavy metals during the thermal plasma treats SW to obtain the interphase structure change regimes of heavy metals. The transformation of SW under high-temperature environments was analyzed by Fluent simulation, and the composition of the crystalline phases and heavy metal content of the post-treatment slags were studied through a combination of XRD, SEM, and heavy metal leaching experiments. The results show that the thermal plasma provides a melting zone temperature of more than 4000 K, and the treated slag is mostly an amorphous solid composed of glassy Si-O mesh, which effectively encapsulates heavy metals and reduces their leaching rate. Additional analysis of the migration and transformation of heavy metals during thermal plasma treatment revealed that solid-phase heavy metals primarily took the form of sulphides and sulphates, while liquid- and gas-phase heavy metals were mostly oxides and chlorides. Simultaneously, Economic analysis results showed that the thermal plasma treats SW economically with an Energetic efficiency of up to 76.7%. The results of this study providing new insights into thermal plasma treatment SW research.

Effects of complex sulphur substrates on sludge bioleaching to improve heavy metal removal and microbial community diversity

In this study, H2S was used as a partial replacement nutrient substrate for sludge bioleaching. The effects of different combinations of H2S/sludge load and monomeric sulphur on heavy metal removal and microbial communities were investigated. Changes in pH, oxidation-reduction potential (ORP), SO42- concentration, heavy metal removal, and the content of heavy metal states during bioleaching were investigated, and community diversity analysis was performed. Daily introduction of H2S three times (at an interval of 8 h) at a gas flow rate of 2 ml/min and an H2S/sludge load of 15 ml/L with 5 g/L FeSO4·7H2O and 2 g/L monomeric sulphur as a nutrient substrate significantly accelerated both the bioleaching process and the pH drop in the sludge system, promoted the production of SO42-, and maintained a higher redox potential. The combination of H2S and monomeric sulphur had a significant effect on the leaching of heavy metals. Compared with the experimental group containing only H2S or monomeric sulphur, the removal rates of Zn, Ni, Pb, and Cr increased by 4.63%/13.8%, 8.5%/20.07%, 3.84%/9.5%, and 4.24%/8.02% respectively, while promoting the transformation of various heavy metal states to labile states, improving heavy metal stability, and reducing sludge ecotoxicity. High-throughput sequencing analysis showed that introducing the H2S gaseous matrix accelerated the decreasing trend of species number, bacterial abundance, and community diversity in the sludge system, promoting Proteobacteria as the dominant phylum, Acidithiobacillus, Metallibacterium, and Thiomonas as the dominant genera, and improving the bioleaching treatment effect.

Effect of acidic erosion on properties of solidified sewage sludge

Solidified sludge can be regarded as a new type of earth cover material for domestic waste landfill. But the acidic environment result from the leachate in landfill is a potential threat to cement-based material. In order to evaluate the deterioration risk of solidified sludge in acidic environment, the leaching process of solidified sludge components under different pH conditions was investigated by taking Ni and Cr as the indexes of semi-dynamic leaching test. Under strongly acid environment (pH = 2), the leaching rate of Cr is significantly higher than that in the weakly acid environment or nearly neutral environment, and the diffusion coefficient increased by an order of magnitudes. The leaching and diffusion coefficients of Ni undergo a small influence from the adding amount of cement and pH value. Both Ni and Cr have relatively low migration ratio.

Production of a new type of cemented paste backfill with solid waste from carbide slag, soda residue, and red mud: mechanism, optimization, and its environmental effects

To solve the disposal problems of carbide slag (CS), soda residue (SR), and red mud (RM) solid wastes, a new type of cemented paste backfill (CPB) was prepared with CS, SR, and RM solid wastes. The mixing proportion for the CPB was optimized by combining the Box‒Behnken design (BBD) response surface method and the satisfaction function method. The strength formation mechanism for the CPB was analyzed with physical and mechanical property tests, X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), etc. The safety of the CPB was evaluated with heavy metal leaching testing. The results showed that the 28-day unconfined compressive strength (UCS) of CPB first increased and then decreased with increasing CS/RM (0.2 ~ 0.6) and SR/RM (0.2 ~ 0.6); the optimum mixing ratios were CS/RM = 0.45 and SR/RM = 0.37, and the solid mass concentration was 64.75%; dense calcium silicate (aluminum) hydrate (C-S-H/C-A-S-H) bound to the solid particles of red mud and filled pores to provide early strength for the CPB, laminar interwoven Friedel's salt (Fs), ettringite and portlandite hydration products provided late strength for the CPB; and the leaching concentrations of five heavy metals (Fe, Mn, Cu, Zn, and Cr) in the solidified CPB were greatly reduced and far below the leaching limits specified in China's Quality Standard for Groundwater (GB/T 14848-2017).

