Stabilization of sewage sludge by different biochars towards reducing freely dissolved polycyclic aromatic hydrocarbons (PAHs) content

Biochar and peat amendments affect nitrogen retention, microbial capacity and nitrogen cycling microbial communities in a metal and polycyclic aromatic hydrocarbon contaminated urban soil

Soil contaminants may restrict soil functions. A promising soil remediation method is amendment with biochar, which has the potential to both adsorb contaminants and improve soil health. However, effects of biochar amendment on soil-plant nitrogen (N) dynamics and N cycling microbial guilds in contaminated soils are still poorly understood. Here, a metal- and polycyclic aromatic hydrocarbon (PAH) contaminated soil was amended with either biochar (0, 3, 6 % w/w) and/or peat (0, 1.5, 3 % w/w) in a full-factorial design and sown with perennial ryegrass in an outdoor field trial. After three months, N and the stable isotopic ratio δ15N was measured in soil, roots and leaves, along with microbial responses. Aboveground grass biomass decreased by 30 % and leaf N content by 20 % with biochar, while peat alone had no effect. Peat in particular, but also biochar, stimulated the abundance of microorganisms (measured as 16S rRNA gene copy number) and basal respiration. Microbial substrate utilization (MicroResp™) was altered differentially, as peat increased respiration of all carbon sources, while for biochar, respiration of carboxylic acids increased, sugars decreased, and was unaffected for amino acids. Biochar increased the abundance of ammonia oxidizing archaea, while peat stimulated ammonia oxidizing bacteria, Nitrobacter-type nitrite oxidizers and comB-type complete ammonia oxidizers. Biochar and peat also increased nitrous oxide reducing communities (nosZI and nosZII), while peat alone or combined with biochar also increased abundance of nirK-type denitrifiers. However, biochar and peat lowered leaf δ15N by 2-4 ‰, indicating that processes causing gaseous N losses, like denitrification and ammonia volatilization, were reduced compared to the untreated contaminated soil, probably an effect of biotic N immobilization. Overall, this study shows that in addition to contaminant stabilization, amendment with biochar and peat can increase N retention while improving microbial capacity to perform important soil functions.

Submerged plant-biochar composite system exhibits effective control over residual organic pollutants in the benthic organisms of aquaculture ponds

As of now, submerged plants and biochar have demonstrated significant benefits in aquaculture pond sediment remediation. However, there is limited research on the synergistic effects of biochar and submerged plants in mitigating hydrophobic organic contaminant (HOC) accumulation in aquaculture benthic organisms and in controlling the nutrient (nitrogen and phosphorus) levels in aquaculture water. This study assesses a submerged plant-biochar system's efficacy in removing HOCs from simulated freshwater aquaculture ponds. Vallisneria natans was planted in sediment with varying levels of wheat straw biochar, while Corbicula fluminea served as the targeted benthic organism. The bioaccumulation experiment identified the optimal biochar ratio for the Vallisneria natans-biochar system in controlling HOCs in aquaculture products. Analyses included final accumulation concentrations in benthic organisms, changes in freely-dissolved concentrations in aquaculture sediment, and a mass balance calculation to explore key factors in their removal from the system. Results indicated that the Vallisneria natans-1.5% biochar composite system achieved optimal control of HOCs in sediment and aquaculture products. Biochar addition to the sediment in the composite system demonstrated a "promotion with low addition, inhibition with high addition" effect on Vallisneria natans growth. Notably, the addition of 1.5% biochar (VN1.5 group) significantly promoted the growth of Vallisneria natans leaves and roots. Comparing the final pollutant proportions in different environmental media, concentrations in water (0.20%-1.8%), clam accumulation (0.032%-0.11%), and plant absorption (0.10%-0.44%) constituted a minimal portion of the overall pollutant load in the system. The majority of pollutants (24%-65%) were degraded in the aquaculture environment, with microbial degradation likely playing a predominant role. Bacterial phyla, particularly Proteobacteria and Firmicutes, were identified as potential direct contributors to pollutant degradation in the Vallisneria natans-biochar system.

The high dose of biochar reduces polycyclic aromatic hydrocarbons losses during co-composting of sewage sludge and wheat straw

The aim of the study was to investigate the effect of the biochar (BC) dose on solvent extractable (Ctot) and freely dissolved (Cfree) polycyclic aromatic hydrocarbons (PAHs) content during co-composting. A significantly better reduction of Σ16 Ctot PAHs after 98 days occurred during composting with BC (for 1% of BC - 44% and for 5% of BC - 23%) than in the control (15%). Despite the relatively high reduction of Ctot PAHs in the experiment with 5% BC rate, the content of the PAHs was still the highest compared to other variants. Regarding Cfree PAHs, 5% rate of BC resulted in the best reduction of PAHs, while the 1% BC dose resulted in a lower reduction of Cfree than the control. For 1% BC, PAHs losses was more effective, and sequestration processes played a less significant role than in the experiment with 5% dose of BC. The total and dissolved organic carbon, and ash were predominantly responsible for Ctot and Cfree losses, and additionally pH for Cfree. The results of the experiment indicate that BC performs a crucial role in composting, affecting the Ctot and Cfree PAHs in the compost but the final effect strictly depends on the BC dose.

Biochar-compost as a new option for soil improvement: Application in various problem soils

Due to the synergistic effects of biochar and compost/composting, the combined application of biochar and compost (biochar-compost) has been recognized as a highly promising and efficient method of soil improvement. However, the willingness to apply biochar-compost for soil improvement is still low compared to the use of biochar or compost alone. This paper collects data on the application of biochar-compost in several problem soils that are well-known and extensively investigated by agronomists and scientists, and summarizes the effects of biochar-compost application in common problem soils. These typical problem soils are classified based on three different characteristics: climatic zones, abiotic stresses, and contaminants. The improvement effect of biochar-compost in different soils is assessed and directions for further research and suggestions for application are made. Generally, biochar-compost mitigates the high mineralization rate of soil organic matter, phosphorus deficiency and aluminum toxicity, and significantly improves crop yields in most tropical soils. Biochar-compost can help to achieve long-term sustainable management of temperate agricultural soils by sequestering carbon and improving soil physicochemical properties. Biochar-compost has shown positive performance in the remediation of both dry and saline soils by reducing the threat of soil water scarcity or high salinity and improving the consequent deterioration of soil conditions. By combining different mechanisms of biochar and compost to immobilize or remove contaminants, biochar-compost tends to perform better than biochar or compost alone in soils contaminated with heavy metals (HMs) or organic pollutants (OPs). This review aims to improve the practicality and acceptability of biochar-compost and to promote its application in soil. Additionally, the prospects, challenges and future directions for the application of biochar-compost in problem soil improvement were foreseen.

