The fate of heavy metals during combustion and gasification of contaminated biomass-a brief review

Implications of fuel poverty for indoor black carbon concentrations from space heating

Despite the global transition towards cleaner energy sources observed over the last decade, disparities in access persist worldwide. The dependence on biomass for household heating exacerbates fuel poverty, as economically vulnerable households face challenges in obtaining certified firewood and often resort to using contaminated biomass as a substitute, either partially or completely. We examined black carbon (BC) particle concentrations -a marker for combustion- during wood stove operation through a five-day case study in a typical Chilean household. BC increased rapidly following the ignition of the stove, with the combustion of dry Eucalyptus globulus logs yielding a substantially lower peak (5.29 μg/m3) than when using unclean biomass: 35.75 μg/m3 with demolition wood and painted furniture, and 87.11 μg/m3 with the addition of a blend of particleboard with polystyrene foam. During the latter two events, BC particles remained indoors for about 20 h before the concentrations reverted to pre-spike levels. The slow decay in BC concentrations was further influenced by the infiltration of outdoor air. The mean indoor BC concentrations were comparable to or even exceeded those observed on busy roads in major cities worldwide. These results highlight the risks associated with limited access to clean fuels for indoor heating, alongside inadequate insulation. This study sheds light on the problem of fuel poverty and its adverse effects on health and well-being.

Characterization of Wildland Firefighters' Exposure to Coarse, Fine, and Ultrafine Particles; Polycyclic Aromatic Hydrocarbons; and Metal(loid)s, and Estimation of Associated Health Risks

Firefighters' occupational activity causes cancer, and the characterization of exposure during firefighting activities remains limited. This work characterizes, for the first time, firefighters' exposure to (coarse/fine/ultrafine) particulate matter (PM) bound polycyclic aromatic hydrocarbons (PAHs) and metal(loid)s during prescribed fires, Fire 1 and Fire 2 (210 min). An impactor collected 14 PM fractions, the PM levels were determined by gravimetry, and the PM-bound PAHs and metal(loid)s were determined by chromatographic and spectroscopic methodologies, respectively. Firefighters were exposed to a total PM level of 1408.3 and 342.5 µg/m3 in Fire 1 and Fire 2, respectively; fine/ultrafine PM represented more than 90% of total PM. Total PM-bound PAHs (3260.2 ng/m3 in Fire 1; 412.1 ng/m3 in Fire 2) and metal(loid)s (660.8 ng/m3 versus 262.2 ng/m3), distributed between fine/ultrafine PM, contained 4.57-24.5% and 11.7-12.6% of (possible/probable) carcinogenic PAHs and metal(loid)s, respectively. Firefighters' exposure to PM, PAHs, and metal(loid)s were below available occupational limits. The estimated carcinogenic risks associated with the inhalation of PM-bound PAHs (3.78 × 10-9 - 1.74 × 10-6) and metal(loid)s (1.50 × 10-2 - 2.37 × 10-2) were, respectively, below and 150-237 times higher than the acceptable risk level defined by the USEPA during 210 min of firefighting activity and assuming a 40-year career as a firefighter. Additional studies need to (1) explore exposure to (coarse/fine/ultrafine) PM, (2) assess health risks, (3) identify intervention needs, and (4) support regulatory agencies recommending mitigation procedures to reduce the impact of fire effluents on firefighters.

Role of Woody Biomass Ash Material in Immobilization of Cadmium, Lead and Zinc in Soil

Nowadays, we have observed the dynamic development of bio-heating plants that use wood biomass for heating or energy purposes. The result of this process is a reduction in carbon dioxide emissions as well as in the production of biomass ash (BA). Despite the waste nature of BA, it should be carefully analyzed and assessed for various applications, including environmental ones. Due to the features attributed to BA, including its alkaline reaction, the high capacity of its sorption complex, relatively low salinity, and significant content of macro- and microelements, a hypothesis was put forward in this work undertaken about the positive role of BA as an immobilizing factor for Cd-, Pb-, and Zn-contaminated soils. This research was based on a pot experiment in which four series were considered: (1) BA; (2) BA + Cd; (3) BA + Pb; and (4) BA + Zn. BA was used at doses of 30, 60, and 90 mg pot-1, and metals at doses of 2 mg Cd, 100 mg Pb, and 300 mg Zn kg-1 of soil. The test plant was corn grown for green mass. The study took into account the influence of BA on the content of the total forms of heavy metals (Metot) and their available forms (Meav). In the soil without the addition of metals, a significant increase in the content of Cdtot and Cdav, and a decrease in the content of Zntot were observed due to the application of BA. The addition of metals against the background of the BA used resulted in a significant increase in Cdtot, Pbtot, and Zntot, as well as an increase in the available forms of Pbav but a decrease in Znav. However, there was no significant increase in the Cdav content. The obtained results may indicate the potentially immobilizing role of BA only in the case of zinc. They may constitute the basis for further, more detailed research aimed at determining the role of BA in the immobilization of various metals in soil.

Entrapment of atmospheric particle bound heavy metals by ferns as evidenced by lead (Pb) isotope and MixSIAR: Implications for improving air quality

Plant metal uptake can occur through both soil-root and atmospheric transfer from leaves. The latter holds potential implications for development of biofiltration systems. To explore this potential, it is crucial to understand entrapment capacity and metal sources within plants. As ferns absorb materials from atmosphere, this study focuses on two abundant fern species growing in densely populated and highly polluted regions of Eastern India. Gravimetric quantification, elemental concentration and Pb isotopic analyses were performed by segregating the ferns into distinct components: foliage dusts (loose dust (LD) and wax-bound dust (WD)) and plant tissue (leaves and roots). To understand metal sources, the study analyzes soil, and atmospheric particulates (PM10 and dust fall (DF)). Results indicate that, while LDs have soil dust influence, wax entraps atmospheric particulates and translocates them inside the leaves. Furthermore, roots demonstrate dissimilar isotopic ratios from soil, while displaying close association with atmospheric particulates. Isotopic composition and subsequent mixing model reveal dominant contribution from DF in leaves (53-73%) and roots (33-86%). Apart from DF, leaf Pb is sourced from PM10 (21-38%) with minimal contribution from soil (6-10%). Conversely, in addition to dominance from DF, roots source Pb primarily from soil (12-62%) with a meagre 2-8% contribution from PM10.

Analysis of trace elements in processed products of grapes and potential health risk assessment

Raisins and grape pekmez are consumed commonly by human all over the globe. Consumption of contaminated foods may be the likely pathway of heavy metal exposure. Therefore, the objectives of the present research were to quantify trace elements concentration in raisins and grape pekmez produced from locally grown grapes in Gonabad and to assess non-carcinogenic (HQ and HI) and carcinogenic (total cancer risk, CRt) health risks caused by trace elements exposure via oral intake of these products for children, teenagers, and adults. For this purpose, a totally 30 (15 raisins and 15 grape pekmez) samples were purchased from the vineyard gardeners and examined for ten trace elements including As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn. The HI values signaled that the studied population consuming these products is at risk. The HQ, HI, and CRt values of the elements were in order of children > teenagers > adults. The highest cancer risk contribution was attributed to As and Ni for all the studied age groups in both raisins and grape pekmez samples. However, it is recommended that the concentration of trace elements in the soil and crops of the study area and its related health risks be regularly monitored to avoid significant health risks in the future.

