Quantification and characterization of chemically-and thermally-labile and recalcitrant biochar fractions

Review on biochar as a sustainable green resource for the rehabilitation of petroleum hydrocarbon-contaminated soil

Petroleum pollution is one of the primary threats to the environment and public health. Therefore, it is essential to create new strategies and enhance current ones. The process of biological reclamation, which utilizes a biological agent to eliminate harmful substances from polluted soil, has drawn much interest. Biochars are inexpensive, environmentally beneficial carbon compounds extensively employed to remove petroleum hydrocarbons from the environment. Biochar has demonstrated an excellent capability to remediate soil pollutants because of its abundant supply of the required raw materials, sustainability, affordability, high efficacy, substantial specific surface area, and desired physical-chemical surface characteristics. This paper reviews biochar's methods, effectiveness, and possible toxic effects on the natural environment, amended biochar, and their integration with other remediating materials towards sustainable remediation of petroleum-polluted soil environments. Efforts are being undertaken to enhance the effectiveness of biochar in the hydrocarbon-based rehabilitation approach by altering its characteristics. Additionally, the adsorption, biodegradability, chemical breakdown, and regenerative facets of biochar amendment and combined usage culminated in augmenting the remedial effectiveness. Lastly, several shortcomings of the prevailing methods and prospective directions were provided to overcome the constraints in tailored biochar studies for long-term performance stability and ecological sustainability towards restoring petroleum hydrocarbon adultered soil environments.

Ligneous amendments increase soil organic carbon content in fine-textured boreal soils and modulate N2O emissions

Organic soil amendments are used to improve soil quality and mitigate climate change. However, their effects on soil structure, nutrient and water retention as well as greenhouse gas (GHG) emissions are still poorly understood. The purpose of this study was to determine the residual effects of a single field application of four ligneous soil amendments on soil structure and GHG emissions. We conducted a laboratory incubation experiment using soil samples collected from an ongoing soil-amendment field experiment at Qvidja Farm in south-west Finland, two years after a single application of four ligneous biomasses. Specifically, two biochars (willow and spruce) produced via slow pyrolysis, and two mixed pulp sludges from paper industry side-streams were applied at a rate of 9-22 Mg ha-1 mixed in the top 0.1 m soil layer. An unamended fertilized soil was used as a control. The laboratory incubation lasted for 33 days, during which the samples were kept at room temperature (21°C) and at 20%, 40%, 70% or 100% water holding capacity. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured periodically after 1, 5, 12, 20 and 33 days of incubation. The application of ligneous soil amendments increased the pH of the sampled soils by 0.4-0.8 units, whereas the effects on soil organic carbon content and soil structure varied between treatments. The GHG exchange was dominated by CO2 emissions, which were mainly unaffected by the soil amendment treatments. The contribution of soil CH4 exchange was negligible (nearly no emissions) compared to soil CO2 and N2O emissions. The soil N2O emissions exhibited a positive exponential relationship with soil moisture. Overall, the soil amendments reduced N2O emissions on average by 13%, 64%, 28%, and 37%, at the four soil moisture levels, respectively. Furthermore, the variation in N2O emissions between the amendments correlated positively with their liming effect. More specifically, the potential for the pulp sludge treatments to modulate N2O emissions was evident only in response to high water contents. This tendency to modulate N2O emissions was attributed to their capacity to increase soil pH and influence soil processes by persisting in the soil long after their application.