Improvement in the physicochemical characteristics of biochar derived from solid digestate of food waste with different moisture contents

The management of digestate from food waste (DFW) has become a worldwide challenge. Pyrolysis is a promising technology to generate biochar from the DFW. However, unlike other biomass, DFW usually has high salt and moisture content, which affects the properties of biochar generated from pyrolysis. The characteristics of biochar derived from DFW with different MCs (5%, 20%, 40%, and 60%) were investigated in the present study. It was found that more micropore and mesopore structures were generated in the biochar with the increase of MC from 5% to 60%, resulting in the Brunauer-Emmett-Teller surface area of the biochar increased from 89.23 m² g^-1 to 117.75 m² g^-1. The MC could also promote the variation of oxygen-containing functional groups and the generation of amorphous carbon structures, which are beneficial for the adsorption property of the biochar. Pyrolysis could stabilize the metals in the biochar, while MC has little effect on the metal speciations. These results provide fundamental information on the impact of MC on the properties of biochar derived from DFW and are important for the optimization of the pre-drying process.

A Comprehensive Insight on Adsorption of Polyaromatic Hydrocarbons, Chemical Oxygen Demand, Pharmaceuticals, and Chemical Dyes in Wastewaters Using Biowaste Carbonaceous Adsorbents

Recent trends in adsorption of hazardous organic pollutants including Polyaromatic Hydrocarbons (PAHs), Chemical Oxygen Demand (COD), Pharmaceuticals, and Chemical Dyes in wastewater using carbonaceous materials such as activated carbon (AC) and biochar (BC) have been discussed in this paper. Utilization of biomass waste in the preparation of AC and BC has gained a lot of attention recently. This review outlines the techniques used for preparation, modification, characterization, and application of the above-mentioned materials in batch studies. The approaches towards understanding the adsorption mechanisms have also been discussed. It is observed that in the majority of the studies, high removal efficiencies were reported using biowaste adsorbents. Regarding the full potential of adsorption, varying values were obtained that are strongly influenced by the adsorbent preparation technique and adsorption method. In addition, most of the studies were concentrated on the kinetic, isotherm equilibrium, and thermodynamic aspects of adsorption, suggesting the dominant isotherm and kinetic models as Langmuir or Freundlich and pseudo-second-order models. Due to development in biosorbents, adsorption has been found to be increasingly economical. However, application of these adsorbents at commercial scale has not been adequately investigated and needs to be studied. Most of the studies have been conducted on synthetic solutions that do not completely represent the discharged effluents. This also needs attention in future studies.

Stabilization of heavy metals in sewage sludge by attapulgite

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

Nitrogen Losses and Potential Mitigation Strategies for a Sustainable Agroecosystem

Nitrogen (N) in the agricultural production system influences many aspects of agroecosystems and several critical ecosystem services widely depend on the N availability in the soil. Cumulative changes in regional ecosystem services may lead to global environmental changes. Thus, the soil N status in agriculture is of critical importance to strategize its most efficient use. Nitrogen is also one of the most susceptible macronutrients to environmental loss, such as ammonia volatilization (NH3), nitrous oxide (N2O) emissions, nitrate leaching (NO3), etc. Any form of N losses from agricultural systems can be major limitations for crop production, soil sustainability, and environmental safeguard. There is a need to focus on mitigation strategies to minimize global N pollution and implement agricultural management practices that encourage regenerative and sustainable agriculture. In this review, we identified the avenues of N loss into the environment caused by current agronomic practices and discussed the potential practices that can be adapted to prevent this N loss in production agriculture. This review also explored the N status in agriculture during the COVID-19 pandemic and the existing knowledge gaps and questions that need to be addressed.

The effect of particle size of bamboo biochar on the phytoremediation of Salix psammophila C. to multi-metal polluted soil

Biochar shows great potential in soil remediation. The benefits of biochar on soil depend onits intrinsic properties and soil characteristics. However, the influence of particle sizes of biochar on soil remediation is not clear. In a pot experiment, we evaluated the effects of bamboo biochar (BBC) particle sizes (P1 < 0.15 mm, 0.15 mm < P2 < 0.25 mm, 0.25 mm < P3 < 0.50 mm) on phytoremediation efficiency of Salix psammophila C. cultivated in multi-metal polluted soil. We added the BBC at 3% (w/w) in tested soil. Next, the BBC was thoroughly mixed with soil and weighting to the pot, and S. psammophila cuttings were planted and grown for six months in the amended soil under model growth condition.Results revealed the addition of different sizes of BBC particles affected soil quality, plant growth, and HMs accumulation in plants. All sizes of BBC treatments improved Cd and Zn accumulation, whereas plants in P2 treatment showed the greatest accumulation, increased by 52.41 and 25.55% compared with the control (1,503 and 19,928 μg·plant^-1). Overall, the results indicated BBC enhanced the phytoremediation efficiency of S. psammophila. Plants cultivated in P2 treatment showed the most significant effect on remediating contaminated soil.

The role of biochar in organic waste composting and soil improvement: A review

Large amounts of organic wastes, which pose a severe threat to the environment, can be thermally pyrolyzed to produce biochar. Biochar has many potential uses owing to its unique physicochemical properties and attracts increasing attentions. Therefore, this review focuses on the agronomic functions of biochar used as compost additives and soil amendments. As a compost additive, biochar provides multiple benefits including improving composting performance and humification process, enhancing microbial activities, reducing greenhouse gas and NH₄ emissions, immobilizing heavy metals and organic pollutants. As a soil amendment, biochar shows a good performance in improving soil properties and plant growth, alleviating drought and salinity stresses, interacting with heavy metals and organic pollutants and changing their fate of being uptaken from soils to plants. Furthermore, combined application of biochar and compost shows a good performance and a high agricultural value when applied to soils. Objectively and undeniably, there are still negative or ineffective cases of biochar amendment on crop yield and heavy metal immobilization, which is worthy of further attention. The medium-long term field monitoring of biochar-specific agricultural functions, as well as the exploration of wider sources for biochar feedstocks, are still needed.

