Effect of alkaline washing treatment on leaching behavior of municipal solid waste incineration bottom ash.

This study aimed to find an effective, inexpensive, and safe washing treatment for municipal solid waste incineration bottom ash (MSWIBA) in order to reduce its potential harmful effects in disposal and recycling. The washing solutions, namely tap water (TW), saturated lime water (SLW), and wastewater from concrete batching plant (WW) were used to wash MSWIBA at different liquid-solid (L/S) ratios and for different durations. Leaching behavior of some heavy metals, chloride, and sulfate from MSWIBA was tested and evaluated. From the TCLP leaching test, when the L/S ratio was above 5, WW was the most effective solution in reducing As, Cd, Se, and Sb emissions from MSWIBA. The calcium and iron ions present in the WW were essential for controlling the leaching of As, Cd, and Sb from MSWIBA due to the formation of stable crystalline pharmacosiderite, cadmium hydroxide sulfate, and hydromeite during the washing process. Using WW showed the best effect in removing sulfate from MSWIBA. At a L/S ratio of 10, about 83% of the sulfate could be removed from MSWIBA after 20 min of washing. The L/S ratio was most influential in removing chloride from MSWIBA. The three washing treatments chosen were effective in reducing the chloride level in MSWIBA to below the level of hazardous waste. Nevertheless, there were still substantial amounts of chloride remaining in the treated MSWIBA. Under the Dutch Building Materials Decree, the treated MSWIBA may be used as a building material in parts which allow isolation, control, and monitoring (ICM).

Recycling of phosphogypsum and red mud in low carbon and green cementitious materials for vertical barrier.

MgO activated slag and bentonite (MASB) slurry is a new and promising vertical barrier material along with excellent performances. Some solid wastes, such as phosphogypsum (PG), red mud (RM), fly ash and so on, show a positive effect on the performances of alkali activated slag. However, few studies focus on the recycling of these solid wastes in the system of MgO activated slag. The purpose of this paper is to study the incorporation of phosphogypsum and red mud on the mechanical property, permeability and hydration process of MASB slurry. The results showed that the addition of PG could significantly improve the mechanical strength and anti-permeability of the MASB slurry at early age (7 days), where the unconfined compressive strength (UCS) increased from 793.1 kPa to 1395.7 kPa and the permeability coefficient declined from 16.1 × 10-7 cm/s to 1.7 × 10-7 cm/s. In contrast, the introduction of RM had some negative effects on its macroscopic properties, resulting the UCS decreased to 580.4 kPa and the permeability coefficient rose to 25.9 × 10-7 cm/s at 7 days. The ettringite formed in the PG blended MASB slurry led to a notable increase in the absolute solid volume, which could satisfactorily fill the pores and block the pore channels. The combined addition of RM and PG had a synergistic effect on the promotion of hydration process and optimization of the pore structure, contributing to establish a low permeability and high mechanical strength matrix. The overall findings indicate that the use of solid wastes in the MASB slurry can not only improve its engineering properties, but also promotes its sustainability and economical efficiency, holding a great potential for popularization and application.

Novel recycling of phosphorus-recovered incinerated sewage sludge ash residues by co-pyrolysis with lignin for reductive/sorptive removal of hexavalent chromium from aqueous solutions.

Incinerated sewage sludge ash (ISSA), a by-product generated from the combustion of dewatered sewage sludge, has been extensively studied as a secondary resource for phosphorus recovery by acid extraction methods. Recycling of the P-recovered ISSA residues is crucial to complete and sustain the whole process. In this study, the ISSA residue rich in iron was reused and co-pyrolyzed with lignin at 650, 850 and 1050 °C under N2 atmosphere for the synthesis of a composite material to remove hexavalent chromium (Cr(VI)) from aqueous solutions. Characterization analysis including XRD, XPS, and FTIR showed that iron oxides in the residue were reduced to zero valent iron at 1050 °C that exhibits the optimal Cr(VI) removal performance. The Cr(VI) removal process was rapid and reached a plateau at around 30 min. The maximum removal rate was obtained at pH 2.0, which was conducive for the treatment of a synthetic Cr(VI)-containing wastewater in fix-bed column experiments, whereby Cr(VI) as well as total Cr were continuously removed. Overall, this study proposed a new routine for the recycling of ISSA residue after phosphorus recovery by the acid extraction method and provided a value-added product for Cr(VI) removal from wastewaters.