The fate of noble metals and rare earth elements during pelletized biomass combustion

The extraction of rare earth elements (REEs) and noble metals (NMs) from unconventional resources is playing a crucial role under the context of industrialization and reserve depletions. Plants used for phytoextraction are promising materials for the recovery of metals, but the biomass needs to be reduced to a manageable amount and volume prior to the extraction process. This paper investigates the combustion process of biomass focusing on NMs and REEs flow. The plants harvested from a brownfield land were pelletized and incinerated in a fixed-grate pilot-scale boiler, meanwhile, solid remains from various points in the combustion and flue gas system were captured and analyzed. The results show that levels of NMs in deposited ash and fly ash are greater than in bottom ash. Meanwhile, the higher REE concentration in bottom ash compared to that in other solid residuals demonstrates the less ability of these compounds to escape from the combustion chamber. Generally, the concentrations of REEs and NMs in the solid residues are significantly higher compared to biomass. SEM-EDS analyses of the contaminated solid remains indicate that gold forms individual particles with purity higher than 95 wt% in the bottom ashes, and this finding adds novel insights into gold phytomining.

Valorization of Textile Sludge and Cattle Manure Wastes into Fuel Pellets and the Assessment of Their Combustion Characteristics

The textile wastewater sludge (TWS) treatment and disposal are environmentally challenging due to toxic organics and metals. At the same time, cattle manure (CM), with better combustion performance, i.e., calorific value and uniform burning capability, is still underutilized in many parts of the world. This study evaluated and assessed the TWS and CM blending compatibility to convert them into fuel pellets for the direct combustion option and to stabilize toxic contaminants in TWS. After initial drying, grinding, and particle size control of the raw TWS and CM, both were blended at different ratios. The blended and nonblended TWS and CM samples were converted into pellets and analyzed for proximate and ultimate analyses, namely, moisture content, fixed carbon, CHNO, gross calorific value (GCV), bulk density, ash content, and metals, to evaluate the efficacy for energy applications. Out of three blended ratios, i.e., 75:25 (W/W%; CM/TWS), 50:50, and 25:75, the 75:25 blended pellet composition was found appropriate for fuel application. For the 75:25 blend, the obtained GCV was 12.77 MJ/kg, elemental carbon was 27.5%, volatiles were 41.7%, and residue ash was 42.8% of the total weight. Moreover, the blending ratios of 75:25 and 50:50 revealed that elemental and metal (Fe, Cu, Zn, Ni, Cr, Na, Mg, Mn) concentrations in TWS were stabilized to below threshold limits in the obtained residue ash for safe handling. The explored methods of TWS and CM waste processing, blending, and pelletization proposed a new technique for their sustainable waste valorization into energy sources.

Beyond Cleansing: Ecosystem Services Related to Phytoremediation

Phytotechnologies used for cleaning up urban and suburban polluted soils (i.e., brownfields) have shown some weakness in the excessive extent of the timeframe required for them to be effectively operating. This bottleneck is due to technical constraints, mainly related to both the nature of the pollutant itself (e.g., low bio-availability, high recalcitrance, etc.) and the plant (e.g., low pollution tolerance, low pollutant uptake rates, etc.). Despite the great efforts made in the last few decades to overcome these limitations, the technology is in many cases barely competitive compared with conventional remediation techniques. Here, we propose a new outlook on phytoremediation, where the main goal of decontaminating should be re-evaluated, considering additional ecosystem services (ESs) related to the establishment of a new vegetation cover on the site. The aim of this review is to raise awareness and stress the knowledge gap on the importance of ES associated with this technique, which can make phytoremediation a valuable tool to boost an actual green transition process in planning urban green spaces, thereby offering improved resilience to global climate change and a higher quality of life in cities. This review highlights that the reclamation of urban brownfields through phytoremediation may provide several regulating (i.e., urban hydrology, heat mitigation, noise reduction, biodiversity, and CO₂ sequestration), provisional (i.e., bioenergy and added-value chemicals), and cultural (i.e., aesthetic, social cohesion, and health) ESs. Although future research should specifically be addressed to better support these findings, acknowledging ES is crucial for an exhaustive evaluation of phytoremediation as a sustainable and resilient technology.

Wildland-urban interface fire ashes as a major source of incidental nanomaterials

Although metal and metalloid concentrations in wildfire ashes have been documented, the nature and concentrations of incidental nanomaterials (INMs) in wildland-urban interface (WUI) fire ashes have received considerably less attention. In this study, the total metal and metalloid concentrations of 57 vegetation, structural, and vehicle ashes and underlying soils collected at the WUI following the 2020 fire season in northern California - North Complex Fire and LNU Lightning Complex Fire - were determined using inductively coupled plasma-time of flight-mass spectrometry after microwave-assisted acid digestion. The concentrations of Ti, Zn, Cu, Ni, Pb, Sn, Sb, Co, Bi, Cr, Ba, As, Rb, and W are generally higher in structural/vehicle-derived ashes than in vegetation-derived ashes and soils. The concentrations of Ca, Sr, Rb, and Ag increased with increased combustion completeness (e.g., black ash < gray ash < white ash), whereas those of C, N, Zn, Pb, and In decreased with increased combustion completeness. The concentration of anthropogenic Ti - determined by mass balance calculations and shifts in Ti/Nb above the natural background ratios - was highest in vehicle ash (median: 30.8 g kg^-1, range: 4.5-41.0 g kg^-1) followed by structural ash (median: 5.5 g kg^-1, range: of 0-77.4 g kg^-1). Various types of carbonaceous INM (e.g., amorphous carbon, turbostratic-like carbon, and carbon associated with zinc oxides) and metal-bearing INMs (e.g., Ti, Cu, Fe, Zn, Mn, Pb, and Cr) with sizes between few nanometers to few hundreds of nanometers were evidenced in ashes using transmission electron microscopy, including energy dispersive X-ray spectroscopy. Overall, this study demonstrates the abundance of a variety of metals and metalloids in the form of INMs in WUI fire ashes. This study also highlights the need for further research into the formation, transformation, reactivity, fate, and effects of INMs during and following fires at the WUI.

Environmental behavior, human health effect, and pollution control of heavy metal(loid)s toward full life cycle processes

Heavy metal(loid)s (HMs) have caused serious environmental pollution and health risks. Although the past few years have witnessed the achievements of studies on environmental behavior of HMs, the related toxicity mechanisms, and pollution control, their relationship remains a mystery. Researchers generally focused on one topic independently without comprehensive considerations due to the knowledge gap between environmental science and human health. Indeed, the full life cycle control of HMs is crucial and should be reconsidered with the combination of the occurrence, transport, and fate of HMs in the environment. Therefore, we started by reviewing the environmental behaviors of HMs which are affected by a variety of natural factors as well as their physicochemical properties. Furthermore, the related toxicity mechanisms were discussed according to exposure route, toxicity mechanism, and adverse consequences. In addition, the current state-of-the-art of available technologies for pollution control of HMs wastewater and solid wastes were summarized. Finally, based on the research trend, we proposed that advanced in-operando characterizations will help us better understand the fundamental reaction mechanisms, and big data analysis approaches will aid in establishing the prediction model for risk management.