Integrating metabolomics and physiological analysis to investigate the toxicological mechanisms of sewage sludge-derived biochars to wheat

Effects of sewage sludge biochars (SSBCs) on the growth of wheat and the specific toxicological mechanisms were investigated from a metabolic perspective for better ecological risk assessment. We observed that conversion of sludge to biochar remarkably changed the properties, and also caused a significant (p < 0.05) reduction of the toxicity towards wheat. Wheat growth under exposure to SSBCs was influenced by the pyrolysis temperature (300 °C, 500 °C and 700 °C), with root length being promoted by SSBCs prepared at higher temperatures (500 °C and 700 °C). In addition to the contaminants, including polycyclic aromatic hydrocarbons (PAHs) and potentially toxic elements (PTEs) detected in SSBCs, the morphological characteristics of biochars contributed substantially to the wheat growth. Metabolomics analysis revealed the remarkable differences in the metabolic profiles among the control (CK), SS300- and SS700-treated samples. The toxicological mechanisms involved were found to be associated with the regulation of metabolisms pathways of protein, fatty acids and carbohydrates, among which protein metabolism was most affected by SSBCs. This work presents an innovative concept that SSBCs produced at a proper temperature may minimize the toxic effects on plant growth by regulating the metabolic fluxes in vivo.

Feasibility of sludge-based biochar for soil remediation: Characteristics and safety performance of heavy metals influenced by pyrolysis temperatures

Sludge-based biochars (SBB) were prepared to evaluate their physiochemical properties and safety performance for the possible application in soil amendments in this study. SBB were produced at the temperatures ranging from 300 to 900 °C at 200 °C intervals. Both the solid fraction and the soluble organic fraction of SBB were analyzed. The pyrolysis temperature was found to affect the characteristics of solid fraction of the SBB greatly, in terms of the pH, surface area and functional groups. The content and composition of dissolved organic matter in SBB were influenced evidently by pyrolysis temperatures, which was mainly comprised of humic-like compounds with the molecular weight in a range of 0.13-2.4 × 10⁵ kDa. The safety performance of heavy metals in SBB at different temperatures were analyzed: (i) The bioavailable fractions (F1+F2+F3) of heavy metals significantly decreased from 91.65% to 9.44% for Cu, from 98.82% to 63.34% for Zn, from 97.91% to 52.11% for As, from 55.91% to 4.87% for Pb, from 78.20% to 12.50% for Cd, and from 73.51% to 9.57% for Cr when sludge was pyrolyzed to biochars at 900 °C.; (ii) Acid and alkaline conditions promoted the leaching of heavy metals from SBB. The luminescence inhibition of Vibrio fischeri was significantly decreased from 81.41% to 6.01% with the increasing pyrolysis temperatures. Compared with the raw sludge addition, the shoot length, root length and activities of soil microbes in sandy soil and loamy soil with pyrolyzed sludge under different pyrolysis temperatures were increased by 37.5-53.32%, 66.81-96.45%, 92.31-157.69% and 154.74-195.76%, respectively. The biotoxicity tests indicated the relatively safe and reliable performance of SBB. The study provided significant perspectives on the application of SBB as the potential soil amendments.

Adsorption and desorption of heavy metals by the sewage sludge and biochar-amended soil

The goal of the study was to evaluate the application of biochar (BC) to the sewage sludge (SL) on the adsorption and desorption capacity of Cd(II), Cu(II), Ni(II) and Zn(II). The effect of biochar contribution in the sewage sludge (2.5, 5 and 10%) was investigated. The isotherms data were fitted to the Langmiur (LM), Freundlich (FM) and Temkin (TM) models. The best fitting for kinetic study was obtained for the pseudo-second-order equation. The best fitting of the experimental data was observed for the LM in the case of SL and BC, and for the FM in the case of SL- and SL/BC-amended soil. SL was characterized by even four-order higher sorption capacity than BC. The addition of the BC to the SL and next to the soil increased the adsorption capacity of the soil and the SL-amended soil. In the case of all investigated potentially toxic elements (PTEs), the highest adsorption capacity was achieved for SL-amended soil in comparison with the control soil. In the case of other experimental variants, the adsorption capacity of metal ions was as follows: 2.5% BC > 5.0% BC > 10% BC. The negative correlation between hydrated radius of metal ions and the kinetics of sorption was observed. However, the desorption of PTEs from BC/SL-amended soil was significantly lower than for SL-amended soil (except of Cd) and non-amended soil. It can be concluded that the addition of the biochar enhanced the immobilization of PTEs and reduced their bioavailability and mobility in the soil amended by the sewage sludge.

Carbon dioxide as a carrier gas and biomass addition decrease the total and bioavailable polycyclic aromatic hydrocarbons in biochar produced from sewage sludge

Organic-solvent extractable (C_tot) and freely dissolved (C_free) polycyclic aromatic hydrocarbons (PAHs) (US EPA 16 PAHs) were determined in biochars produced from the mixture of sewage sludge and sewage sludge and willow (8:2 or 6:4, w/w). The pyrolysis was carried out at 500, 600, and 700 °C using two different carrier gases (N₂ or CO₂). Addition of willow and the change of carrier gas from N₂ to CO₂ reduced C_tot PAHs (from 7.0 to 52%) and C_free PAHs (15-29%) content. Co-application of willow and SSL and the use of CO₂ as a carrier gas also beneficially affected the PAHs profile. The biochars produced with willow addition and/or in a CO₂ atmosphere were characterized by a lower (from 9.0 to 62.8%) percentage of 3-6-ring PAHs (C_tot) than the biochars derived from sewage sludge alone in N₂ atmosphere. The contribution of individual C_free PAHs did not differ significantly between biochars. The presence of willow during pyrolysis influenced the direction of the changes in the C_tot PAH content depending on the pyrolysis temperature. For SSL alone, regardless of the carrier gas used, the content of C_tot PAHs was observed to decrease with increasing temperature, whereas in the presence of willow temperature did not affect significantly (P ≥ 0.05) the C_tot PAHs content. The change of carrier gas from N₂ to CO₂ increased the affinity of the biochars to PAHs as confirmed by the distribution coefficients log K_TOC.

Combined Effects of Plant Cultivation and Sorbing Carbon Amendments on Freely Dissolved PAHs in Contaminated Soil

We report freely dissolved concentrations ( C_free) of PAHs in soils amended with 2.5% biochar and activated carbon (AC) during a long-term (18-months) field experiment. The study evaluates also the impact of different plants (clover, grass, willow) on C_free PAHs. The cumulative effect of treatments on nitrogen and available forms of phosphorus, potassium, and magnesium is also assessed. The direct addition of biochar to soil did not cause any immediate reduction of the sum of 16 C_free PAHs, while AC resulted in a slight reduction of 5- and 6 ring compounds. The efficiency of binding of C_free PAHs by biochar and AC increased with time. For biochar, the maximum reduction of 4-6-ring PAHs (18-67%) was achieved within 6 months. For 2- and 3-ring PAHs, a gradual decrease of C_free was observed which reached 60-66% at 18 months. AC proved to be better in reducing C_free PAHs than biochar, though for 2- and 3-ring PAHs, the differences in AC and biochar performances were smaller than those for 4-6-ring PAHs. After 18 months, a significantly lower content of C_free PAHs was observed in the soil with plants compared to the unplanted soil. Except for potassium, AC or biochar did not negatively impact nutrient availability.