Alkaline modification of the acid residue of incinerated sewage sludge ash after phosphorus recovery for heavy metal removal from aqueous solutions.

Enriched in phosphorus, sewage sludge ash has been extensively studied and applied as a secondary source for phosphorus recovery. Wet extraction, especially acid washing, is one of the most feasible methods to recover phosphorus from the ash due to its ease of operation, high efficiency and low cost. However, the management of the resultant acid residue was seldom addressed. In this study, special focus was paid to the reuse and recycling of the acid residue by an alkaline activation method. Its adsorption performance towards four different heavy metals in aqueous solutions was evaluated by batch and fixed-bed column adsorption experiments. The obtained material showed a high BET specific area (98.29 m2/g) and a total pore volume (0.114 cm3/g), and effectively removed Cd(II), Cu(II), Pb(II) and Zn(II) from aqueous solutions with the maximum adsorption capacity of around 26.8, 22.2, 53.3 and 13.5 mg/g respectively. It could be loaded in a fixed-bed column to continuously remove heavy metals especially for Pb(II). The proposed method to recycle the acid residue makes the wet extraction methods designing to recover phosphorus from incinerated sewage sludge complete without the generation of waste, which contributes to circular economy and a sustainable future.

Tailoring acidity and porosity of alumina catalysts via transition metal doping for glucose conversion in biorefinery.

Efficient conversion of food waste to value-added products necessitates the development of high-performance heterogeneous catalysts. This study evaluated the use of Al2O3 as a low-cost and abundant support material for fabricating Lewis acid catalysts, i.e., through the in-situ doping of Cu, Ni, Co, and Zr into Al2O3 followed by calcination. The characterisation results show that all catalysts were mainly amorphous. In particular, adding the transition metals to the Al2O3 matrix resulted in the increase of acidity and meso-/micro-pores. The catalysts were evaluated in the conversion of glucose, which can be easily derived from starch-rich food waste (e.g., bread waste) via hydrolysis, to fructose in biorefinery. The results indicate that the Ni-doped Al2O3 (Al-Ni-C) achieved the highest fructose yield (19 mol%) and selectivity (59 mol%) under heating at 170 °C for 20 min, of which the performance falls into the range reported in literature. In contrast, the Zr-doped Al2O3 (Al-Zr-C) presented the lowest fructose selectivity despite the highest glucose conversion, meaning that the catalyst was relatively active towards the side reactions of glucose and intermediates. The porosity and acidity, modified via metal impregnation, were deduced as the determinants of the catalytic performance. It is noteworthy that the importance of these parameters may vary in a relative sense and the limiting factor could shift from one parameter to another. Therefore, evaluating physicochemical properties as a whole, instead of the unilateral improvement of a single parameter, is encouraged to leverage each functionality for cost-effectiveness. This study provides insights into the structure-performance relationships to promote advance in catalyst design serving a sustainable food waste biorefinery.

Green remediation of contaminated sediment by stabilization/solidification with industrial by-products and CO2 utilization.

Navigational dredging is an excavation of marine/freshwater sediment to maintain channels of sufficient depth for shipping safety. Due to historical inputs of anthropogenic contaminants, sediments are often contaminated by metals/metalloids, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and other contaminants. Its disposal can present significant environmental and financial burdens. This study developed a novel and green remediation method for contaminated sediment using stabilization/solidification with calcium-rich/low-calcium industrial by-products and CO2 utilization. The hydration products were evaluated by quantitative X-ray diffraction analysis and thermogravimetric analysis. The incorporation of calcium carbide residue (CCR) facilitated hydration reaction and provided relatively high 7-d strength. In contrast, the addition of Class-F pulverized fly ash (PFA) and ground granulated blast furnace slag (GGBS) was beneficial to the 28-d strength development due to supplementary pozzolanic and hydration reactions. The employment of 1-d CO2 curing was found to promote strength development (98%) and carbon sequestration (4.3wt%), while additional 7-d air curing facilitated cement rehydration and further carbonation in the sediment blocks. The leachability tests indicated that all studied binders, especially CCR binder, effectively immobilized contaminants in the sediments. The calcium-rich CCR and GGBS were regarded as promising candidates for augmenting the efficacy of CO2 curing, whereas GGBS samples could be applicable as eco-paving blocks in view of their superior 28-d strength. This study presents a new and sustainable way to transform contaminated sediment into value-added materials.