Heavy metal removal using an advanced removal method to obtain recyclable paper incineration ash

Various agents, including ethylenediaminetetraacetic acid, oxalic acid, citric acid, and HCl, were applied to remove heavy metals from raw paper incineration ash and render the ash recyclable. Among these prepared agent solutions, ethylenediaminetetraacetic acid showed the highest efficiency for Pb removal, while oxalic acid showed the highest efficiencies for Cu, Cd, and As removal. Additionally, three modes of an advanced removal method, which involved the use of both ethylenediaminetetraacetic acid and oxalic acid, were considered for use at the end of the rendering process. Among these three modes of the advanced removal method, that which involved the simultaneous use of ethylenediaminetetraacetic acid and oxalic acid, i.e., a mixture of both solutions, showed the best heavy metal removal efficiencies. In detail, 11.9% of Cd, 10% of Hg, 28.42% of As, 31.29% of Cu, and 49.19% of Pb were removed when this method was used. Furthermore, the application of these three modes of the advanced removal method resulted in a decrease in the amounts of heavy metals eluted and brought about an increase in the CaO content of the treated incineration ash, while decreasing its Cl content. These combined results enhanced the solidification effect of the treated incineration ash. Thus, it was confirmed that the advanced removal method is a promising strategy by which recyclable paper incineration ash can be obtained.

Phytomining of rare earth elements - A review

The increasing demand for rare earth elements (REEs) for modern industry has led to a surge in mining activities and consequently has released these metals into the environment. Intensifying REEs in a habitat has impacts on its ecosystem, but on the other side, it also provides the opportunity to recover REEs from low-grade minerals. Phytomining has emerged as an ecologically sound technique to extract these valuable elements from contaminated soils where traditional mining is not competitive. This paper presents and reviews the concept of REE phytomining from three scientific areas. The accumulation of rare earth metals in plants is the first stage, referred to as the phytoextraction process. This is followed by elevating REE concentrations into bio-ores via the enrichment phase. Ultimately, extraction is the final step to complete the phytomining pathway for reclaiming REEs in brownfield land.

Thermochemical conversion of heavy metal contaminated biomass: Fate of the metals and their impact on products

With the rapid depletion of fossil energy and increasingly severe environmental pollution, the development of biomass resources for biorefineries has become a new research focus. However, heavy metals may be released during the thermochemical treatment when the biomass materials used in biomass conversion are contaminated by heavy metals. This can cause secondary environmental pollution or transference to the target products, reducing product quality. Therefore, having a systematic understanding of the fate of heavy metals in biomass conversion is necessary for alleviating potential risks. This study presents the current status of contaminated biomass and conversion products involving thermochemical processes, the migration, transformation, and impact of heavy metals in biomass conversion was investigated, and the utilization of heavy metals in contaminated biomass was briefly outlined. This review aims to link biomass conversion to the fate of heavy metals, avoid existing risks as much as possible to produce cleaner products efficiently, and promote the sustainable development of heavy metal contaminated biomass resources.

Remarkable enrichment of heavy metals in baghouse filter dust during direct-fired thermal desorption of contaminated soil

This study focuses on the widely applied technology of direct-fired thermal desorption, taking a site contaminated by polycyclic aromatic hydrocarbons (PAHs) as a typical test case. The entire thermal desorption process of contaminated soil is considered in the analysis. The concentration levels and occurrence characteristics of heavy metals in dust traditionally considered to be clean are evaluated, and possible secondary pollution and environmental impacts are explored. The results indicate that, compared with the thermal desorption soil, the dust samples generated in the baghouse filter during the ex situ direct-fired thermal desorption process have higher amounts of heavy metal accumulation as well as altered speciation. In addition, the enrichment characteristics and origins of the heavy metals are analyzed according to the process flow and particle size composition as well as the results of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and other microscopic research methods. Phenomenon further reveals enrichment of arsenic (As), nickel (Ni), and chromium (Cr). The findings of this study can provide a scientific basis for the proper disposal and risk management of the dust collected after direct-fired thermal desorption treatment of contaminated soil.

Novel combination of bioleaching and persulfate for the removal of heavy metals from metallurgical industry sludge

The objective of this study was to remove heavy metals from metallurgical industry sludge by bioleaching alone and bioleaching combined with persulfate (PDS). The results showed that the removal of Cu, Zn, Pb, and Mn reached 70%, 83.8%, 25.2%, and 76.9% by bioleaching alone after 18 days, respectively. The experiment of bioleaching combined with PDS was carried out in which the optimal additive dosage of K₂S₂O₈, 8 g/L, was added to bioleaching after 6 d. After 1 h, the removal of four heavy metals reached 75.1, 84.3, 36.7, and 81.6%, respectively. Compared with bioleaching alone, although the increase in removal efficiency was only slightly increased, the treatment cycle was distinctly shortened from 18 to 6 days + 1 h. The scanning electron microscopy (SEM) results showed that the surface morphology of the sludge was changed significantly by the combined treatment. The content of heavy metals was significantly reduced after bioleaching combined with PDS by energy dispersive X-ray spectroscopy (EDX). Through electron paramagnetic resonance (EPR) and free radical quenching experiments, it was indicated that sulfate radicals [Formula: see text] plays a leading role in the combined treatment. The treated sludge mainly existed in a stable form, and the bioavailability was reduced with European Community Bureau of Reference (BCR) morphology analysis. This study proved that the combination of bioleaching and PDS could not only shorten the treatment cycle but also further improve the efficiency of heavy metal leaching. It provides a novel treatment method for the removal of heavy metals from metallurgical industry sludge.

Hazardous emissions and concentrations of toxic metalloids and trace elements in charcoals from six commonly used tropical timbers for carbonization

Carbonized wood is a biofuel from cellulose pyrolysis with frequent smoke and life-threatening carcinogenic emissions. Carbon monoxide (CO), particulate matter (PM_2.5), metalloids and trace elements from charcoals from six commonly used tropical timbers for carbonization in Donkorkrom (Ghana) were assessed. During combustion, Anogeissus leiocarpa charcoal emitted the least CO (4.28 ± 1.08 ppm) and PM_2.5 (3.83 ± 1.57 μg/m³), while particulate matter was greatest for Erythrophleum ivorense (28.05 ± 3.08 ppm) and Azadirachta indica (27.67 ± 4.17 μg/m³) charcoals. Erythrophleum ivorense charcoal produced much lead (16.90 ± 0.33 ppm), arsenic (1.97 ± 0.10 ppm) and mercury (0.58 ± 0.003 ppm) but the least chromium (0.11 ± 0.01 ppm) and zinc (2.85 ± 0.05 ppm). Nickel was greatest for A. indica charcoal (0.71 ± 0.01 ppm) and least for Vitellaria paradoxa (0.07 ± 0.004 ppm). Trace elements ranged from 342.01 ± 2.54 ppm (A. indica) to 978.47 ± 1.80 ppm (V. paradoxa) for potassium and 1.74 ± 0.02% (V. paradoxa) to 2.24 ± 0.10% (A. indica) for sulphur. Besides A. leiocarpa charcoal, which ranked safest during combustion, the high PM_2.5 and CO emissions make the other biofuels hazardous indoors. Kitchens need air filters to absorb these emissions together with the use of improved cook stoves. These carcinogenic metalloids would necessitate that their ashes be properly discarded without human contact. Yet, the charcoals would be much suitable as soil amendment bio-char for plant growth quality improvement.

Water and soil contaminated by arsenic: the use of microorganisms and plants in bioremediation

Owing to their roles in the arsenic (As) biogeochemical cycle, microorganisms and plants offer significant potential for developing innovative biotechnological applications able to remediate As pollutions. This possible use in bioremediation processes and phytomanagement is based on their ability to catalyse various biotransformation reactions leading to, e.g. the precipitation, dissolution, and sequestration of As, stabilisation in the root zone and shoot As removal. On the one hand, genomic studies of microorganisms and their communities are useful in understanding their metabolic activities and their interaction with As. On the other hand, our knowledge of molecular mechanisms and fate of As in plants has been improved by laboratory and field experiments. Such studies pave new avenues for developing environmentally friendly bioprocessing options targeting As, which worldwide represents a major risk to many ecosystems and human health.