Influence of immobilization on phenanthrene degradation by Bacillus sp. P1 in the presence of Cd(II)

Suspended microbes gradually lost advantages in practical applications of PAHs and heavy metals bioremediation. Therefore this study investigated the effect of immobilization on phenanthrene degradation by Bacillus sp. P1 in the presence of different Cd(II) concentrations. Condensed Bacillus sp. P1 was immobilized with polyvinyl alcohol and sodium alginate and PVA-SA-cell cryogel beads were prepared. The results indicated that the use of gel beads increased the number of adsorption sites thus accelerating phenanthrene degradation. In addition, changes in detoxification indices, including superoxide dismutase (SOD), catalase (CAT) and glutathione (GSH), were determined to elucidate the immobilization mechanisms related to cells protection from Cd(II) when degrading phenanthrene. By protecting the gel membrane, oxidative damage was minimized, while SOD activity increased from 55.72 to 81.33 U/mgprot as Cd(II) increased from 0 to 200 mg/L but later dropped to 44.29 U/mgprot as Cd(II) increased to 300 mg/L for the non-immobilized system. On the other hand, the SOD activity kept increasing from 52.23 to 473.35 U/mgprot for the immobilized system exposed to Cd(II) concentration between 0 and 300 mg/L. For CAT and GSH, immobilization only slowed down the depletion process without any change on the variation trends. The changes in surface properties and physiological responses of microbes caused the differences of immobilization effect on phenanthrene biodegradation in the presence of Cd(II), which is a novel finding.

Four decades since the ban, old urban wastewater treatment plant remains a dominant source of PCBs to the environment

Despite the ban on new manufacture and commercial use of PCBs, municipal sewer systems continue to serve as ongoing secondary sources for contamination in receiving water bodies. Ongoing PCB sources have made it difficult to achieve desired recovery after implementation of sediment cleanup efforts. We report on a 16-month surveillance to determine the inputs, fate, and export of PCBs within a municipal waste collection/treatment system by strategic sampling of the freely-dissolved and biosolids-associated PCBs. The total PCBs entering the treatment plant was found to be 170 g/day of which 100 g/day exited the plant associated with the biosolids and 5.2 g/day was discharged in the form of freely-dissolved PCBs in the effluent. A net loss of 68 g/day was calculated for the plant, attributable to volatilization and biodegradation. Freely dissolved PCBs in the treated effluent was an order of magnitude higher than the water quality criteria for the protection of human health through fish consumption and found to be a major contributor to the dissolved concentration in the receiving river. Predicted bioaccumulation in fish from dissolved PCBs in the effluent exceeded the threshold for human consumption. The biosolids, currently land-applied as fertilizer, contained an average PCB concentration of 760 μg/kg. The sludge produced in this treatment plant is processed in large anaerobic digesters and changes to the homolog distribution point to some microbial dechlorination. Application of biosolids to clean agricultural soil resulted in a 6-fold increase in PCB levels in the earthworm E. fetida which could be eliminated by the amendment of 1% by weight of activated carbon.

Particle size dependence of the physicochemical properties of biochar

The inadequate definition of different particle sizes of biochar impedes the establishment of guidelines concerning the selection of suitable particle sizes. In this study, various properties of a series of ten biochar samples of varying particle size (5-5000 μm) from the same origin have been investigated and compared. Cluster analysis and principal component analysis based on all of the parameters were executed to evaluate the dissimilarity of different particle sizes of biochar, and to classify the size ranges determining the relevant properties. The results showed that the series of ten biochar samples with different particle sizes could be separated into four groups, indicating that there were significant characteristic differences between different particle sizes of biochar. The greater the difference in particle size, the more disparate the properties. Moreover, 75-150 μm has been identified as a turning point along the continuous particle size range. A deviation in the properties of biochar particles smaller than 5 μm confirmed that different preparation methods would lead to highly significant differences between different particle sizes of biochar, even if the pristine materials were the same. The need for definition of the characteristics of biochar required for a specific environmental application should be emphasized.

Impact of sludge conditioning treatment on the bioavailability of pyrene in sewage sludge

Conditioning is an indispensable step to improve mechanical dewatering of municipal sewage sludge. However, it is still unclear how sludge conditioning treatments impact the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in sewage sludge that potentially influences the biodegradation of PAHs during the composting of dewatered sludge cake. In the present study, five sludge conditioning treatments, including chemical acidification, bioleaching driven by Acidithiobacillus ferrooxidans, chemical conditioning with Fe[III] and CaO, and chemical conditioning with either aluminum polychloride (PACl) or polyacrylamide (PAM), were investigated to reveal their respective impacts on the bioavailability of pyrene in sewage sludge. The bioavailability of pyrene in conditioned sludge was evaluated by using the n-butanol extraction method. The results showed that the bioavailable fraction of pyrene increased from 59.1% in raw sludge to 68.7% in chemically acidified sludge and 79.3% in bioleached sludge, while the other three conditioning approaches did not significantly change the bioavailability of pyrene. During chemical acidification or bioleaching of sludge, cellular membrane damage of sludge microbial cells induced changes in sludge chemical and physical properties. Ridge regression analysis revealed that during these two conditioning processes the contribution rates of the changes in sludge chemical properties and physical properties on the enhancement of pyrene bioavailability were 33.0% and 67.1%, respectively. Therefore, chemical acidification and bioleaching conditioning treatments can enhance the bioavailability of pyrene in sewage sludge, mainly through changing the relative hydrophobicity and particle size of sludge flocs.

Application of biochar to sewage sludge reduces toxicity and improve organisms growth in sewage sludge-amended soil in long term field experiment

The aim of the present study was to determine changes in the physicochemical properties and toxicity of soil amended with sewage sludge (10t_dw/ha) or sewage sludge (10t_dw/ha) with biochar addition (2.5, 5 or 10% of sewage sludge). The study was carried out as a field experiment over a period of 18months. Samples for analysis were taken at the beginning of the experiment as well as after 6, 12 and 18months. The study investigated toxicity of the unamended soil, sewage sludge-amended soil and sewage sludge-amended soil with biochar addition towards Folsomia candida (collembolan test) and Lepidium sativum (Phytotoxkit F). Moreover, toxicity of aqueous extracts obtained from the tested soils towards Vibrio fischeri (Microtox®) and Lepidium sativum (elongation test) was determined. The study showed that addition of biochar to the sewage sludge and soil reduced leaching of nutrients (mainly phosphorus and potassium) from the amended soil. Biochar significantly reduced sewage sludge toxicity, exhibiting a stimulating effect on the tested organisms. The stimulating effect of biochar addition to the sewage sludge persisted throughout the entire experiment. Apart from the remediatory character of biochar, this is also evidence of its fertilizing character. In the tests with L. sativum (leachates and solid phase) and V. fischeri (leachates), increasing the rate of biochar in the sewage sludge increased root growth stimulation (L. sativum) and bacteria luminescence (V. fischeri). However, increasing biochar rate decreased F. candida reproduction stimulation, which could have been an effect of reduced nutrient bioavailability due to the biochar.