Physical properties, chemical composition, and toxicity leaching of incineration fly ash by multistage water washing

Incineration fly ash contains a large amount of chloride, which limits the scope of its resource utilization. Water washing effectively removes chlorides and soluble substances, increasing the ability to dispose of them. The properties of incineration fly ash after multi-level water washing have been studied, providing theoretical guidance for the safe disposal of water-washed ash at all levels. Taking a practical project as an example, this paper analyzed the impact of three-stage countercurrent water washing on the physicochemical properties and toxicity leaching of incineration fly ash with different washing grades by XRD, BET, XRF, SEM, and ICP-MS. The results showed that with the improvement of washing grade, the removal rate of chloride ions was more than 86.96%. However, due to the removal of soluble substances, dioxins enriched from 98 ng-TEQ/kg of raw ash to 359 ng-TEQ/kg of tertiary washed incineration fly ash. Cr, Cu, and Zn also increased from 40.35 mg/L, 356.55 mg/L, and 3290.58 mg/L of raw ash to 136.30 mg/L, 685.75 mg/L, and 5157.88 mg/L, respectively. Pozzolanic activity had increased from 40.56% of the raw ash to 74.12% of the tertiary-washed incineration fly ash. There was no risk of excessive heavy metal leaching, and the dioxin content was lower than the raw ash in the primary washed incineration fly ash. After multi-stage water washing, incineration fly ash accumulated heavy metals, so more attention must be paid to the issue of heavy metal content in the safe disposal process.

Feasibility analysis of resource application of waste incineration fly ash in asphalt pavement materials

To confirm the feasibility of waste incineration fly ash applied in asphalt pavement materials, the waste fly ash from the waste incineration station in Dongguan (DG-FA) and Guangzhou (GZ-FA) were selected as mineral filler replacing limestone powder (LF) to prepare asphalt mortar. The physical properties, chemical composition, and thermal characteristics of FA were analyzed. The effect of FA on the physical properties and rheological properties of asphalt binder was investigated systematically. The micromorphology of FA asphalt mortar was characterized. Finally, the blocking effect of asphalt binder on the leaching of toxic elements from FA was evaluated through XRF test. The results showed that the granular composition of FA particles was similar to that of LF. Furthermore, compared to LF, the specific surface area and the pore structure of FA were more developed, and the high active amorphous material was higher. Adding FA to asphalt mortar and increasing replacing amount improved its high-temperature rutting resistance as well as its ability to adhere to asphalt, while the low-temperature crack resistance was decreased. Further, the asphalt binder provided the good shielding effect against the migration of heavy metals in FA. While the leaching concentration of Pb element slightly exceeded the hazardous waste leaching standard (GB5085.3-2007), the remaining elements met the standard requirements. Overall, FA improved asphalt mortar performance, and the asphalt had a good curing and stabilizing effect on the toxic elements in fly ash, indicating that FA could be used as a filler in asphalt pavements.

Biochar as additive for improved building performances and heavy metals solidification of sediment-based lightweight concrete

The sustainable disposal of large volumes of contaminated dredged river sediment has become a challenge for municipal management. In this study, a cutting-edge biochar application method was innovated, which converted the polluted dredged sediment into a low-carbon and environmentally friendly building material through an autoclave-free method. As the amount of biochar addition increased from 0 to 2% (w/w), the compressive strength of the dredged sediment-based lightweight concrete (DS-LC) increased from 3.92 to 4.61 MPa. Accordingly, the thermal conductivity decreased from 0.237 to 0.222 W/(m K), the water absorption decreased by 6%, and the water resistance coefficient increased by 33%. Results of X-ray diffraction (XRD) and thermogravimetric (TG) analysis showed that biochar promoted the hydration reaction and the carbonation process. Scanning electron microscopy (SEM) attached with energy-dispersive X-ray spectroscopy (EDX) showed that biochar addition changed the microstructure of the DS-LCs, which made the pore distribution more uniform and densified. Biochar addition also strengthened the immobilization of heavy metals (Cu, Zn, Cr, and As) by approximately 18-27% and combination of biochar and silica fume could increase the heavy metal immobilization by 28-44%. Compared with the traditional concrete material, the DS-LC with biochar addition could not only reduce the carbon emission but also has potential economic benefit for the treatment and utilization of dredged sediment.