Phytomining of noble metals - A review

Phytomining of noble metals (NMs) offers a promising possibility of metal extraction at sites where traditional mining activities or recovering NMs from low-grade minerals are not competitive. In addition to conventional mining, producing NMs from secondary resources strengthening the circular economy has been paid worldwide attention. The review presented in this paper links three scientific areas as the essential elements to form the phytomining chain of NMs. The accumulation of NMs in plants is the first step, referred as the phytoextraction process. This is followed by heightening the concentration of NMs via the enrichment stage. Eventually, although less well understood, extraction methods of NMs from biomass solid remains as well as from diverse secondary sources particularly incineration ashes are discussed that assist to visualize the potential pathways in phytomining.

Brassica Species in Phytoextractions: Real Potentials and Challenges

The genus Brassica is recognized for including species with phytoaccumulation potential and a large amount of research has been carried out in this area under a variety of conditions, from laboratory experiments to field trials, with spiked or naturally contaminated soils, using one- or multi-element contaminated soil, generating various and sometimes contradictory results with limited practical applications. To date, the actual field potential of Brassica species and the feasibility of a complete phytoextraction process have not been fully evaluated. Therefore, the aim of this study was to summarize the results of the experiments that have been performed with a view to analyzing real potentials and limitations. The reduced biomass and low metal mobility in the soil have been addressed by the development of chemically or biologically assisted phytoremediation technologies, the use of soil amendments, and the application of crop management strategies. Certain issues, such as the fate of harvested biomass or the performance of species in multi-metal-contaminated soils, remain to be solved by future research. Potential improvements to current experimental settings include testing species grown to full maturity, using a greater amount of soil in experiments, conducting more trials under real field conditions, developing improved crop management systems, and optimizing solutions for harvested biomass disposal.

Selenium uptake, volatilization, and transformation by the cyanobacterium Microcystis aeruginosa and post-treatment of Se-laden biomass

With a narrow margin between beneficial and toxic effects, selenium (Se) is of great concern due to its increasing level in aquatic environments. The accumulation and transformation of Se by the cyanobacterium Microcystis aeruginosa and effects of nutrients, particularly sulfate, were investigated. The nutrient-deprived cyanobacterium removed water-borne selenate (82.2 ± 0.93%) faster than selenite (58.9 ± 1.77%), with 86.0 ± 1.41% and 77.2 ± 1.00%, respectively, of the Se accumulated in the biomass and the rest volatilized. When supplied with excess nutrients, the Se accumulation and volatilization rates were significantly inhibited, with the removal efficiency dropping to 50.2 ± 2.59% and 7.37 ± 0.93% for selenite and selenate, respectively. When M. aeruginosa was tested with inadequate, appropriate, and adequate levels of sulfate, Se uptake decreased with increasing sulfate concentrations, particularly for selenate (from 34.1 to 4.81%). Using X-ray absorption near-edge structure to speciate biomass Se, selenite and selenate were transformed to organo-Se (87.3-100%), with or without nutrients present, suggesting M. aeruginosa could efficiently reduce Se oxyanions to more bioavailable forms. With increasing sulfate levels (5.0 and 10.0 mg S/L), percentages of SeMet converted from selenite decreased by 28.2-33.0%, with 19.1-33.2% as elemental Se, while organo-Se remained dominant (93.6-95.1%) in selenate-treated M. aeruginosa. Transmission electron microscopy shows structural damage in the cell wall at exposure to selenite (1600 μg Se/L), with the intracellular structure intact. To prevent Se biomagnification along aquatic food chains, the Se-laden biomass was combusted as a post-treatment, leading to a significant reduction in Se content (∼99.2%) and Se bioavailability, with inorganic Se (45.0-70.5%) predominant in the residue.

Dual lignin-derived polymeric systems for hazardous ion removals

The functionalization of lignin derivatives for ion removals is a promising method to expedite their use in treating industrial wastewater. In this work, kraft lignin (KL) was polymerized with [2-(methacryloyloxy)ethyl]trimethylammonium methyl sulfate (METAM) or acrylic acid (AA) in an acidic aqueous suspension system to produce cationic and anionic water-soluble lignin polymers with high molecular weights. Then, the interaction of soluble ions and KL-METAM and KL-AA was investigated using a Quartz crystal microbalance (QCM) and a vertical scan analyzer (VSA). The QCM, X-ray photoelectron spectroscopy (XPS) and contact angle measurement results showed that the adsorption efficiency of KL-AA was better than KL-METAM for ions due to the stronger electrostatic interaction, cationic π-interaction, and chelation between ions and KL-AA. Based on adsorption, sedimentation, and aggregate size analyses, the dual polymer systems of KL-AA/KL-METAM were more effective than KL-METAM/KL-AA in removing ions. Among Zn²⁺, Cu²⁺, and K⁺; Zn²⁺ interacted more effectively with polymers in all scenarios because it has higher reactivity for interacting with other elements. As the efficiency of ion removals was more remarkable than past reported findings, the system of KL-AA/KL-METAM may be a promising alternative for the removal of dissolved ions from solutions.

Migration and transformation of heavy metals in hyperaccumulators during the thermal treatment: a review

The pollution of heavy metals (HMs) in the soil has become one of the important factors affecting the national environment and human health. Phytoremediation, as a technology to deal with HM pollution in soil, has been extensively studied and applied due to its sustainability and environmental friendliness. However, hyperaccumulators polluted by HMs need to be properly treated to avoid secondary pollution to the environment. This paper reviews the migration and transformation of HMs during the incineration, pyrolysis, gasification, and hydrothermal treatment of hyperaccumulators; comprehensively evaluates the advantages and disadvantages of each technology in the treatment of HM-enriched hyperaccumulators; and analyzes the current development status and unsolved problems in detail for each technology. Generally speaking, thermal treatment technology can fix most of the HMs of exchangeable fraction in biochar, reducing its bioavailability and biotoxicity. In addition, the application direction and research focus of the target product are discussed, and it is clarified that in the future, it is necessary to further optimize the reaction conditions and explore the mechanism of HM immobilization to maximize the immobilization of HMs and improve the quality and output of the target product.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

Influence of different fluidization and gasification parameters on syngas composition and heavy metal retention in a two-stage fluidized bed gasification process

Herein, we investigated the influence of gasification and fluidization parameters on the H2 content of syngas and the retention of heavy metals (Cu and Pb) in a bed material during a two-stage fluidized bed gasification process. The results indicated that a temperature of 900 °C in both stages resulted in the highest H2 content (32.4 mol%) in syngas. When different equivalence ratios (ERs) were investigated, it was found that the highest H2 content in syngas (25.4 mol%) was achieved at an ER of 0.3. A particle size of 0.46 mm in the fluidized bed led to an increase in the H2 content of syngas. Moreover, increasing the operating gas velocity led to an increase in the H2 content of syngas. The heavy metal concentration in the bed material was the highest at 500 °C. When the influences of different particle sizes and operating gas velocities were compared, it was observed that a particle size of 0.46 mm and gas velocity of 1.5 U/Umf resulted in increased heavy metal concentrations in the bed material, which indicates that the reduction in the particle size and the increase in the operating gas velocity enhanced gasification and improved the retention of heavy metals.