A field study of bioavailable polycyclic aromatic hydrocarbons (PAHs) in sewage sludge and biochar amended soils

The bioavailable PAHs (freely dissolved concentration, Cfree) were determined in sewage sludge (SL) or sewage sludge and biochar (BC) amended soil. SL or SL with a 2.5, 5 or 10% of BC was applied to the soil. The study was conducted as a long-term field experiment. Addition of BC to SL at a dose of 2.5 and 5% did not affect the content of Cfree PAHs in soils. However a significant difference (by 13%) in Cfree PAHs content was noted in experiment with 10% addition of BC. During the experiment, the concentration of Cfree PAHs in SL- and SL/BC-amended soil decreased. In particular sampling terms the content of Cfree PAHs in SL/BC-amended soil was significantly lower comparing to the Cfree PAHs content in SL-amended soil. After 18 months, Cfree PAH content was significantly lower in SL/BC-amended soil than in the experiment with SL alone, and did not differ significantly from the Σ16 Cfree content in the control soil. The largest decrease relative to the soil with sewage sludge alone was observed for 3-, 5- and 6-ring PAHs. This is the first field-based evidence that biochar soil amendment was effective to reduce of the Cfree of PAHs in sewage sludge-amended soils.

Insight into Multiple and Multilevel Structures of Biochars and Their Potential Environmental Applications: A Critical Review

Biochar is the carbon-rich product of the pyrolysis of biomass under oxygen-limited conditions, and it has received increasing attention due to its multiple functions in the fields of climate change mitigation, sustainable agriculture, environmental control, and novel materials. To design a "smart" biochar for environmentally sustainable applications, one must understand recent advances in biochar molecular structures and explore potential applications to generalize upon structure-application relationships. In this review, multiple and multilevel structures of biochars are interpreted based on their elemental compositions, phase components, surface properties, and molecular structures. Applications such as carbon fixators, fertilizers, sorbents, and carbon-based materials are highlighted based on the biochar multilevel structures as well as their structure-application relationships. Further studies are suggested for more detailed biochar structural analysis and separation and for the combination of macroscopic and microscopic information to develop a higher-level biochar structural design for selective applications.

Co-application of sewage sludge with biochar increases disappearance of polycyclic aromatic hydrocarbons from fertilized soil in long term field experiment

The application of sewage sludge with biochar as fertilizer may be a new method improves soil properties. Biochar increases of the crops productivity and reduction of bioavailability of contaminants. In the present study the persistence of sum of 16 (Σ16) PAHs (US EPA 16 PAHs) in a sewage sludge-amended soil (11t/h) and in a sewage sludge-amended soil with the addition of biochar (at a rate of 2.5, 5 or 10% of sewage sludge (dry weight basis)) was determined. This study was carried out as a plot experiment over a period of 18months. Samples for analysis were taken at the beginning of the study and after 6, 12 and 18months from the beginning of the experiment. Application of sewage sludge as a soil amendment did not cause a significant change (P≥0.05) in the soil content of Σ16 PAHs. In turn, the addition of biochar with sewage sludge to the soil, regardless of the contribution of biochar in the sewage sludge, resulted in a significant decrease in PAH content already at the beginning of the experiment. Throughout the experiment, in all treatments the PAH content varied, predominantly showing a decreasing trend. Ultimately, after 18months the content of Σ16 PAHs decreased by 19% in the experiment with sewage sludge alone and by 45, 35 and 28% in the experiment with sewage sludge and the 2.5%, 5.0% and 10% biochar rates, respectively. After 18months of the study, the largest losses in the sewage sludge-amended soil were observed for 2- and 3-ring PAHs. In the sewage sludge- and biochar-amended soil, compared to the beginning of the study and the sewage sludge-amended soil, the highest losses were found for 5- and 6-ring PAHs (2.5 and 5.0% rates) as well as for 5- and 2-ring PAHs (10% rate).

Biochar for composting improvement and contaminants reduction. A review

Biochar is characterised by a large specific surface area, porosity, and a large amount of functional groups. All of those features cause that biochar can be a potentially good material in the optimisation of the process of composting and final compost quality. The objective of this study was to compile the current knowledge on the possibility of biochar application in the process of composting and on the effect of biochar on compost properties and on the content of contaminants in compost. The paper presents the effect of biochar on compost maturity indices, composting temperature and moisture, and also on the content and bioavailability of nutrients and of organic and inorganic contaminants. In the paper note is also taken of the effect of biochar added to composted material on plants, microorganisms and soil invertebrates.

Role of biochar on composting of organic wastes and remediation of contaminated soils-a review

Biochar is produced by pyrolysis of biomass residues under limited oxygen conditions. In recent years, biochar as an amendment has received increasing attention on composting and soil remediation, due to its unique properties such as chemical recalcitrance, high porosity and sorption capacity, and large surface area. This paper provides an overview on the impact of biochar on the chemical characteristics (greenhouse gas emissions, nitrogen loss, decomposition and humification of organic matter) and microbial community structure during composting of organic wastes. This review also discusses the use of biochar for remediation of soils contaminated with organic pollutants and heavy metals as well as related mechanisms. Besides its aging, the effects of biochar on the environment fate and efficacy of pesticides deserve special attention. Moreover, the combined application of biochar and compost affects synergistically on soil remediation and plant growth. Future research needs are identified to ensure a wide application of biochar in composting and soil remediation. Graphical abstract ᅟ.

