Mechanism of aging of biochars obtained at different temperatures from sewage sludges with different composition and character

Innovative solutions for coastal wetlands pollution: Application of Spartina alterniflora Loisel. - biochar for cadmium removal and carbon sequestration

Coastal wetland ecosystems are extremely fragile, with heavy metal contamination and uncontrolled growth of invasive plants (e.g. Spartina alterniflora Loisel.) could result in threaten the ecological security and resource wastage. In this study, Spartina alterniflora-biochars at two different temperatures (400 and 600 °C), as well as phosphorus-modified biochars (400BC, 600BC, 400PBC, and 600PBC) were produced, and evaluated for their removal capacity in cadmium (Cd)-contaminated sediments. The maximum sorption capacity, as determined by the Langmuir equation, was in the order of 600BC (838.81 mg/g) > 400BC (707.25 mg/g) > 600PBC (581.69 mg/g) > 400PBC (455.82 mg/g) for Cd, with adsorption occurred through single-layer chemical processes. In the Cd removal tests, increasing incubation times led to a significant reduction in bioavailable Cd (from 84.74 mg/kg at 1 h to 34.79 mg/kg at 7 day). There were significantly negative correlation (p < 0.01) and highly linear regression correlation (R2 > 0.81) between bioavailable Cd and three enzymes' activities (urease, sucrase, and acid phosphatase). Moreover, 63.52%-98.75% of the desorbed Cd can be reabsorbed and fixed in the presence of biochar after 56 d, which effectively reducing the potential for secondary release of heavy metals into the environment. Notably, a comprehensive evaluation through cost-benefit analysis (CBA) and life cycle assessment (LCA) revealed that the biochar system can generate an overall profit of $302.8/tonne and result in a net reduction of CO2 emissions by 1455.479 kg/tonne. The application of biochar could reduce carbon footprints and promote carbon neutrality, and be an effective in-situ remediation strategy for Cd-contaminated coastal wetland.

Effect of zinc-biochar composite aging on its physicochemical and ecotoxicological properties

The stability of Zn-biochar composites is determined by environmental factors, including the aging processes. This paper focused on the ecotoxicological evaluation of Zn-biochar (Zn-BC) composites subjected to chemical aging. Pristine biochars and composites produced at 500 or 700 °C were incubated at 60 and 90 °C for six months. All biochars were characterized in terms of their physicochemical (elemental composition, Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and porous structure), ecotoxicological properties (tested with Folsomia candida and Aliivibrio fischeri) and contaminant content (polycyclic aromatic hydrocarbons (PAH), heavy metals (HM) and environmentally persistent free radicals (EPFR)). An increase in the number of surface oxygen functional groups and increased hydrophilicity and polarity of all Zn-BC composites were observed due to oxidation during aging. It was also found that Zn-BC aging at 90 °C resulted in a 28-30% decrease in solvent-extractable PAHs (Ʃ16 Ctot PAHs) compared to nonaged composites. The aging process at both temperatures also caused a 104 fold reduction in EPFRs in Zn-BC composites produced at 500 °C. The changes in the physicochemical properties of Zn-BC composites after chemical aging at 90 °C (such as pH and HM content) caused an increase in the toxicity of the composites to Folsomia candida (reproduction inhibition from 19 to 24%) and Aliivibrio fischeri (luminescence inhibition from 96 to 99%). The aging of composites for a long time may increase the adverse environmental impact of BC-Zn composites due to changes in physicochemical properties (itself and its interactions with pollutants) and the release of Zn from the composite.