Mobility and environmental fate of heavy metals in fine fraction of dumped legacy waste: Implications on reclamation and ecological risk

The study evaluates mobility and chemical speciation of heavy metals (HMs) in fine fraction (<4 mm) collected from municipal solid waste dumpsite located in Mumbai, India to assess its reclamation feasabilty. A total of fifteen samples were collected from five zones (named chronologically, zone A to zone E, with the increasing waste age) at 1-m depth interval to understand temporal variation in mobility and potential pollution risk of heavy metals. The results depicted that Zn had the highest concentration in the fine fraction, followed by Cu > Cr > Pb > Ni > Cd. Furthermore, HMs concentration showed increasing trend with the waste age. According to hiererachial cluster analysis, two cluster were observed in the analyzed samples which could be differentiated with age of the waste. Further, the chemical speciation differed for all assessed HMs and had a significant change with age. HMs were dominant in non-bioavailable forms, except for Cd that had significant distribution in all forms. In addition, Cd (23%) and Zn (17%) showed high mobility amongst all analyzed HM, while Cr (0.4%) had the least. According to pollution assessment and chemical speciation results, Cd was identified as the most polluting and mobile HM. The study shows that the mobile form of HMs is low and needs to be considered while deciding the remediation routes and environment hazards.

Influence of speciation distribution and particle size on heavy metal leaching from MSWI fly ash

Fly ash from municipal solid waste incineration (MSWI-FA) contains leachable heavy metals. In the present study the correlations between heavy metal content, particle size, speciation distribution with respect to water leaching are investigated, using a combination of solid-state bulk analytical techniques, leaching treatments, sequential extractions and thermodynamic geochemical modelling. Among the analyzed heavy metals, Zn and Pb are the most abundant in any grain size class, followed by Cu, Cr, Cd and Ni, with concentration that tends to increase with a decrease of the grain size. The phase composition is constituted of salt (halite, sylvite, anhydrite and syngenite), which provide the main minerals regardless of the particle size class; calcite, quartz and gehlenite occur in comparatively lower amounts, while 50% wt is composed of amorphous fraction. Heavy metal leaching is strongly correlated to speciation distribution, and in particular to the fraction (F1) associated with salt, carbonate and weak surface sorption. Leaching from speciation due to surface complexation on Al/Fe (hydr)oxide becomes relevant at acidic regime. Particle size and heavy metal content, in turn, moderately correlate with leaching. The F1-speciation as a function of particle size does not exhibit a definite trend shared by all heavy metals under investigation. This suggests that i) differences in speciation distribution, rather than bare heavy metal content or particle size, govern leaching from MSWI-FA; ii) F1 can be regarded as a marker of the potential heavy metal leaching; iii) a comparatively modest efficiency in managing MSWI-FA is expected from grain size separation strategies.

Comparison of MSWI fly ash from grate-type and circulating fluidized bed incinerators under landfill leachate corrosion scenarios: the long-term leaching behavior and speciation of heavy metals

In this study, the long-term leaching behaviors of Cd, Cr, Cu, Ni, Pb, and Zn in municipal solid waste incineration (MSWI) fly ash samples from grate-type (GT) and circulating fluidized bed (CFB) incinerators were investigated and compared under the simulated landfill leachate corrosion scenario, which was determined to be more severe than the acid rain corrosion scenario. The total heavy metal contents showed increasing hierarchies of Ni<Cr<Cd<Cu<Pb<Zn in the GT fly ash samples and Cd<Ni<Cr<Pb<Cu<Zn in the CFB fly ash samples. During the leaching processes, all heavy metals followed the two-stage leaching mode, including quick accumulation in stage 1 and then stable release in stage 2. The heavy metals with the highest accumulative leaching amounts were Cd, Pb, and Zn in GT fly ash and Cr, Cu, and Ni in CFB fly ash. In the landfill leachate corrosion scenario, Cd and Cr showed cationic patterns while Pb, Zn, and Cu showed amphoteric patterns. The leaching of Cd, Ni, and Cr arose from the dissolution of the salts they formed (solubility control), while the leaching of Cu, Pb, and Zn was controlled by the Ca-bearing compounds (sorption and precipitation control). A large difference in Pb leaching was observed: the cumulative leaching amount of GT fly ash (707.59-3072.36 mg/kg) was an order of magnitude higher than that of CFB fly ash (22.47-407.314 mg/kg), as a result of the higher primary content and larger proportion of the residual fraction in CFB fly ash. The acid-soluble and reducible fractions exhibited higher percentages than those of other fractions representing higher levels of environmental toxicity and risk. Therefore, more emphasis should be placed on the conversion of bioavailable fractions into stable fractions for the stabilization and utilization of MSWI fly ash.