The co-application of biochar with bioremediation for the removal of petroleum hydrocarbons from contaminated soil

Soil pollution from petroleum hydrocarbon is a global environmental problem that could contribute to the non-actualisation of the United Nations Sustainable Development Goals. Several techniques have been used to remediate petroleum hydrocarbon-contaminated soils; however, there are technical and economical limitations to existing methods. As such, the development of new approaches and the improvement of existing techniques are imperative. Biochar, a low-cost carbonaceous product of the thermal decomposition of waste biomass has gained relevance in soil remediation. Biochar has been applied to remediate hydrocarbon-contaminated soils, with positive and negative results reported. Consequently, attempts have been made to improve the performance of biochar in the hydrocarbon-based remediation process through the co-application of biochar with other bioremediation techniques as well as modifying biochar properties before use. Despite the progress made in this domain, there is a lack of a detailed single review consolidating the critical findings, new developments, and challenges in biochar-based remediation of petroleum hydrocarbon-contaminated soil. This review assessed the potential of biochar co-application with other well-known bioremediation techniques such as bioaugmentation, phytoremediation, and biostimulation. Additionally, the benefits of modification in enhancing biochar suitability for bioremediation were examined. It was concluded that biochar co-application generally resulted in higher hydrocarbon removal than sole biochar treatment, with up to a 4-fold higher removal observed in some cases. However, most of the biochar co-applied treatments did not result in hydrocarbon removal that was greater than the additive effects of individual treatment. Overall, compared to their complementary treatments, biochar co-application with bioaugmentation was more beneficial in hydrocarbon removal than biochar co-application with either phytoremediation or biostimulation. Future studies should integrate the ecotoxicological and cost implications of biochar co-application for a viable remediation process. Lastly, improving the synergistic interactions of co-treatment on hydrocarbon removal is critical to capturing the full potential of biochar-based remediation.

Characteristics and Applications of Biochar in Soil–Plant Systems: A Short Review of Benefits and Potential Drawbacks

The available literary data suggest the general applicability and benefits of different biochar products in various soil–plant–environment systems. Due to its high porosity, biochar might generally improve the physicochemical and biological properties of supplemented soils. Among the direct and indirect effects are (i) improved water-retention capacity, (ii) enhanced soil organic matter content, (iii) pH increase, (iv) better N and P availability, and (v) greater potential uptake of meso- and micronutrients. These are connected to the advantage of an enhanced soil oxygen content. The large porous surface area of biochar might indirectly protect the survival of microorganisms, while the adsorbed organic materials may improve the growth of both bacteria and fungi. On the other hand, N2-fixing Rhizobium bacteria and P-mobilizing mycorrhiza fungi might respond negatively to biochar’s application. In arid circumstances with limited water and nutrient availability, a synergistic positive effect was found in biochar–microbial combined applications. Biochar seems to be a valuable soil supplement if its application is connected with optimized soil–plant–environment conditions. This work aims to give a general review of the potential benefits and drawbacks of biochar application to soil, highlighting its impacts on the soil–plant–microbe system.

Influence of coal treatments on the Ni loading mechanism of Ni-loaded lignite char catalysts

Many kinds of lignite coals have been used as catalyst supporters for preparing the Ni-loaded lignite char catalyst. However, these coals have different properties; especially ash content. The ash in coal affects the mechanism of Ni-loading on the position of the functional group structures in coal. In catalyst preparation, it is interesting that the difference in coal properties might directly influence the mechanism of Ni loading. To prove this point, the coal sample needed to be treated before the catalyst preparation. MM coal (original coal) was treated with acid (HClMM and AceMM coals), alkali (NaMM coal) and alkali followed by acid treatments (NaHClMM and NaAceMM coals). Then, Ni was loaded on the five treated coals by the ion-exchange technique. The Ni-loaded lignite coals were pyrolyzed at 650 °C under a N₂ atmosphere to prepare the Ni-loaded lignite char catalysts. The Ni-loading mechanisms were studied via FTIR, XRD, AAS and SEM-EDS analyses. The results showed that the different treatments affected the ash content and the functional groups in the coals. The decreases in the ash contents of HClMM, AceMM, NaHClMM and NaAceMM coals indicated that the exchangeable metallic species were removed by transforming metal-carboxylates into carboxyl groups. The transformations of metal-carboxylates were confirmed by the increased Δϑ(COO⁻) value. For acid treatment, the ion exchange of Ni was controlled by carboxyl groups, while in alkali treatment it occurred through hydroxyl and metal-carboxyl groups. In alkali followed by acid treatment, Ni ions were exchanged with hydroxyl and carboxyl groups. The Ni ion forms, Ni(NH₃)₆²⁺ and/or Ni(H₂O)₆²⁺, appeared on the modified coals. Through pyrolysis, the Ni ion was reduced to Ni metal that was observed in the XRD patterns of the catalysts. The Ni contents of the catalysts were in the range of 16.51–20.07 wt%. The thermal behaviours of the catalysts were presented via TGA-DTG.