The effect of biotransformation of sewage sludge- and willow-derived biochars by horseradish peroxidase on total and freely dissolved polycyclic aromatic hydrocarbon content

This study analyzed the effect of enzymatic aging (horseradish peroxidase) of biochars on their content of solvent extractable (Ctot) and freely dissolved (Cfree) polycyclic aromatic hydrocarbons (PAHs). Physicochemical properties and phytotoxicity of pristine and aged biochars were also compared. The study used biochars obtained at 500 or 700 °C from sewage sludges (SSLs) or willow. Compared to SSL-derived biochars, willow-derived biochars were more susceptible to enzymatic oxidation. Aging increased the specific surface area and pore volume of most SSL-derived biochars. An opposite direction, however, was found in the willow-derived biochars. Low-temperature biochars, regardless of their feedstock, underwent physical changes, such as removal of labile ash components or degradation of aromatic structures. The enzyme caused an increase in the content of Ctot light PAHs in biochars (by 34-3402 %) and heavy PAHs (≥4 rings) in the low-temperature SSL-derived biochars (by 46-713 %). In turn, the content of Cfree PAHs decreased in aged SSL-derived biochars (by 32-100 %). In the willow-derived biochars the bioavailability of acenaphthene increased (by 337-669 %), while the immobilization degree of some PAHs was lower (25-70 %) compared to the SSL-derived biochars (32-83 %). Nevertheless, aging positively affected the ecotoxicological properties of all biochars by increasing their stimulation effects or removing their phytotoxic effects on both Lepidium sativum seed germination and root growth. Significant relationships between the changes in Cfree PAH content, pH and salinity of SSL-derived biochars and seed germination/root growth inhibition were found. The study demonstrates that the risk associated with application of SSL-derived biochars, regardless of the type of SSL and pyrolysis temperature, can be lower in terms of Cfree PAHs than in the case of willow-derived biochars. Regarding to Ctot PAHs, high-temperature SSL-derived biochars are safer than low-temperature ones. In the case of application of high-temperature SSL-derived biochars, these with moderate alkalinity and salinity will not bring risks for plants.

The impact of cyclic freezing and thawing (physical aging) on properties and polycyclic aromatic hydrocarbon content in biochars

In this study, we investigated the effect of freeze-thaw cycles of sewage sludge (SSL)- and willow-derived biochars obtained at different temperatures on their physicochemical properties and total (Ctot) and freely dissolved (Cfree) polycyclic aromatic hydrocarbon (PAH) content. In our research, we showed that the fate of Ctot PAHs depended on the type of SSL and may differ, which is determined mainly by the properties of the feedstock. The properties affect low-temperature SSL-derived biochars (500 °C) especially, in which the interactions based on π-π bonds between the PAHs and matrix during aging were weakened by newly created functional groups on the biochar surface. Additionally, the removal of ash or biochar particle fragmentation during aging increased the availability of solvent non-extractable PAHs, which resulted in an increase in the content of high molecular weight Ctot PAHs after aging. In turn, the Σ16 Cfree PAHs in biochars gradually decreased with their progressive surface functionalization, increase in specific surface area and pore volume and generally were independed on properties of feedstock and biochar. The obtained results evidence that the environmental risk associated with the use of SSL as a feedstock for biochar production is similar to or even lower than in the case when willow is used and depending on SSL type, the potential risk may be reduced or increased for low-temperature biochars.

Potential Mechanism of Long-Term Immobilization of Pb/Cd by Layered Double Hydroxide Doped Chicken-Manure Biochar

Layered double hydroxide (LDH)-doped chicken-manure biochar (CMB) with long-term stability was synthesized to immobilize Pb/Cd. MgAl-Cl-LDH-doped CMB (MHs) showed prominent long-term oxidation resistance and the least biodegradation sensitivity. Efficient Pb/Cd adsorption was observed on MHs, and the maximum adsorption capacities of Pb(II)/Cd(II) reached 1.95 mmol/g and 0.65 mmol/g, respectively. Precipitation and isomorphous substitution were identified as the key adsorption mechanisms, which formed highly stable Pb/Cd species (PbAl-CO₃-LDH, Pb₃(OH)₂CO₃, CdAl-Cl-LDH and CdCO₃). Pb(II) and Cd(II) precipitated with CO₃^2- in MHs; meanwhile, Mg(II) and Ca(II) in LDH layers were substituted by Pb(II) and Cd(II) respectively. Therefore, MHs had the potential for long-term stability of Pb/Cd. Moreover, complexation and electrostatic adsorption also contributed to the Pb/Cd immobilization to a certain extent. When 5% MHs (w/w) was applied to Pb/Cd contaminated smelting site soils, the soil pH increased from 5.9 to 7.3. After applying MHs for 25 d, the content of bioavailable Pb(II) and Cd(II) decreased by 98.8% and 85.2%, respectively, and the content of soluble Pb and Cd dropped by 99.5% and 96.7%. This study paves the way for designing a novel LDH doped CMB as efficient Pb/Cd immobilizers for smelting site soils.