Ecological and human health risk assessment of metals leached from end-of-life solar photovoltaics

Photovoltaic industry has shown tremendous growth among renewable energy sector. Though, this high installation rate will eventually result in generation of large volume of end-of-life photovoltaic waste with hazardous metals. In present study, reported leached metal contents from different photovoltaics in previous investigations were utilized for (i) potential fate and transport analysis to soil and groundwater and, (ii) estimating ecological and human health risks via dermal and ingestion pathways for child and adult sub-populations. The results indicate that the children are at highest risk, mainly due to lead (hazard quotient from 1.2 to 2.6). Metals, such as cadmium, lead, indium, molybdenum and tellurium pose maximum risks for child and adult sub-populations via soil-dermal pathway followed by soil-ingestion pathway. This is further proved by calculated high values of contamination factor and geo-accumulation index for cadmium (102.4), indium (238.9) and molybdenum (16.12). The estimated soil contamination is significant with respect to aluminium, silver, cadmium, iron, lead, however, groundwater contamination was insignificant. Exposure to polluted soils yields an aggregate hazard index (for non-cancer effects) > 1 for all four pathways, with soil dermal pathway as the major contributor. Lead poses significant cancer risk for all scenarios (average risk: 0.0098 to 0.047 (soil) and 2.1 × 10^-5 to 3.5 × 10^-5 (groundwater)), whereas acceptable non-cancer risk was observed for other metals from groundwater exposure. Further, variance contribution and spearman correlation coefficient analysis show that metal concentration, exposure frequency and ingestion rate are the main contributors towards overall uncertainty in risk estimates. More detailed assessment for environmentally-sensitive metals should be carried out by considering other field breakage scenarios also, although the assessment suggests low risk for majority of metals examined.

Nitrogen induced DOC and heavy metals leaching: Effects of nitrogen forms, deposition loads and liming

Atmospheric nitrogen (N) deposition is believed to accelerate dissolved organic carbon (DOC) production and could lead to increased heavy metal mobility into water resources. We sampled intact soil cores from the Isle of Skye with low background N deposition history and having Serpentine rock known for its higher heavy metal concentrations including zinc (Zn), copper (Cu), nickel (Ni) and lead (Pb). The effects of 16 (16kgN) and 32 kg N ha^-1 year^-1 (32kgN), and liming with 32kgN (32kgN+Lime) on soil solution chemistry and heavy metal mobilization were investigated over the 15-month study. Nitrogen in deposition load was added at five ammonium (NH₄⁺) to nitrate (NO₃^-) ratios of 9:1, 5:1, 1:1, 1:5 and 1:9 along NO₃^-dominance. We found significant effects of load on Cu and NH₄⁺/NO₃^- ratio on pH, DOC and Zn in soil solution. However, under lime and ratio experimental factors, liming significantly influenced pH, DOC, Cu and Pb, and NH₄⁺/NO₃^- ratio pH, DOC, Ni and Zn whereas interactions between lime and ratio was significant for Ni and Cu. pH and DOC increased with N load, liming and NO₃^- dominance, and both correlated significantly positively. Liming under NH₄⁺ dominance enhanced DOC production due to supply of base cations in lime. Mobilization of Cu, Ni and Pb was driven by DOC concentrations and, therefore, increased with load, liming and NO₃^- dominance in deposition. However, in contrast, low pH and high NH₄⁺ dominance was associated with Zn mobilization in soil solution. On the contrary, despite of some patterns, heavy metals in soil HNO₃ extracts were devoid of any load, lime and NH₄⁺/NO₃^- ratio effects. Our study suggests that the effects of N load and forms in deposition on sites with high accumulated loads of metals need to be better quantified through soil solution partitioning models.