Experimental and DFT studies on the characteristics of PbO/PbCl2 adsorption by Si/Al- based sorbents in the simulated flue gas

Mineral oxides are effective in-furnace sorbents used to control lead (Pb) emissions in flue gas at high temperatures. In this paper, the PbO/PbCl₂ adsorption characteristics of sorbents were investigated via experimental and density functional theory (DFT) methods. The results show that Si/Al-based compounds can chemically adsorb Pb, and the adsorption is related to the Si-O and Al-O bonds in the sorbents. Exposed Si and O atoms on SiO₂ surfaces and exposed Al and O atoms on Al₂O₃ surfaces are the active sites for Pb adsorption, and PbO is easier to remove than PbCl₂. Pb adsorption is promoted in a mixture of SiO₂ and Al₂O₃. Doping Si atoms into Al₂O₃(100) promotes PbO adsorption, and doped three-coordinate Si atoms have a more obvious promotion effect than doped two-coordinate Si atoms. Doping Al atoms into SiO₂(001) has no obvious effect on PbO adsorption. The effect of temperature on Pb adsorption was studied by thermodynamic analysis. The Gibbs free energy difference for PbO adsorption on SiO₂(001) increases from -373.04-32.42 kJ/mol as the temperature increases from 300 to 1800 K. High-temperature calcination changes the bond length and bond angle of the system, affecting the stabilities of atomic configurations and decreasing the Pb adsorption capacity.

Organic acid leaching was an efficient approach for detoxification of metal-containing plant incineration ash

Metal-containing plant incineration ash (MPIA), which was the by-product for metal extraction from soil by phytoextraction process, contains various kinds of heavy metal that have post potential risk to the environment. This study investigated the leaching efficiency and metal redistribution of MPIA using organic acid as leaching agents. The MPIA before and after leaching was characterized using the toxicity characteristic leaching procedure (TCPL) test, X-ray diffraction, scanning electronic microscopy, and Fourier transform infrared spectroscopy. The results showed that all tested organic acids resulted in the dissolution of metals, especially 1 mol L-1 citric acid leaching achieved for the dissolution efficiency of 84% Mn, 87.01% Cd, 66.97% Zn, and 55.83% Pb. During leaching progress, the synergetic of chelation and acid soluble action accelerated the metal release and redistribution, and the dissolution of Mn, Zn, Pb, and Cd fit best to the shrinking core model of chemical control. Meanwhile, the leaching residue reached the regulatory standard. Thus, organic acid leaching may be a feasible strategy for detoxification of MPIA.

Insights into thermal hydrolyzed sludge liquor - Identification of plant-growth-promoting compounds

This study proposes a new path to utilize thermal hydrolyzed sludge (TH sludge) as fertilizer given high value chemical compounds that can promote plant growth were identified in the liquid fraction of TH sludge (TH liquor). Together with micro- and macro-nutrients released/synthesized during thermal hydrolysis, the feasibility of using TH liquor as organic fertilizer was evaluated. Besides high contents of N, P and K, total free amino acids (FAAs) and plant-growth-promoting FAAs (including glutamic acid, leucine and cystine) also presented in high concentration (4.98-6.48 and 1.12-2.73 g/100 g) in the TH liquor. For the first time, phytohormone compound, indole-3-acetic acid, was observed and the content was the highest in TH liquor with 165 °C treatment (165 °C TH liquor). Meantime, 165 °C TH liquor did not have negative impact on the growth of soil microbes, and this product, instead, demonstrated stimulating effect on the plant growth. These results suggest that 165 °C TH liquor has a great potential to be an organic fertilizer. The remaining solids of TH sludge could be converted to valuable biochar. The holistic approach of using TH liquor as organic fertilizer and producing biochar could realize nearly zero-waste discharge in sludge management.

Application of Laser-Induced Breakdown Spectroscopy in Detection of Cadmium Content in Rice Stems

The presence of cadmium in rice stems is a limiting factor that restricts its function as biomass. In order to prevent potential risks of heavy metals in rice straws, this study introduced a fast detection method of cadmium in rice stems based on laser induced breakdown spectroscopy (LIBS) and chemometrics. The wavelet transform (WT), area normalization and median absolute deviation (MAD) were used to preprocess raw spectra to improve spectral stability. Principal component analysis (PCA) was used for cluster analysis. The classification models were established to distinguish cadmium stress degree of stems, of which extreme learning machine (ELM) had the best effect, with 91.11% of calibration accuracy and 93.33% of prediction accuracy. In addition, multivariate models were established for quantitative detection of cadmium. It can be found that ELM model had the best prediction effects with prediction correlation coefficient of 0.995. The results show that LIBS provides an effective method for detection of cadmium in rice stems. The combination of LIBS technology and chemometrics can quickly detect the presence of cadmium in rice stems, and accurately realize qualitative and quantitative analysis of cadmium, which could be of great significance to promote the development of new energy industry.

The migration, transformation and control of trace metals during the gasification of rice straw

This research investigates the trace metals speciation, partitioning and removal in rice straw gasification equipped with an integrated hot gas cleaning (HGC) system. The experiments were conducted by fluidized bed gasifier and controlled at 800 °C with equivalence ratio (ER) varied between 0.2 and 0.4. The experimental results indicated that the concerned trace metals Zn, Cr, Cd, and Pb partitioning in the gas phase were increased significantly with an increase in ER. This is because the exothermic reaction could enhance the trace metals reacted with chlorine and/or sulfur as well as correspondingly formed highly volatile metals compounds. However, other tested metals Cu, Na, K, Ca, Mg partitioning was obviously decreased in the gas phase with ER increasing. These tested metals tend to form oxides speciation leading the variation in their partitioning characteristics. The XRD identification and thermodynamic equilibrium simulation results were also confirmed the tested metals speciation and partitioning characteristics. The dominant gaseous species produced from rice straw gasification, such as KCl_(g), NaCl_(g), KO_(g), K₂O_(g), ZnCl_2(g), CrO₂Cl_2(g), CuCl_2(g), PbCl_2(g), PbO_(g), and Cd_(g), were predicted by thermodynamic equilibrium model. The tested metals removal by adsorbents of hot gas cleaning system was found to be adsorbed in decreasing order as: K > Cr > Ca > Pb > Mg > Cd > Na > Zn > Cu. Activated carbon was used in hot gas cleaning system and showed a good performance for adsorbing tested metals, especially for Pb, Cd, Cr, Ca, K, and Mg. In summary, HGC system is proposed as an effective way for improving the syngas quality and reducing trace contaminants emission in rice straw gasification.

Contaminated Land by Wildfire Effect on Ultramafic Soil and Associated Human Health and Ecological Risk

The purpose of this study is the evaluation of fire effect on contaminated land and the assessment of the associated risk of human health and terrestrial ecological receptors. Ash and soil samples were gathered from burned and unburned areas (central Evia, Greece) which are adjacent with a Natura 2000 area. The geochemical dataset includes 20 sampling sites and 35 elements. The wildfire severity was investigated by applying a macroscopic approach and field observations. Statistical and spatial analysis were applied for delineating the distribution of elements in ash and soil. Elemental balance approach was performed for estimating net gain (+) or loss (−) to the ash. Element contents in sampling sites were compared to screening values proposed by the literature. Hundreds of hectares of burned land including wildland areas in central Evia are contaminated with (contents in mg Kg−1), Co (up to 43.5), Cr (up to 244), Mn (up to 1158), Ni (up to 463) associated with geogenic sources such as serpentinite peridotites and Ni-laterite deposits. Aluminum, As, Cd, Co, Cr, Fe, Mn, Ni, Pb, V and Zn contents recorded in the sampling sites are posing a potential risk to human health and ecological receptors.