Effect of carrier gas during pyrolysis on the persistence and bioavailability of polycyclic aromatic hydrocarbons in biochar-amended soil

In this study the persistence (based on extractable, C_tot) and bioavailability (based on freely dissolved content, C_free) of polycyclic aromatic hydrocarbons (PAHs) in biochar-amended soil was investigated. Biochar produced at 500 or 700 °C from sewage sludge (BC) or sewage sludge and willow (W) mixture (BCW) in an atmosphere of nitrogen (N₂) or carbon dioxide (CO₂) was evaluated. The biochars were applied to the real soil (podzolic loamy sand) at a dose of 2% (w/w). The content of C_tot and C_free PAHs was monitored for 180 days. The biochar production conditions determined the C_tot and C_free PAHs in the soil. A change of carrier gas from N₂ to CO₂ caused an increase in C_tot PAH losses in the soil from 19 to 75% for the biochar produced from SL and from 49 to 206% for the co-pyrolyzed biochar. As regards C_free PAHs, the change from N₂ to CO₂ increased the losses of C_free PAHs only for the biochar derived from SL at a temperature of 500 °C (by 21%). In the soil with the other biochars (produced at 700 °C from SL as well as at 500 and 700 °C from SL/W), the C_free increased from 17 to 26% compared to the same biochars produced in an atmosphere of N₂.

Long-term physical and chemical aging of biochar affected the amount and bioavailability of PAHs and their derivatives

Biochar applied into the soil is recommended as an effective tool for increasing its properties and crop productivity. However, biochar can contain some potentially toxic compounds such as polycyclic aromatic hydrocarbons (PAHs). Moreover, during biochar production or environmental application (e.g. as soil fertilizer), more toxic PAHs derivatives containing nitrogen, oxygen or sulfur can be formed. There is a lack of information on how the environmental factors affect the bioavailability of such compounds during the long-term application of BC into the soil. In the presented studies the effects of physical (freeze-thaw cycles) and chemical aging (temperatures 60 °C and 90 °C) on the total and bioavailable content of PAHs and their derivatives were estimated. The results indicate that long-term (6 months) aging affected the physicochemical characteristic of biochars promoting the formation of new C and O-containing species on the BC surface increasing their polarity and hydrophilicity. Physical and chemical aging promoted the formation of compounds with higher molecular weight and a significant (up to 550 %) increase in the bioavailability of PAHs and their derivatives. The results of this study highlight the importance of the bioavailable fraction of PAHs and their derivatives for evaluation of the toxicity of aged biochar.

Assessing the influence of sewage sludge and derived-biochar in immobilization and transformation of heavy metals in polluted soil: Impact on intracellular free radical formation in maize