Pyrolytic valorization of water treatment residuals containing powdered activated carbon as multifunctional adsorbents

This study investigated the possibility of applying pyrolysis as an alternative method to recycle powdered activated carbon-containing water treatment residuals (PAC-WTRs) discharged from the Cheongju water treatment plant as a multifunctional adsorbent. WTRs pyrolyzed for 1 h at 200-700 °C were compared with raw material. The carbon content of the PAC-WTR reaches 19.27%, with about 25% Al and 17% Si. Changes in PAC through pyrolysis imparted new adsorbent properties to WTR. As the pyrolysis temperature increased, the purity of PAC increased, and pores were regenerated to recover the Brunauer-Emmett-Teller (BET) from 6.5 m² g^-1 to 131.8 m² g^-1. In addition, the basicity increased as the carboxylic and phenolic groups on the carbon surface were decomposed, which increased the cation (methylene blue) adsorption capacity and reduced heavy metal leaching. As the coagulant regenerated with increasing pyrolysis temperature, the amount of aluminum leached and phosphate removal efficiency were increased. In the case of simultaneous removal of cations (MB⁺) and anions (PO₄^3-), the removal efficiency was higher than that for single adsorption without competition through multi-layer adsorption by Al complex and PAC complex. Therefore, the pyrolyzed PAC-WTR is capable of adsorbing and removing anions and cations simultaneously without the peril of substance leaching. The regenerated WTRs containing PAC is expected to be utilized as a multi-functional remediation material for wastewater containing various pollutants.

Mobility of arsenic, chromium and copper arising from soil application of stabilised aggregates made from contaminated wood ash

Stabilized cementitious aggregates AG were produced from wood ashes containing ∼10,000 mg kg^-1 As, Cr and Cu, then amended to two agricultural pasture soils. Metal(loid) leaching (column tests), mobility (pore water extracts) and uptake to ryegrass was determined, comparing raw ashes with aggregates. Risk modeling was applied to selected data to inform wider discussion of the experimental results. Under rapid leaching (7 h) AG 2 (pre-strengthened with CO₂) outperformed AG 1 in suppressing soluble metal(loid) removal. During prolonged leaching (12d) both aggregates were susceptible to mild dissolution/release of metal(loid)s upon acidification. Pore water sampled from the pot test indicated that Cr was generally most mobile, As least so, reduced furthest by AG 2. Risk modelling, based on pot experimental data, demonstrated soil specific accumulation of As in beef muscle and milk, being furthest reduced (compared to the raw ash addition) by AG 2 in soil A, but increased in soil B by the same treatment. The results of this study indicate that a reduction in soluble As, Cr and Cu can be achieved through cementitious aggregation of wood ashes, though the extent is metal(loid) specific when amended to soils. Pre-testing under local soil conditions before field application would be required to ensure that metal(loid) mobility remained suppressed.

Recycled concrete aggregate as road base: Leaching constituents and neutralization by soil Interactions and dilution

Recycled Concrete Aggregate (RCA) is often used as a replacement for natural aggregate in road construction activities because of its excellent mechanical properties, and this trend should increase as more transportation departments include RCA in specifications and design manuals. Concerns raised by some engineers and contractors include impacts from leachate generated by RCA, both from transport of metals to water sources and the impact of a high pH leachate on corrosion of underlying metal drainage pipes. In this study, RCA collected from various regions of Florida exhibited pH ranging from 10.5 to 12.3. Concentrations of Al, Ba, Cr, Fe, Mo, Na, Ni, Sb, and Sr measured using batch leaching tests exceeded applicable risk-based thresholds on at least some occasions, but the concentrations measured suggest that risk to water supplies should be controlled because of dilution and attenuation. Two mechanisms of pH neutralization were evaluated. Soil acidity plays a role, but laboratory testing and chemical modeling found that at higher liquid-to-solid ratios the acidity is exhausted. If high pH leachate did reach groundwater, chemical modeling indicated that groundwater dilution and carbonation would mitigate groundwater pH effects.