Change in re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus.

The potential of six different extractants to recover phosphorus (P) from incinerated sewage sludge ash (ISSA) was evaluated. Secondary effects such as the co-dissolution of Zn and Cu were also considered. The residual ISSA from each study was assessed in particular detail, focusing on the leachability of remaining Zn and Cu, major element composition, crystalline phases and overall degree of crystallinity and particle size distribution. The residual ISSA was also evaluated as a pozzolanic material using a Strength Activity Index (SAI) test with mortars containing Portland cement with a 20% substitution by ISSA. All results were compared to tests with untreated ISSA. Overall, the use of 3 of the 6 extractants could be ruled out due to poor P recovery potential and/or a serious compromise of the potential reuse of residual ISSA in Portland cement-based materials. The results highlight the added value of considering the potential reuse of residual ISSA when trying to optimize P recovery from ISSA by wet methods.

Phosphorus recovery and leaching of trace elements from incinerated sewage sludge ash (ISSA).

Chemical extraction of phosphorus (P) from incinerated sewage sludge ash (ISSA) is adversely influenced by co-dissolution of metals and metalloids. This study investigated P recovery and leaching of Zn, Cu, Pb, As and Ni from ISSA using inorganic acids (sulphuric acid and nitric acid), organic acids (oxalic acid and citric acid), and chelating agents (ethylenediaminetetraacetic acid (EDTA) and ethylene diamine tetramethylene phosphonate (EDTMP)). The aim of this study was to optimize a leaching process to recover P-leachate with high purity for P fertilizer production. The results show that both organic and inorganic acids extract P-containing phases but organic acids leach more trace elements, particularly Cu, Zn, Pb and As. Sulphuric acid was the most efficient for P recovery and achieved 94% of total extraction under the optimal conditions, which were 2-h reaction with 0.2 mol/L H2SO4 at a liquid-to-solid ratio of 20:1. EDTA extracted only 20% of the available P, but the leachates were contaminated with high levels of trace elements under optimum conditions (3-h reaction with EDTA at 0.02 mol/L, pH 2, and liquid-to-solid ratio of 20:1). Therefore, EDTA was considered an appropriate pre-treatment agent for reducing the total metal/metalloid content in ISSA, which produced negligible changes in the structure of ISSA and reduced contamination during subsequent P extraction using sulphuric acid.

Comparative environmental evaluation of construction waste management through different waste sorting systems in Hong Kong.

This study aimed to compare the environmental performance of building construction waste management (CWM) systems in Hong Kong. Life cycle assessment (LCA) approach was applied to evaluate the performance of CWM systems holistically based on primary data collected from two real building construction sites and secondary data obtained from the literature. Different waste recovery rates were applied based on compositions and material flow to assess the influence on the environmental performance of CWM systems. The system boundary includes all stages of the life cycle of building construction waste (including transportation, sorting, public fill or landfill disposal, recovery and reuse, and transformation and valorization into secondary products). A substitutional LCA approach was applied for capturing the environmental gains due to the utilizations of recovered materials. The results showed that the CWM system by using off-site sorting and direct landfilling resulted in significant environmental impacts. However, a considerable net environmental benefit was observed through an on-site sorting system. For example, about 18-30kg CO2 eq. greenhouse gases (GHGs) emission were induced for managing 1 t of construction waste through off-site sorting and direct landfilling, whereas significant GHGs emission could be potentially avoided (considered as a credit -126 to -182kg CO2 eq.) for an on-site sorting system due to the higher recycling potential. Although the environmental benefits mainly depend on the waste compositions and their sortability, the analysis conducted in this study can serve as guidelines to design an effective and resource-efficient building CWM system.

Fate of metals before and after chemical extraction of incinerated sewage sludge ash.