Effect of co-application of nano-zero valent iron and biochar on the total and freely dissolved polycyclic aromatic hydrocarbons removal and toxicity of contaminated soils

The aim of this study was to investigate co-application of biochar and nano zero-valent iron (nZVI) in order to increase the degradation of PAHs and reduce the toxicity of soils historically contaminated with these compounds. To performed the experiment biochar, biochar with nZVI (2 g kg^-1 or 10 g kg^-1 soil), or nZVI alone (2 g kg^-1 or 10 g kg^-1 soil) were added to the PAHs contaminated soils. The soils alone and soils with amendments were aged by mixing for 7 and 30 days. After that the chemical analysis were carried out and total (C_tot) and C_free PAH content in the samples were determined. Moreover, the toxicity of aqueous extracts were investigated using the Microtox^® (Vibrio fischeri) method. Results showed that any of used nZVI dose did not reduce the content of C_tot or C_free PAHs in contaminated soils, but biochar applied both alone and together with the nZVI significantly reduced C_tot and C_free PAHs. However, no significant differences in PAH reduction were found between biochar alone and biochar with nZVI addition. This indicates that the observed reduction was mostly associated with the sorption properties of biochar. Moreover, only in the case of co-application of biochar and nZVI reduction of the toxicity of nZVI to V. fischeri was observed. The toxic effect was different and depend on the type of soil and their properties including total organic carbon and black carbon content, which may affect the PAHs reduction efficiency.

Biochar amendment for integrated composting and vermicomposting of sewage sludge - The effect of biochar on the activity of Eisenia fetida and the obtained vermicompost

Sewage sludge derived biochar (SSDB) was used as a supplementary material for municipal sewage sludge (SS) and wood chips mixtures (WC) treated by combined composting and vermicomposting. SSDB added to the mixture before composting resulted in significantly higher reproduction rate: on week 4 the number of cocoons increased by 213% when compared to the mixture with no biochar. On week 6 the average number of juveniles increased 11-fold in the mixture with biochar added before composting and 5-fold in the mixtures with biochar added after composting when compared to the mixture with no biochar. Biochar added before composting reduced bioavailability of Cd and Zn to E. fetida. The biochar-added vermicomposts showed good fertilizing properties except for elevated concentrations of Cr. The pH of all vermicomposts was in the range of 5.27-5.61. The obtained vermicomposts can be used as a growing medium for horticultural purposes or as an amendment in calcareous soils.

Addition of biochar to sewage sludge decreases freely dissolved PAHs content and toxicity of sewage sludge-amended soil

Due to an increased content of polycyclic aromatic hydrocarbons (PAHs) frequently found in sewage sludges, it is necessary to find solutions that will reduce the environmental hazard associated with their presence. The aim of this study was to determine changes of total and freely dissolved concentration of PAHs in sewage sludge-biochar-amended soil. Two different sewage sludges and biochars with varying properties were tested. Biochars (BC) were produced from biogas residues at 400 °C or 600 °C and from willow at 600 °C. The freely dissolved PAH concentration was determined by means of passive sampling using polyoxymethylene (POM). Total and freely dissolved PAH concentration was monitored at the beginning of the experiment and after 90 days of aging of the sewage sludge with the biochar and soil. Apart from chemical evaluation, the effect of biochar addition on the toxicity of the tested materials on bacteria - Vibrio fischeri (Microtox^®), plants - Lepidium sativum (Phytotestkit F, Phytotoxkit F), and Collembola - Folsomia candida (Collembolan test) was evaluated. The addition of biochar to the sewage sludges decreased the content of C_free PAHs. A reduction from 11 to 43% of sewage sludge toxicity or positive effects on plants expressed by root growth stimulation from 6 to 25% to the control was also found. The range of reduction of C_free PAHs and toxicity was dependent on the type of biochar. After 90 days of incubation of the biochars with the sewage sludge in the soil, C_free PAHs and toxicity were found to further decrease compared to the soil with sewage sludge alone. The obtained results show that the addition of biochar to sewage sludges may significantly reduce the risk associated with their environmental use both in terms of PAH content and toxicity of the materials tested.

Effect of steam activated biochar application to industrially contaminated soils on bioavailability of polycyclic aromatic hydrocarbons and ecotoxicity of soils

The aim of this study was to determine the effect of steam activation of biochars on the immobilization of freely dissolved (Cfree) and bioaccessible fraction (Cbioacc) of PAHs in soils. Additionally, the toxicity to various organisms like Vibrio fischeri, Lepidium sativum and Folsomia candida was measured before and after the amendment of biochars to soils. Three biochars produced from willow, coconut and wheat straw were steam activated and added to three different soils with varying content and origin of PAHs (coke vs. bitumen). The soils with the addition of the biochars (activated and non-activated) were incubated for a period of 60days. Steam activation of the biochars resulted in more pronounced reduction of both Cfree and Cbioacc. The range of the increase in effectiveness was from 10 to 84% for Cfree and from 50 to 99% for Cbioacc. In contrast, the effect of activation on the toxicity of the soils studied varied greatly and was specific to a particular test and soil type. Essentially, biochar activation did not result in a change of phytotoxicity, but it increased or decreased (depending on the parameter, type of biochar, contaminant source, and soil and soil type) the toxic effect to F. candida, and decreased the toxicity of leachates to V. fischeri.

Wheat straw biochar amendments on the removal of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil

Soil amendments of wheat straw biochar (BC), lignocellulosic substrate (LS), BC+LS, and BC+LS+BR (surfactant Brij30) were investigated for the first time in order to remedy polycyclic aromatic hydrocarbons (PAHs)-polluted soil using pilot scale microcosm incubation. We hypothesized that the removal of PAHs could be inhibited due to the adsorption and immobilization of biochar and the inhibition depends on the molecular-weight of PAHs. The removal rates of phenanthrene (PHE) and Benzo[a]pyrene (BaP) ranked as C=BC>LS=LS+BC=LS+BC+BR and C=BC=LS+BC+BR>LS=LS+BC. Wheat straw biochar inhibited the removal of PHE and accelerated BaP removal. The activity of Dehydrogenase (DH) was depressed by the addition of the biochar while the activity of polyphenol oxidase (PPO) was stimulated. Lignocellulose and surfactant are favourable to sustain soil microbiological activity and the removal of PAHs although the diversity of bacterial community was not significantly changed. The findings implied that the components of PAHs are necessary to consider when the amendments are implemented by associated biochar in PAH-polluted soil.