A Biosorption-Pyrolysis Process for Removal of Pb from Aqueous Solution and Subsequent Immobilization of Pb in the Char

The application of biosorption in the removal of heavy metals from water faces a challenge of safe disposal of contaminated biomass. In this study, a potential solution for this problem was proposed by using a biosorption-pyrolysis process featured by pretreatment of biomass with phosphoric acid (PA). The PA pretreatment of biomass increased the removal efficiency of heavy metal Pb from water by sorption, and subsequent pyrolysis helped immobilize Pb in the residual char. The results indicate that most (>95%) of the Pb adsorbed by the PA-pretreated biomass was retained in the char, and that the lower pyrolysis temperature (350 °C) is more favorable for Pb immobilization. In this way, the bioavailable Pb in the char was hardly detected, while the Pb leachable in acidic solution decreased to <3% of total Pb in the char. However, higher pyrolysis temperature (450 °C) is unfavorable for Pb immobilization, as both the leachable and bioavailable Pb increased to >28%. The reason should be related to the formation of elemental Pb and unstable Pb compounds during pyrolysis at 450 °C, according to the X-ray diffraction study.

Mutual effects of selenium and chromium on their removal by Chlorella vulgaris and associated toxicity

The release of selenium (Se) and chromium (Cr) into the environment from anthropogenic activities has posed a hazard to aquatic ecosystems. In this study, we used Chlorella vulgaris for Se/Cr bioremediation and evaluated their mutual effects on the removal efficiency. Our results found C. vulgaris highly effective in removing selenite-Se(IV) (49.5 ± 1.9%), selenate-Se(VI) (93.0 ± 0.5%), chromic nitrate-Cr(III) (89.0 ± 3.2%) and dichromate-Cr(VI) (88.1 ± 1.3%) over a 72 h period. Cr(VI) significantly impeded Se removal, particularly for selenate, due to competition between both for algal uptake, whereas Cr(III) obviously enhanced Se removal, increasing Se volatilization by ~29%. Similarly, Se significantly increase Cr removal rates, with a maximum of 94.6 ± 0.2% for the algal co-exposed to Se(IV) and Cr(III). To reduce residual pollutants in the alga, we applied combustion as a post-treatment to burn off >99% of the biomass Se for all Se treatments, whereas most of the biomass Cr (54.7-81.6%) remained in the ash at significantly higher levels (~7430 μg Cr/g DW). For toxicity, our speciation analysis found organo-Se (SeCys and SeMet) dominant in the alga exposed to Se, particularly selenite. No Cr(VI) but Cr(III) forms were detected in all Cr-exposed alga. Elemental Se disappeared from all Se-exposed alga in the presence of Cr(VI), while Se resulted in the emergence of Cr-acetate in all Cr(III)-treated alga. After combustion, mineral Se, particularly elemental Se dominated most of the ash; likewise, elemental Cr, along with Cr₂O₃, was found in all the ash. Overall, our research would contribute to developing a low ecotoxic algal treatment system for Se/Cr contaminated water.

Chemical Composition of PM10 in 16 Urban, Industrial and Background Sites in Italy

Italy is characterized by a very variable configuration in terms of altitude, proximity to the sea, latitude and the presence of industrial plants. This paper summarizes the chemical characterization of PM10 obtained from 38 sampling campaigns carried out in 16 sites in Italy during the years 2008–2018. Chemical determinations include all macro-components (six macro-elements, eight ions, elemental carbon and organic carbon). The sum of the individual components agrees well with the PM10 mass. The chemical composition of the atmospheric aerosol clearly reflects the variety in the Italian territory and the pronounced seasonal variations in the meteoclimatic conditions that characterize the country. Macro-sources reconstruction allowed us to identify and evaluate the strength of the main PM10 sources in different areas. On 10 sampling sites, the soluble and insoluble fractions of 23 minor and trace elements were also determined. Principal Component Analysis was applied to these data to highlight the relationship between the elemental composition of PM10 and the characteristics of the sampling sites.

Comparison of sewage sludge mono-incinerators: Mass balance and distribution of heavy metals in step grate and fluidized bed incinerators

We investigated the distribution of 18 elements including non-volatiles (Al, P, Ca, Fe, Mg, K, Mn, Cu, Na, Cr, and Ni), semi-volatiles (Zn, Pb, Ag, As, and Cd), and volatiles (Hg and S) and compared their behaviors in two types of full-scale sewage sludge mono-incinerators, namely, a step-grate stoker (GS) and two fluidized bed incinerators (F-types), with the same feed sludge. Most of the non-volatile elements were enriched five-fold in all incinerated sewage sludge ash (ISSA), while the volatile S and Hg were barely enriched in ash due to the combustion components generated in the gas phase. While the semi-volatile elements were also enriched five-fold in the F-types, a different enrichment behavior was observed in the GS. Boiler and multi-cyclone dust in the GS showed higher enrichments of Pb and Cd compared to ash due to the combined effects of lower temperature and smaller particle size. Compared to the F-types, the GS generated ashes with lower toxicity as the major component (99.7%) and hazardous dust as the minor component. In the future, more attention should be paid to grate stokers in terms of recycling ISSA.

Liquid and solids separation for target resource recovery from thermal hydrolyzed sludge

This study proposed an integrated process for biogas generation and biochar production from thermal hydrolysis pretreated sludge (THP sludge). In this study, the liquid and solids fractions of THP sludge were separately processed for the first time. The liquid fraction of THP sludge (THP-L) reached the biodegradability (262.6 ± 5.1 mL CH₄/g tCOD_feed) on the 15th day during anaerobic treatment, while the solids fraction of THP sludge (THP-S) only contributed 31.0% to the total methane production and required more than 30 days digestion time. We investigated the feasibility to convert THP-S into biochar to realize the higher value of the solids fraction. The results prove the produced biochar can be used as slow-release fertilizer. Preliminary energy analysis was performed to evaluate the energy efficiency of the integrated approach, namely, methane generation from THP-L coupled with biochar production from THP-S. The process realized energy surplus of 0.81 MWh/tonne dry sludge. In addition, THP-L digested sludge showed better dewaterability, lower yield stress and reduced viscosity during digestion. The proposed new sludge treatment process therefore has lower operating cost and higher value returns.

Stabilization and solidification of arsenic and iron contaminated canola meal biochar using chemically modified phosphate binders

Adsorption is a widely used process for removal of heavy metals, but the management of spent adsorbent containing concentrated amounts of heavy metals is a problem due to potential risk of groundwater contamination from leaching of heavy metals. Generally, cementitious binder and additives are used for stabilization and solidification treatment, however heavy metals tend to leach from such matrices. Therefore, this research investigated the effectiveness of chemically modified phosphate biochar (CMPB) composite for the simultaneous solidification and stabilization of arsenic (As) and iron (Fe) contaminated canola meal biochar. Results showed that the performance of spent biochar added CMPB composites was significantly better than the pure composites (without biochar) due to filling of inter-aggregate pores using biochar and availability of sufficient amount of MgKPO₄ for binding of biochar particles. Moreover, leaching test and risk assessment studies indicated that there is no potential adverse effect as the concentrations of As and Fe in TCLP leachate were well below the Universal Treatment Standard (UTS) in optimized CMPB composites. In conclusion, chemically modified phosphate binders were found effective in stabilization and solidification of As and Fe contaminated biochar into thermodynamically stable material with high immobilization capacity and low leachability.