Chemical extraction of incinerated sewage sludge ash (ISSA) can effectively recycle P, but it may change the speciation and mobility of the remaining metals. This study investigated the changes of the leaching potential and distribution of metals in the chemically extracted ISSA. Batch extraction experiments with different extractants, including inorganic acids, organic acids, and chelating agents, were conducted on the ISSA collected from a local sewage sludge incinerator. The extraction of Zn, Cu, Pb, Ni, Cd, Ba, Cr and As from the ISSA and the corresponding changes of the mobility and speciation were examined. The results showed that the metals in ISSA were naturally stable because large portions of metals were associated with the residual fraction. The inorganic (HNO3 and H2SO4) and organic acids (citric acid and oxalic acid) significantly co-dissolved the metals through acid dissolution, but the reduction in the total concentrations did not tally the leaching potential of the residual metals. The increase in the exchangeable fraction due to destabilization by the extractants significantly enhanced the mobility and leachability of the metals in the residual ISSA. Chelating agents (EDTA and EDTMP) only extracted a small quantity of metals and had a marginal effect on the fate of the residual metals, but they significantly reduced the Fe/Mn oxide-bound fraction. In comparison, the bioaccessibility of residual metals were reduced to varying extent. Therefore, the disposal or reuse of chemically extracted ISSA should be carefully evaluated in view of possible increase in mobility of residual metals in the environment.

Arsenic-containing soil from geogenic source in Hong Kong: Leaching characteristics and stabilization/solidification.

Geogenic sources of arsenic (As) have aroused extensive environmental concerns in many countries. This study evaluated the vertical profiles, leaching characteristics, and surface characteristics of As-containing soils in Hong Kong. The results indicated that elevated levels of As (486-1985 mg kg-1) were mostly encountered in deeper layer (15-20 m below ground). Despite high concentrations, geogenic As displayed a high degree of chemical stability in the natural geochemical conditions, and there was minimal leaching of As in various leaching tests representing leachability, mobility, phytoavailability, and bioaccessibility. Microscopic/spectroscopic investigations suggested that As in the soils was predominantly present as As(V) in a coordination environment with Fe oxides. Sequential extraction indicated that the majority of As were strongly bound with crystalline Fe/Al oxides and residual phase. Yet, uncertainties may remain with potential As exposure through accidental ingestion and abiotic/biotic transformation due to changes in geochemical conditions. Hence, the effectiveness of stabilization/solidification (S/S) treatment was evaluated. Although the leached concentrations of As from the S/S treated soils increased to varying extent in different batch leaching tests due to the increase in alkalinity, the mobility of As was considered very low based on semi-dynamic leaching test. This suggested that As immobilization in the S/S treated soils was predominantly dependent on physical encapsulation by interlocking framework of hydration products, which could also prevent potential exposure and allow controlled utilization of S/S treated soils as monolithic materials. These results illustrate the importance of holistic assessment and treatment/management of As-containing soils for enabling flexible future land use.

Mixture design and treatment methods for recycling contaminated sediment.

Conventional marine disposal of contaminated sediment presents significant financial and environmental burden. This study aimed to recycle the contaminated sediment by assessing the roles and integration of binder formulation, sediment pretreatment, curing method, and waste inclusion in stabilization/solidification. The results demonstrated that the 28-d compressive strength of sediment blocks produced with coal fly ash and lime partially replacing cement at a binder-to-sediment ratio of 3:7 could be used as fill materials for construction. The X-ray diffraction analysis revealed that hydration products (calcium hydroxide) were difficult to form at high sediment content. Thermal pretreatment of sediment removed 90% of indigenous organic matter, significantly increased the compressive strength, and enabled reuse as non-load-bearing masonry units. Besides, 2-h CO2 curing accelerated early-stage carbonation inside the porous structure, sequestered 5.6% of CO2 (by weight) in the sediment blocks, and acquired strength comparable to 7-d curing. Thermogravimetric analysis indicated substantial weight loss corresponding to decomposition of poorly and well crystalline calcium carbonate. Moreover, partial replacement of contaminated sediment by various granular waste materials notably augmented the strength of sediment blocks. The metal leachability of sediment blocks was minimal and acceptable for reuse. These results suggest that contaminated sediment should be viewed as useful resources.