The concentration and changes in freely dissolved polycyclic aromatic hydrocarbons in biochar-amended soil

The presence of polycyclic aromatic hydrocarbons (PAHs) in biochars hinders their environmental use. The aim of this study was to determine the freely dissolved (Cfree) PAH content in soil amended with biochar in a long-term (851 days) field experiment. Biochar was added to the soil at a rate of 30 and 45 t/ha. The addition of biochar to the soil resulted in a decrease in Σ13 Cfree PAHs by 25 and 22%, in the soil with the addition of biochar at the rate of 30 and 45 t/ha, respectively. As far as individual PAHs are concerned, in most cases a reduction in Cfree was also observed (from 3.6 to 66%, depending on the biochar rate). During the first 105 days of the experiment, the content of Σ13 Cfree in the biochar-amended soil significantly decreased by 26% (30 t/ha) and 36% (45 t/ha). After this period of time until the end of the experiment, no significant changes in Cfree were observed, regardless of the biochar rate. However, the behavior of individual PAH groups differed depending on the number of rings and experimental treatment. Ultimately, after 851 days of the experiment the content of Σ13 Cfree PAHs was lower by 29% (30 t/ha) and 35% (45 t/ha) compared to the beginning of the study as well as lower by 40% (30 t/ha) and 42% (45 t/ha) than in the control soil. The log KTOC coefficients calculated for the biochar-amended soils were higher immediately after adding biochar and subsequently they gradually decreased, indicating the reduced strength of the interaction between biochar and the studied PAHs. The obtained results show that the addition of biochar to soil does not create a risk in terms of the content of Cfree PAHs.

Effect of activated carbon and biochars on the bioavailability of polycyclic aromatic hydrocarbons in different industrially contaminated soils

Coal production negatively affects the environment by the emission of polycyclic aromatic hydrocarbons (PAHs). Two soils (KOK and KB) from a coking plant area was investigated and their total PAH concentration was 40 and 17 mg/kg for the sum (∑) 16 US EPA PAHs, respectively. A third soil was sampled from a bitumen plant area and was characterized by 9 mg/kg ∑16 US EPA PAHs. To reduce the freely dissolved concentration (Cfree) of the PAHs in the soil pore water, active carbon (AC) and two biochars pyrolysed from wheat straw (biochar-S) and willow (biochar-W) were added to the soils at 0.5-5 % (w/w), each. The AC performed best and reduced the Cfree by 51-98 % already at the lowest dose. The biochars needed doses up to 2.5 % to significantly reduce the Cfree by 44-86 % in the biochar-S and by 37-68 % in the biochar-W amended soils. The high black carbon (BC) content of up to 2.3 % in the Silesian soils competed with the sorption sites of the carbon amendments and the performance of the remediation was a consequence of the contaminant's source and the distribution between the BC and the AC/biochars. In contrast, the carbon amendment could best reduce the Cfree in the Lublin soil where the BC content was normal (0.05 %). It is therefore crucial to know the contaminant's source and history of a sample/site to choose the appropriate carbon amendment not only for remediation success but also for economic reasons.

Effect of pyrolysis temperatures on freely dissolved polycyclic aromatic hydrocarbon (PAH) concentrations in sewage sludge-derived biochars

The aim of this study was to evaluate the effect of sewage sludge pyrolysis on freely dissolved (Cfree) polycyclic aromatic hydrocarbon (PAH) contents in biochars. Four sewage sludges with varying properties and PAH contents were pyrolysed at temperatures of 500 °C, 600 °C or 700 °C. Cfree PAH contents were determined using polyoxymethylene (POM). The contents of Cfree PAHs in the sludges ranged from 262 to 294 ng L(-1). Sewage sludge-derived biochars have from 2.3- to 3.4-times lower Cfree PAH contents comparing to corresponding sewage sludges. The Cfree PAH contents in the biochars ranged between 81 ng L(-1) and 126 ng L(-1). As regards agricultural use of biochar, the lower contents of Cfree PAHs in the biochars compared to the sewage sludges makes biochar a safer material than sewage sludge in terms of PAH contents.

Reduced sulfentrazone phytotoxicity through increased adsorption and anionic species in biochar-amended soils

Burning straw in the field is a common agricultural practice. The effects of adding biochar derived from rice straw to soils on the phytotoxicity of sulfentrazone to Oryza sativa L. were observed. Overall, when 1 % biochar was added to three different soils, the phytotoxicity of sulfentrazone to O. sativa L. decreased, and the concentration that inhibits growth by 50 % (IC50) increased by 1.4 to 7.6 times. To illuminate the influencing mechanisms, the changes in sulfentrazone adsorption to the soil, the soil pH, and the bioavailable sulfentrazone extracted from the soil solution using hollow fiber-based liquid-phase microextraction were studied. The Freundlich constant (K f ) of sulfentrazone to the soil increased 1.5 to 25 times relative to the K f in the three unamended soils, and the soil pH increased by 0.36 to 1.36 units resulted in a fraction of dissociated sulfentrazone increased by 10.2-17.4 %. In addition, the average concentrations of sulfentrazone in the three unamended soil solutions were 1.3-6.1 times relative to those in the three biochar-amended soil solutions. These results suggest that the sulfentrazone adsorption and soil pH increased when soils were amended with biochar, which decreased the bioavailable concentrations and reduced its phytotoxicity to O. sativa L.

The total and freely dissolved polycyclic aromatic hydrocarbons content in residues from biogas production

In the situation of increasing agricultural utilization of residues from biogas production (RBP) it is important to determine the concentration of contaminants, which could occur in these materials. The group of contaminants that requires special attention are polycyclic aromatic hydrocarbons (PAH). The objective of the study was to determine the total and freely dissolved (Cfree) of PAHs in RBP from 6 different biogas plants operating under various temperature conditions and without or with the separation into the solid and liquid fractions. The freely dissolved PAHs were determined using polyoxymethylene (POM method). The total content of the Σ16 PAHs in RBP varied from 449 to 6147 μg/kgdw, while that of Cfree PAHs was at the level from 57 to 653 ng/L. No significant differences were noted in the content of the Σ16 PAHs (total) between the solid and the liquid fractions. This indicates that in the course of the separation, the PAHs are distributed proportionally between the fractions. However in the case of Cfree, PAHs content in the solid fraction was over twice as high as in the liquid fraction. This was probably due to the greater affinity of the particles present in the liquid fraction to the analysed PAHs than to the particles of the solid fraction. Higher affinity to liquid fraction was also confirmed by the distribution coefficients KTOC determined on the basis of Cfree.

The influence of various biochars on the bioaccessibility and bioaccumulation of PAHs and potentially toxic elements to turnips (Brassica rapa L.)