Impact of wood combustion on indoor air quality

The incomplete wood combustion in appliances operated in batch mode is a recognised source of both in- and outdoor airborne pollutants, especially particulate matter (PM). Data on pollutant levels and PM characteristics in households with wood burning devices in developed countries are scarce with most studies describing stove change out programmes or other intervention measures. The aim of the present study was to simultaneously evaluate indoor and outdoor concentrations of CO, CO₂ and PM₁₀ during the operation of wood burning appliances (open fireplace and woodstove) in unoccupied rural households. PM₁₀ samples were analysed for water soluble inorganic ions, major and trace elements, organic carbon (OC), elemental carbon (EC), and detailed organic speciation. The CO 8-hour average concentrations did not exceed the protection limit despite the sharp increases observed in relation to background levels. During the open fireplace operation, PM₁₀ levels rose up 12 times compared to background concentrations, while the airtight stove resulted in a 2-fold increase. The inhalation cancer risk of particulate bound PAHs in the room equipped with woodstove was estimated to be negligible while the long-term exposure to PAH levels measured in the fireplace room may contribute to the development of cancer. The excess lifetime cancer risk resulting from the particle-bound Cr(VI) exposure during the fireplace and woodstove operation was higher than 1.0 × 10^-6 and 1.0 × 10^-5, respectively. Levoglucosan was one of the most abundant individual species both indoors and outdoors. This study underlines air pollution hazards and risks arising from the operation of traditional wood burning appliances.

Characterization of fecal sludge as biomass feedstock for thermal treatment in the southern Indian state of Tamil Nadu

Background: Transformative sanitation technologies aim to treat fecal sludge (FS) by thermal processes and recover resources from it. There is a paucity of data describing the relevant properties of FS as viable feedstock for thermal treatment in major geographical target areas, such as India. Methods: This study characterized FS collected from septic tanks in two cities located in the Indian southern state of Tamil Nadu. FS samples were obtained at the point of discharge from trucks in Tiruppur (n=85 samples) and Coimbatore (n=50 samples). Additionally, biosolids obtained from sewage treatment plants (STP) in the cities of Coimbatore and Madurai were characterized. Total solids (TS) were measured, and proximate and ultimate analysis were conducted according to methods used by the fuel industry. Additionally, the ash content was analyzed for heavy metal using standard methods. Results: The average higher heating value (HHV) across all FS samples in Tiruppur (13.4 MJ/kg) was significantly higher than in Coimbatore (5.4 MJ/kg), which was partially attributed to the high ash content of 69% in the latter samples.  The HHV of the biosolids samples ranged from 10 to 12.2 MJ/Kg. The average total solids (TS) content for FS was 3.3% and 2.0% for Tiruppur and Coimbatore respectively, while the median TS content for the two cities was 2.3% and 1.2%. The heavy metal content of the ash was found to be below the thresholds for land disposal. Conclusions: This is one of the first studies that has systematically characterized the calorific and mineral content of septage and biosolids in several cities in India. We expect these data to serve as input data in the design of thermal processes for fecal sludge treatment.

Temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust

The temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust was studied using an on-line analysis system. The arsenic release amount in combustion atmosphere was higher than that in CO₂ gasification and argon pyrolysis. The derived values of activation energy followed the order: combustion < gasification < pyrolysis. Furthermore, the enhancement effect of chlorine species on arsenic release percentage in air combustion was also higher than that in gasification and pyrolysis conditions. The total proportion of arsenic release in combustion with additive chlorine is bigger than the case in gasification and pyrolysis, especially when 20% chlorine is added. According to equilibrium analysis, arsenic oxides were identified as the main gaseous arsenic species and their formation were decreased in the oxygen-deficient environment, mainly accounting for lesser arsenic release proportion in gasification and pyrolysis than combustion. The release of arsenic was promoted to a different extent with additive chlorine, mainly caused by the AsCl₃ (g) formation. By the findings of the experiments and theoretical analyses, the possible reaction pathways and release mechanisms of arsenic species were proposed.

Analyses for Synthesis Gas from Municipal Solid Waste Gasification under Medium Temperatures

Municipal solid waste (MSW) gasification could be a novel method that shows the various advantages over traditional MSW treatments in China. Other research concluded that MSW gasification was operating by the assistant heat, and the gasification may occur under medium temperature. So, this study is aimed to investigate MSW gasification and pyrolysis behavior and analyze the syngas evolution and reaction mechanism. The MSW samples were collected in daily life and the experiments were carried out in a fixed tubular reactor below 650 °C. The effects of medium temperature and oxygen content on syngas quality were elucidated in depth. The results have shown that temperature can promote the syngas quality in the range of 550–650 °C, because the increasing temperature strengthens the reaction rate. The oxygen content should be controlled in a certain range, or oxidation reactions will be more prominent during gasification. The optimal gasification condition in this study was obtained at 650 °C and an oxygen concentration of 1.25%, the combustible gas yield and the lower heating value (LHV) of syngas of this condition were 0.296 L/g and 10.98 kJ/L, respectively. This study provides insights for MSW gasification under medium temperature, and a practical gasification system can be designed under a certain condition.

Release and transformation mechanisms of trace elements during biomass combustion

The release and transformation mechanisms of trace elements (TEs) during biomass combustion and the environmental impact of TEs in biomass ash were evaluated in this study. Three types of biomass were selected as fuels, with a type of bituminous coal used as a comparison. The quantitative contents and chemical fractions of the TEs in the fuels, ash and char obtained after combustion and pyrolysis were analyzed. The results indicated that during the biomass combustion, the amounts of TEs released were ordered as follows: As > Cd, Pb, Zn > Cu. The TEs in the different chemical fractions both in the fuels and the solid products were shown to be in the forms of different specific compounds, and the transformation of each TE was discussed in depth. Cd, Cu and Zn were found to be released in higher quantities during pyrolysis than combustion and the reason may be that more nonvolatile TE oxides but less volatile TEs chlorides were formed during combustion. The assessment of the environmental impact of the TEs in biomass ash indicated that the environmental risks of Zn and Cu in biomass ash are higher than those in coal ash.

Investigation of controlling factors on toxic metal leaching behavior in municipal solid wastes incineration fly ash

Municipal solid wastes incineration (MSWI) fly ash has drawn worldwide attention for its substantial annual generation capacity and high toxic metals leachability. Although many factors have been shown to affect the leachability of metals in fly ash, the controlling factors, which guide the selection of appropriate risk reduction method, remain unclear. The purpose of this study was to evaluate the effects of the two most important factors, total metal content, and remaining alkaline substances of MSWI fly ash, on the leaching behavior of toxic metals. In this work, a series of leaching tests and sequential extraction procedures were performed for seven fly ash samples collected from one MSWI plant. Results show that particulate size distribution, morphology, and mineralogy of all samples are similar, indicating the effects of these properties on metal leaching behavior can be ignored. In leaching tests, although the leaching behavior in terms of metal species and concentration levels vary as expected, only the leachate Pb concentration in four samples (up to 17.32 mg/L) exceeds the threshold in Chinese regulation (0.25 mg/L). The variation of the leachate Pb concentration is not consistent with the change of the total Pb concentration in fly ash. Further correlation analysis evidences that the acid-soluble Pb, which is highly correlated to the calcium content of fly ash, dominates the concentration of leachate Pb. Notably, when the addition of lime is about 1.5 times over the theoretical value, the concentration of leachate Pb would exceed the threshold regardless of the total Pb concentration in fly ash. Overall, this study demonstrates that the remaining alkaline substances (mainly calcium-bearing compounds), rather than the total content of metals, are the controlling factor of metal leaching behavior in fly ash. Thus, strategies to delicately optimize the quantity of lime addition in acid gas purification process should be considered to minimize MSWI fly ash environmental risks in the future.