As one of the most common ways to get rid of municipal waste, landfill leachate, waste with complicated compositions and high levels of contaminants, has become a significant threat to the world's environment. Here, the impact of sewage sludge (SS) and derived-biochar (SSB) amendments on the immobilization and potential mobility of heavy metals in a contaminated soil-plant system was investigated. The sequential fractionation findings showed that using SS-2%, SSB-2%, and SSBC-1% reduced the potential mobility of heavy metals while increasing the residual fraction in polluted soils. The translocation and bioconcentration factors showed that heavy metals were slightly transferred into shoots from roots and lowered accumulation in roots from contaminated soils. Fourier transform infrared (FTIR) and X-ray photoelectron spectrum (XPS) comprehensive characterization results indicated the significant role of applied amendments for heavy metals transformation from the exchangeable-soluble fractions to the least available form by lowering their mobility to confirm the adsorption-based complexes, which results in the surface adsorption of heavy metals with functional groups. The electron paramagnetic resonance (EPR) results indicated the dominance of reactive oxygen species (ROS) in the intracellular formation of hydroxyl radicals (•OH) in maize plant roots and shoots. ROS (•OH) generation plays a critical influence in the interaction between the physiological processes of plants and heavy metals. Moreover, all the amendments increased maize growth and biomass production. Our study suggests that alone and combined application of SS and SSB have great potential to remediate heavy metals contaminated soil for environmental sustainability.

Coupling of desorption of phenanthrene from marine sediments and biodegradation of the sediment washing solution in a novel biochar immobilized-cell reactor

The recurrent dredging of marine sediments needs the use of ex-situ technologies such as sediment washing (SW) to effectively remove polycyclic aromatic hydrocarbons. Notwithstanding, the large volumes of generated spent SW effluents require adequate treatment by employing highly-efficient, inexpensive and environmentally-friendly solutions. This study proposes the phenanthrene (PHE) desorption from sediments using Tween® 80 (TW80) as extracting agent and the treatment of the resulting spent SW solution in a biochar (BC) immobilized-cell bioreactor. The SW process reached the highest PHE removal of about 91% using a surfactant solution containing 10,800 mg L^-1 of TW80. The generated amount of spent PHE-polluted SW solution can be controlled by keeping a solid to liquid ratio of 1:4. A PHE degradation of up to 96% was subsequently achieved after 43 days of continuous reactor operation, aerobically treating the TW80 solution in the BC immobilized-cell bioreactor with a hydraulic retention time of 3.5 days. Brevundimonas, Chryseobacterium, Dysgonomonas, Nubsella, and both uncultured Weeksellaceae and Xanthobacteraceae genera were mainly involved in PHE biodegradation. A rough economic study showed a total cost of 342.60 € ton^-1 of sediment, including the SW operations, TW80 and BC supply and the biological treatment of the SW solution.

Ecotoxicity of sewage sludge- or sewage sludge/willow-derived biochar-amended soil

Co-pyrolysis of sewage sludge (SL) with plant biomass gains attention as a way to minimize SL-derived biochar drawbacks, such as high amount of toxic substances, low specific surface area and carbon content. The toxicity of soil amended with SL- (BCSL) or SL/biomass (BCSLW)-derived biochar was evaluated in long-term pot experiment (180 days). The results were compared to SL-amended soil. Biochars produced at 500, 600, or 700 °C were added to the soil (podzolic loamy sand) at a 2% (w/w) dose. Samples were collected at four different time points (at the beginning, after 30, 90 and 180 days) to assess the potential toxicity of SL-, BCSL- or BCSLW-amended soil. The bacteria Aliivibrio fischeri (luminescence inhibition - Microtox), the plant Lepidium sativum (root growth and germination inhibition test - Phytotoxkit F), and the invertebrate Folsomia candida (mortality and reproduction inhibition test - Collembolan test) were used as the test organisms. Depending on the organism tested and the sample collection time point variable results were observed. In general, SL-amended soil was more toxic than soil with biochars. The leachates from BCSLW-amended soil were more toxic to A. fischeri than leachate from BCSL-amended soil. A different tendency was observed in the case of phytotoxicity. Leachate from BCSL-amended soil was more toxic to L. sativum compared to BCSLW-amended soil. The effect of biochars on F. candida was very diversified, which did not allow a clear trend to be observed. The toxic effect of SL-, BCSL- or BCSW-amended soil to particular organisms was observed in different time, point's periods, which may suggest the different factors affecting this toxicity.