The influence of amending a contaminated soil with different dry-pyrolyzed biochars on the bioaccessibility and biouptake of polycyclic aromatic hydrocarbons (PAH) and potentially toxic elements (PTE) in turnip (Brassica rapa L.,) was investigated. This is the first study to examine the influence of biochar amendments on turnips grown in a contaminated soil. The biochars came from different local feedstocks, including sewage sludge biochar (SSBC), soybean straw biochar (SBBC), rice straw biochar (RSBC) and peanut shell biochar (PNBC). The biochars were applied to soil at 2% and 5% amendments, and the resulting influence on various soil and porewater properties were quantified. The bioaccessible concentrations of PAHs in soil and their bioaccumulation in B. rapa L. significantly (P < 0.05) decreased in the biochar amended soils. Biochar additions significantly (P ≤ 0.05) reduced the bioaccumulation of PTEs (As, Cd, Cu, Pb and Zn) in B. rapa L, though not as much as for PAHs. The most effective biochar at reducing both PAHs and PTEs was PNBC (P ≤ 0.05). Amendments of 5% biochar were more effective at reducing contaminant bioaccessibility than amendments at 2% (P < 0.05). Crop yield, however, increased the most for the 2% biochar amendments, in particular for SSBC (with a 49% increase in crop yield compared to the non-amended soil). Therefore, which biochar would be the most advantageous in this system would require a cost-benefit analysis between increasing crop yield (best achieved with 2% SSBC amendments) and decreasing the PAH and PTE uptake (best achieved with 5% PNBC amendments).

Heavy metal removal from MSS fly ash by thermal and chlorination treatments

The thermal behavior of heavy metals in the co-incineration of municipal solid waste-sludge incinerator fly ash (MSS fly ash) was studied using a laboratory-scale tube furnace. The results indicate that without the addition of chlorinating agents, temperature was an important parameter and had significantly influenced on heavy metal removal, whereas the residence time had a weak effect. Between 900 and 1000 °C for 60 to 300 min, heavy metals reacted with chloride-inherent in the fly ash, and approximately 80 to 89% of Pb, 48% to 56% of Cd, 27% to 36% of Zn and 6% to 24% of Cu were removed. After the adding chlorinating agents, the evaporation rate of the heavy metals improved dramatically, where the evaporation rates of Cu and Zn were larger than that of Pb and Cd. As the amount of added chlorinating agents increased, the removal rate of heavy metals increased. However, the effect of the type of chlorinating agent on the chlorination of heavy metals differed considerably, where NaCl had the weakest effect on the removal rate of Cu, Cd and Zn. In terms of resource recovery and decontamination, MgCl₂ and CaCl₂ are the best choices due to their efficient removal of Zn.

Application of sewage sludge and sewage sludge biochar to reduce polycyclic aromatic hydrocarbons (PAH) and potentially toxic elements (PTE) accumulation in tomato

The effects of sewage sludge (SS) and its derived biochar (SSBC) on the availability and uptake of polycyclic aromatic hydrocarbons (PAHs) and potential toxic elements (PTEs) by Solanum lycopersicum (tomato) fruits grown in contaminated urban soil were investigated. Increasing application rates of SS and SSBC (2, 5, and 10%) decreased PAH availability and, correspondingly, PAH accumulation (22-39 and 48-62%, respectively) into tomato. SSBC was more effective in this regard. The available concentrations of PAHs (Σ16PAH) in the SSBC treatments were significantly reduced (from 30.0-47.3%) as compared to the control treatment. The availability of high-molecular-weight PAHs (containing four to six benzene rings) was greatly affected, while low-molecular-weight PAHs (containing two to three benzene rings) was less affected by SSBC amendments. The addition of SSBC showed the least effect on bioaccumulation of naphthalene (two-ring PAH; 24.5-32.6%), while the highest effect was observed for benzo(b)fluoranthene (five-ring PAH; 3.1-86.8%) and benzo(g,h,i)perylene (six-ring PAH; 51.8-84.2%). In contrast, increasing application rates of SS successively increased PTE (As, Cd, Cu, Pb, and Zn) availability and accumulation (15-139%) into tomato while SSBC successively decreased PTE availability and accumulation (17-91%). Changes in accumulation varied with PTE and the extent to which PTE concentrations in soil was elevated.

Effects of sulfur on lead partitioning during sludge incineration based on experiments and thermodynamic calculations

Experiments in a tubular furnace reactor and thermodynamic equilibrium calculations were conducted to investigate the impact of sulfur compounds on the migration of lead (Pb) during sludge incineration. Representative samples of typical sludge with and without the addition of sulfur compounds were combusted at 850 °C, and the partitioning of Pb in the solid phase (bottom ash) and gas phase (fly ash and flue gas) was quantified. The results indicate that three types of sulfur compounds (S, Na2S and Na2SO4) added to the sludge could facilitate the volatilization of Pb in the gas phase (fly ash and flue gas) into metal sulfates displacing its sulfides and some of its oxides. The effect of promoting Pb volatilization by adding Na2SO4 and Na2S was superior to that of the addition of S. In bottom ash, different metallic sulfides were found in the forms of lead sulfide, aluminosilicate minerals, and polymetallic-sulfides, which were minimally volatilized. The chemical equilibrium calculations indicated that sulfur stabilizes Pb in the form of PbSO4(s) at low temperatures (<1000 K). The equilibrium calculation prediction also suggested that SiO2, CaO, TiO2, and Al2O3 containing materials function as condensed phase solids in the temperature range of 800-1100 K as sorbents to stabilize Pb. However, in the presence of sulfur or chlorine or the co-existence of sulfur and chlorine, these sorbents were inactive. The effect of sulfur on Pb partitioning in the sludge incineration process mainly depended on the gas phase reaction, the surface reaction, the volatilization of products, and the concentration of Si, Ca and Al-containing compounds in the sludge. These findings provide useful information for understanding the partitioning behavior of Pb, facilitating the development of strategies to control the volatilization of Pb during sludge incineration.

Effect of different sulfides on cadmium distribution during sludge combustion based on experimental and thermodynamic calculation approaches

The effects of sulfur compounds on the migration of a semi-volatile heavy metal (cadmium) during sludge incineration were investigated with two methods, i.e., experiments in a tubular furnace reactor and thermodynamic equilibrium calculations. The representative typical sludge with and without the addition of sulfur compounds was incinerated at 850 °C. The partitioning of Cd among the solid phase (bottom ash) and gas phase (fly ash and flue gas) was quantified. The results indicate that sulfur compounds in the elemental form and a reduced state could stabilize Cd in the form of CdS, aluminosilicate minerals, and polymetallic sulfides, whereas sulfur in the oxidized forms slightly increases Cd volatilization during incineration. For Cd solidification points, the inhibition effect on the volatilization of Cd is as follows: S > Na2SO4 > Na2S. Chemical equilibrium calculations indicate that sulfur binds with Cd and alters Cd speciation at low temperatures (<950 K). Furthermore, SiO2- and Al2O3-containing minerals can function as sorbents stabilizing Cd as condensed phase solids (CdSiO4 and CdAl2O4) according to the results of equilibrium calculations. These findings provide useful information for understanding the partitioning of Cd and thus facilitate the development of strategies to control Cd volatilization during sludge incineration.