Feasibility of anaerobic digestion on the release of biogas and heavy metals from rice straw pretreated with sodium hydroxide

The feasibility of anaerobic digestion on the release of biogas and heavy metals from contaminated rice straw pretreated with NaOH solution was studied. The results show that NaOH pretreatment can significantly boost the release of biogas and heavy metals from rice straw using anaerobic digestion. Under the optimal conditions for biomass pretreated 6% (w/w) NaOH with a solid-to-solution ratio of 1:20, total biogas and methane yields of 446.3 mL/g and 263.5 mL/g volatile solids were achieved, which were 22.18% and 41.59% higher than those of the control without NaOH pretreatment, respectively, and the release percentages of Cd, Pb, Cu, and Zn from rice straw reached 86.95-97.69%. The release of heavy metals from rice straw can contribute to both the degradation of lignin by NaOH pretreatment and the utilization/transformation of lignocellulose via anaerobic digestion. The acidification levels and total volatile fatty acid contents significantly influence on the release of heavy metals. Based on the Illumina HiSeq sequencing analysis, the dominant phyla in the biogas residues were proteolytic (Bacteroidetes) and hydrogen-producing (Firmicutes) bacteria, while the growth of Methanospirillum and Methanosaeta in anaerobically digested effluent was promoted. The results revealed that anaerobic digestion combined with NaOH pretreatment is suitable for the disposal of heavy metal-contaminated biomass.

Fate and transport of estrogenic compounds in an integrated swine manure treatment systems combining algal-bacterial bioreactor and hydrothermal processes for improved water quality

An integrated manure treatment system, including a mixed algal-bacterial bioreactor (MABB) and hydrothermal processing of biomass solids, was found to remove 76.4-97.0% of the total estrogenic hormones (estrone (E1), 17β-estradiol (E2), and estriol (E3)) from the liquid portion of animal manure (LPAM). The mixed biomass was converted into either biocrude oil with a yield up to 40% via hydrothermal liquefaction (HTL) or syngas with a yield up to 54% yield via catalytic hydrothermal gasification (CHG). Adding granular activated carbon (GAC) in the MABB enhanced the removal of estrogenic hormones (+ 7.2%), cytotoxicity (+ 58%), and heavy metals (+ 10.5%). After the integrated system with the MABB, HTL, and CHG processes, the overall percent removal of heavy metals from the LPAM ranged from 27.1 to 40.3%. The concentrations of potentially toxic heavy metals (lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), chromium (Cr)) in the aqueous phase after HTL and CHG tests ranged from 0.01 to 25.3 mg/L.

Partition of Cu and Pb in a Two-Stage Fluidized-Bed Waste Gasification System

In this study, a two-stage fluidized-bed gasification system was used to determine the distribution of heavy metal contaminants in simulative waste and evaluate the effect of the bed operating temperature, Equivalence ratio (ER), and Steam/Biomass ratio (S/B) in stage 1. The heavy metal concentrations in bed materials and fly ash were measured in two stages. The results show that as the operating temperature of stage 1 increased, the quantity of heavy metals entrapped by stage 1 bed material decreased, while that captured by stage 2 bed material increased. The less volatile Cu was mostly entrapped in stage 1 bed material. The ER and S/B results show that increasing the ER and S/B caused a slight increase in the concentrations of heavy metals entrapped in stage 1 and stage 2 bed materials. However, the influences of ER and S/B were less than that of temperature. The major factors affecting the heavy metal distribution were the operating temperature and the heavy metal volatility. Stage 2 bed material was able to entrap gaseous heavy metals or particles containing heavy metals by particle filtration and chemical adsorption. Thus, the heavy metals (Cu and Pb) had a downward trend after passing through stage 2.

Effectively immobilizing lead through a melanotekite structure using low-temperature glass-ceramic sintering

This work evaluated the feasibility of using low-temperature thermal immobilization based on the reaction mechanism forming the melanotekite (Pb2Fe2Si2O9) crystal phase to stabilize lead (Pb) containing waste. X-ray diffraction demonstrated that Pb could be incorporated into the melanotekite structure at easily attainable treatment temperatures (less than 500 °C) by magnetite and SiO2 precursors. The γ-Fe2O3 intermediate was found to play a key role in initializing melanotekite crystallization at a much lower temperature than that in traditional thermal immobilization techniques. Although a higher sintering temperature may increase Pb incorporation efficiency, amorphization occurred at temperatures higher than 950 °C. In addition, Pb was found to partition more in the amorphous phase of the SiO2-rich matrix. The results of the prolonged toxicity characteristic leaching procedure revealed a substantial improvement in the acid resistance of the targeted crystallized product sintered at 850 °C compared with the amorphous product and the other oxide products. The results of batch adsorption and subsequent thermal treatment verified the possibility of using the melanotekite structure to stabilize aqueous Pb with the Fe3O4@SiO2 residue. The study demonstrated that the melanotekite structure can be used to immobilize both solid and aqueous Pb through low-temperature thermal stabilization.

Development of an algal treatment system for selenium removal: Effects of environmental factors and post-treatment processing of Se-laden algae

In developing an algal treatment system, selenium (Se) removal efficiency by Chlorella vulgaris was evaluated under various conditions such as Se concentration, algal density, temperature and pH. A maximum removal efficiency plateau of ∼90% was observed between 1000-3000 μg Se/L while the tolerance of Se toxicity was found at 6000 μg Se/L. C. vulgaris of 0.75 g DW/L showed the highest removal efficiency (84%), and volatilization was dominant below 1.37 g DW/L. Se volatilization was two times higher at 25 °C than at 20 °C in the first 24 h. Moreover, the highest removal efficiency (77%) was obtained at pH 8.0, compared to 66.5% at pH 6.5 and 40% at pH 10.0. To prevent ecotoxicity, Se laden algae were further burned to ashes or filtered out by Anodonta woodiana. After burning, biomass Se was reduced by 99%, with organo-Se entirely converted into inorganic Se, lowering Se bioavailability. A. woodiana removed 54% of Se in 24 h, leading to Se bioaccumulation in soft tissues, which may serve as dietary Se supplements for human health. Our results suggest the cleanup of Se-contaminated water from either agricultural runoff or industrial discharge could be achieved using an algal treatment system with minimum potential ecotoxicity.

Mineralogy, chemical composition and leachability of ash from biomass combustion and biomass–coal co-combustion

Ash samples from biomass combustion or co-combustion with coal were analysed. The aim of this study of ash was to determine its mineral and chemical composition, and the chemical composition of solutions obtained during one-step water extraction. Besides the chemical analysis, X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) were applied.

The mineral and chemical composition of ash samples differ strongly. The content of heavy metals in the ash is generally low, but in some samples the limits of the content of some elements determined for fertilizers or soil amendments are exceeded. The relatively poor correlation between the concentration in leachate and bulk content in ash indicates that numerous elements are present in different forms in the studied samples. The results indicate that the potential use of biomass ash, or ash from biomass–coal co-combustion, requires complex studies that explore ash and leachates.