Evaluation of recycled concrete aggregates for their suitability in construction activities: An experimental study

Construction and demolition waste disposal is a major challenge in developing nations due to its ever increasing quantities. In this study, the recycling potential of waste concrete as aggregates in construction activities was studied. The metal leaching from the recycled concrete aggregates (RCA) collected from the demolition site of a 50year old building, was evaluated by performing three different leaching tests (compliance, availability and Toxic Characteristic Leaching Procedure). The metal leaching was found mostly within the permissible limit except for Hg. Several tests were performed to determine the physical and mechanical properties of the fine and coarse aggregates produced from recycled concrete. The properties of recycled aggregates were found to be satisfactory for their utilization in road construction activities. The suitability of using recycled fine and coarse aggregates with Portland pozzolanic cement to make a sustainable and environmental friendly concrete mix design was also analyzed. No significant difference was observed in the compressive strength of various concrete mixes prepared by natural and recycled aggregates. However, only the tensile strength of the mix prepared with 25% recycled fine aggregates was comparable to that of the control concrete. For other mixes, the tensile strength of the concrete was found to drop significantly. In summary, RCA should be considered seriously as a building material for road construction, mass concrete works, lightly reinforced sections, etc. The present work will be useful for the waste managers and policy makers particularly in developing nations where proper guidelines are still lacking.

Comparative characterization of sewage sludge compost and soil: Heavy metal leaching characteristics

The leaching and accumulation of heavy metals are major concerns following the land application of sewage sludge compost (SSC). We comparatively characterized SSC, the reference soil, and the SSC amended soil to investigate their similarities and differences regarding heavy metal leaching behavior and then to evaluate the effect of SSC land application on the leaching behavior of soil. Results showed that organic matter, including both of particulate organic matter (POM) and dissolved organic matter (DOM), were critical factors influencing heavy metal leaching from both of SSC and the soil. When SSC was applied to soil at the application rate of 48t/ha, the increase of DOM content slightly enhanced heavy metal leaching from the amended soil over the applicable pH domain (6<pH<8) and at alkaline pH, and led to more DOM-bound species in the liquid phases. However, the increase of POM content with the SSC application had less influence on the leaching behavior of heavy metals. The geochemical speciation modeling revealed that heavy metal speciation in the solid phase were similar between the reference soil and the amended soil.

Variation of the phytotoxicity of municipal solid waste incinerator bottom ash on wheat (Triticum aestivum L.) seed germination with leaching conditions

Municipal solid waste incinerator bottom ash (MSWIBA) has long been regarded as an alternative building material in the construction industry. However, the pollutants contained in the bottom ash could potentially leach out and contaminate the local environment, which presents an obstacle to the reuse of the materials. To evaluate the environmental feasibility of using MSWIBA as a recycled material in construction, the leaching derived ecotoxicity was assessed. The leaching behavior of MSWIBA under various conditions, including the extractant type, leaching time, liquid-to-solid (L/S) ratio, and leachate pH were investigated, and the phytotoxicity of these leachates on wheat (Triticum aestivum L.) seed germination was determined. Moreover, the correlation between the germination index and the concentrations of various chemical constituents in the MSWIBA leachates was assessed using multivariate statistics with principal component analysis and Pearson's correlation analysis. It was found that, heavy metal concentrations in the leachate were pH and L/S ratio dependent, but were less affected by leaching time. Heavy metals were the main pollutants present in wheat seeds. Heavy metals (especially Ba, Cr, Cu and Pb) had a substantial inhibitory effect on wheat seed germination and root elongation. To safely use MSWIBA in construction, the potential risk and ecotoxicity of leached materials must be addressed.

Release of soluble elements from biochars derived from various biomass feedstocks

Biochar as soil amendment can increase soil carbon (C) sequestration and mineral nutrients; however, some of its soluble elements may also be unintentionally released during the application. In this work, eight types of biochars were derived from herbaceous, woody, and waste (tailing, manure, sludge) biomass feedstocks through slow pyrolysis at 600 °C in N2. The elemental composition, specific surface area, morphology, crystalline phases, thermal stability, surface functional groups, and pH of the point of zero charge of the biochars were determined using various methods. These properties varied significantly among the tested biochars, suggesting that feedstock type played an important role in controlling their properties. Laboratory release and toxicity characteristic leaching procedure extraction experiments were conducted to evaluate the potential release of nutritious and toxic element from biochars. Results showed that all the biochars released nutritious elements and thus, may be beneficial to plants when amended in soils. In general, biochars produced from herbaceous and woody biomass feedstocks showed low risks of releasing toxic elements. Biochar derived from sludge, however, might present ecotoxicological challenges for its environmental applications due to the release of toxic elements, such as heavy metals.