Coal treatments remarkably affected ash contents and the functional groups in coals. The changes of functional groups were the key factor in controlling Ni loading mechanism and capacity of Ni/lignite char catalysts.

The Role of Pyrolysis and Gasification in a Carbon Negative Economy

The International Panel on Climate Change and the 2015 Climate Summit in Paris have recommended that efforts to reduce carbon emissions be coupled with carbon removal from the atmosphere. Carbon negative energy combines net carbon removal with the production of energy products or other revenue-generating products beyond sequestered carbon. Even though both biochemical and thermochemical approaches to carbon negative energy can be envisioned, this paper considers the prospects for the latter including pyrolysis and gasification. The fundamentals of these two processes are described to better understand how they would be integrated with carbon removal. Characteristics of pyrolysis and gasification are related to the kinds of sequestration agents they would produce, the scale of their deployment, the fraction of biomass carbon that could ultimately sequestered, the challenges of effectively sequestering these different forms of carbon and the economics of thermochemical carbon negative energy.

Estimating the organic oxygen content of biochar

The organic O content of biochar is useful for assessing biochar stability and reactivity. However, accurately determining the organic O content of biochar is difficult. Biochar contains both organic and inorganic forms of O, and some of the organic O is converted to inorganic O (e.g., newly formed carbonates) when samples are ashed. Here, we compare estimates of the O content for biochars produced from pure compounds (little or no ash), acid-washed biomass (little ash), and unwashed biomass (range of ash content). Novelty of this study includes a new method to predict organic O content of biochar using three easily measured biochar parameters- pyrolysis temperature, H/C molar ratio, and %biochar yield, and evidence indicating that the conventional difference method may substantially underestimate the organic O in biochar and adversely impact the accuracy of O:C ratios and van Krevelen plots. We also present evidence that acid washing removed 17% of the structural O from biochars and significantly changes O/C ratios. Environmental modelers are encouraged to use biochar H:C ratios.

Temporal physicochemical changes and transformation of biochar in a rice paddy: Insights from a 9-year field experiment

Biochar application to soil has attracted extensive attention worldwide due to its carbon (C) sequestration and fertility-enhancing properties. However, the lack of biochar accumulation in highly disturbed agroecosystems challenges the perceived long-term stability of biochars in soil. This 9-year field experiment was conducted in rice paddy fields to understand the temporal degradation of biochars produced from two contrasting feedstocks (rice straw vs. bamboo) at a high temperature (600 °C). Obvious physical alterations, surface oxidation, and transformation of condensed aromatic C occurred in biochars in the disturbed paddy field with frequent redox cycles. Increase in O/C atomic ratio, levels of high-temperature-sensitive degradable components, H/C ratio, and linear alkyl-C content were observed, which were indicative of time-dependent molecular changes and degradative transformation of biochars. Biochar degradation was characterized by the loss of labile C at an early stage and the degradation of aromatic C at a later stage. Based on the massive loss of C content in biochars (10.3-11.8%) and considerable degradation of aromatic C (5.0-8.7%) in 9 years, we argue that current biphasic C dynamic models probably overestimate the stability of biochars in agroecosystems such as rice paddy fields. Long-term field experiments (>5 years) are required to assess biochar's potential for C sequestration. This study provides long-term field data regarding the temporal changes in biochar physicochemical properties, which may facilitate the development of a robust assessment scheme on the long-term persistence of biochars in agroecosystems.

Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar

Biochar has been presented as a multifunctional material with short- and long-term agro-environmental benefits, including soil organic matter stabilization, improved nutrient cycling, and increased primary productivity. However, its turnover time, when applied to soil, varies greatly depending on feedstock and pyrolysis temperature. For sewage sludge-derived biochars, which have high N contents, there is still a major uncertainty regarding the influence of pyrolysis temperatures on soil carbon mineralization and its relationship to soil N availability. Sewage sludge and sewage sludge-derived biochars produced at 300 °C (BC300), 400 °C (BC400), and 500 °C (BC500) were added to an Oxisol in a short-term incubation experiment. Carbon mineralization and nitrogen availability (N-NH4+ and N-NO3−) were studied using a first-order model. BC300 and BC400 showed higher soil C mineralization rates and N-NH4+ contents, demonstrating their potential to be used for plant nutrition. Compared to the control, the cumulative C-CO2 emissions increased by 60–64% when biochars BC300 and BC400 were applied to soil. On the other hand, C-CO2 emissions decreased by 6% after the addition of BC500, indicating the predominance of recalcitrant compounds, which results in a lower supply of soil N-NH4+ (83.4 mg kg−1) in BC500, being 67% lower than BC300 (255.7 mg kg−1). Soil N availability was strongly influenced by total N, total C, C/N ratio, H, pore volume, and specific surface area in the biochars.

Biochar stability assessment methods: A review

Biochar is being developed as a candidate with great potential for climate change mitigation. Sequestering biochar carbon in soil contributes greatly to the reduction of greenhouse gases emissions, and biochar stability is the most decisive factor that determines its carbon sequestration potential. However, methods that can be used universally for direct or indirect assessment of biochar stability are still under investigation. This present review aims to give comprehensive and detailed up-to-date information on the development of biochar stability assessment methods. The method details, advantages and disadvantages, along with the correlations between different methods were reviewed and discussed. Three stability assessment method categories were identified: I) biochar C structure analysis, II) biochar oxidation resistance determination, and III) biochar persistence evaluation by biochar incubation and mineralization rate modelling. Biochar persistence value (e.g., mean residence time, MRT) obtained from incubation and modelling and biochar elemental ratios such as H/C_org and O/C_org are the current most commonly used biochar stability indicators. Incubation and modelling method is too time-consuming while H/C_org and O/C_org ratios are qualitative and conservative, although the effectiveness of these two methods can be further improved. On the other hand, biochar C structures such as aromaticity and degree of aromatic condensation obtained from nuclear magnetic resonance (NMR) analysis and benzene polycarboxylic acids (BPCA) molecular markers and biochar oxidation/degradation recalcitrance obtained from proximate analysis (volatile matter and fixed carbon yields), thermal recalcitrance index (R₅₀), and H₂O₂- and heat-assisted oxidation (Edinburgh stability tool) are being developed as promising proxies to indicate biochar stability.

Effects of chemical oxidation on surface oxygen-containing functional groups and adsorption behavior of biochar

Biochar is a beneficial soil amendment but the changes in its surface properties during the aging process, especially the oxygen-containing functional groups and the associated adsorption behaviors, are not well documented. In this paper, the aged wheat straw biochar was simulated by chemical oxidation with HNO₃-H₂SO₄ and NaOH-H₂O₂ systems. Characterization results showed that carbon loss and oxygen incorporation ran throughout the aging process. Surface oxygen-containing functional groups were found to be increased in all treated biochars, especially for carboxyl. Much more developed mesopores were observed in aging biochar, specific surface area was increased by 126% for biochar treated with NaOH-H₂O₂, and 226% for biochar treated with 40% of HNO₃-H₂SO₄. Thermogravimetric analysis showed that the increasing oxygen-containing functional groups led to 14% and 30% mass loss by treating biochar with alkali and acid, respectively. The improved biochar surface through the increase of oxygen-containing functional groups enhanced the cadmium sorption capacity, and the sorption capacity increased by 21.2% in maximum. Roughed surface from oxidation was another reason for increasing cadmium adsorption. Results indicated that the adsorption performance of biochar on pollutant would be changed during aging process along with the changing surface properties.