Biochar, derived from thermal pyrolysis of biomass, has been regarded as a low-cost, sustainable and beneficial material and widely applied in agriculture, environment and energy during the last two decades. To elucidate the research status timely and future trends in biochar field, CiteSpace is used to systematically analyze the related literature retrieved from the Web of Science core collection in 2019. Based on the keywords clustering analysis, it was found that “biochar production”, “organic pollutants removal”, “heavy metals immobilization”, “bioremediation” were the main hotspots in research covering biochar. “Bioremediation” is an emerging topic and deserves extensive attention due to its highly effective and environmentally friendly treatment of pollutants. Improving the phytoremediation effect, immobilizing functional microorganisms on biochar, and using microorganisms as raw materials to produce biochar were the common methods of biochar-assisted bioremediation. While studies focused on “soil quality and plant growth” and “biochar and global climate change” decreased, investigations concentrated in the toxicity of biochar to soil biota and ruminants are sustainably growing. Research on direct and catalytic thermal pyrolysis of green waste (mainly microalgae) for biofuels (bio-oil, biodiesel, syngas, etc.) and biochar production is increasing. Converting municipal wastes (e.g., sewage sludge, fallen leaves) into biochar through pyrolysis was a suitable treatment for municipal waste and became a popular topic in recent time. Moreover, the biochar produced from these municipal wastes exhibited excellent performance in the removal of pollutants from wastewater and soil. This review may help to identify future directions in biochar research and applications.
| • | Sorption properties of biochar for heavy metals was assessed through a literature review using random forest and multi-objective optimization analyses |
| • | Feedstock was the most important variable determining sorption capacity and affinity, with best results obtained for nutrient-dense feedstocks (animal biowaste, sludge, and manure) |
| • | The best performing biochar had lower C and higher N and O content, as well as lower C/N and higher O/C and H/C ratios, higher pore volumes, and higher pH. |
| • | Post-pyrolysis chemical treatment of biochar increased sorption properties more effectively than washing and magnetization. |
Fe-impregnated biochar (Fe-BC) as high-efficiency heterogeneous Fenton catalyst was synthesized and evaluated in detail for its catalytic activity, stability and reusability under various conditions. The optimal conditions for the Fenton oxidation of methylene blue (MB) as model dye were determined as 0.075 g/L H2O2, 0.5 g/L Fe-BC for 0.1 g/L MB, which resulted in optimum Dye:Fecat:H2O2 ratio of 1:5:0.75 (on g/L basis) or [Dye]:[Fetotal]:[H2O2] molar ratio of 1:6.2:7.0 respectively. The effective degradation of MB was identified over a wider pH range, and even after four consecutive runs Fe-BC maintained above 95% MB removal rate within 3 min of treatment with low Fe release, indicating strong stability and reusability. Under the optimum Dye:Fecat:H2O2 (g/L) condition at initial pH 4, the Fe-BC achieved 99.9% removal efficiency of MB within 3 min in heterogeneous Fenton reaction (HEFR) with much less H2O2 concentration and low catalyst dosage, demonstrating its efficiency and cost-effectiveness compared to other Fenton reaction catalysts. The removal velocity of MB showed two rate steps: a fast first stage followed by a slow stage with the rate in the order of H2O2/Fe-BC ⋙ H2O2/biochar > biochar > H2O2. Overall, the developed Fe-BC is more economical with strong stability and recyclability for use in HEFR for treating recalcitrant pollutants.
Hydrochar (HC), produced by hydrothermal carbonization, offers technical advantages over biochar (BC) produced by pyrolysis, and is suitable for soil amelioration, carbon sequestration, and enhanced plant growth. BC grain size has been shown to influence nutrient retention, microbial colonization and aggregate formation; however, similar research for HC is lacking. Pot trials were conducted to investigate the influence of HC grain size [coarse (6.3–2 mm), medium (2–0.63 mm) and fine (< 0.63 mm)], produced from biogas digestate, for soil improvement in three soils: loamy Chernozem, sandy Podzol, and clayey Gleysol, at a 5% HC application rate (w/w). All soils including two controls (with and without plants) were analysed for water holding capacity (WHC), cation exchange capacity (CEC), wet aggregate stability, pH, plant available nutrients (PO4–P, K and Nmin) and germination and biomass success using standard laboratory and statistical methods. Soil pH showed a compensatory shift toward the HC pH (7.2) in all soils over the course of the study. For example, the pH of the medium grained HC treatment for the Chernozem decreased from 7.9 to 7.2 and increased in the Podzol and Gleysol from 5.9 to 6.1 and 4.9 to 5.5, respectively. The nutrient-rich HC (2034 ± 38.3 mg kg−1 PO4–P and 2612.5 ± 268.7 mg kg−1 K content) provided only a short-term supply of nutrients, due to the relatively easily mineralized fraction of HC, which allowed for quick nutrient release. The pH and PO4–P effects were most pronounced in the fine grained HC treatments, with a ~ 87%, ~ 308% and ~ 2500% increase in PO4–P content in the Chernozem, Podzol and Gleysol, respectively, compared to the controls at the beginning of the study. The same trend was observed for the K and NH4+ content in the fine and medium grained HC treatments in all soils. No seed germination inhibition of Chinese cabbage was observed, with average germination rates > 50% in all soils. An effect on NO3− content was indeterminable, while there was little to no effect on biomass production, WHC, CEC and aggregate stability. In conclusion, the application of 5% fine grained HC significantly influenced the nutrient content over a short-term. However, the application rate was insufficient to substantially improve plant growth, nor to sustain a longer-term nutrients supply, regardless of grain size.
Biochar is known for its effects on carbon sequestration and soil fertility. However, there is a lack of information about its effects on soil physical and hydraulic properties for tropical soils. We assessed the effects of biochar (BC) plus sugar cane filter cake (FC) rate, and time of interaction on soil physical and hydraulic properties under humid tropical conditions. For this purpose, a field experiment was installed at a loamy sandy soil with five treatments and four replicates: control (only soil), 25 Mg ha−1 sugar cane filter cake, and 25 Mg ha−1 sugar cane filter cake plus 6.25, 12.5, and 25 Mg ha−1Miscanthus biochar, respectively, two soil depths (0–10 and 10–20 cm) and two times of interaction (9 and 18 months). Physical properties (aggregate stability, bulk density, total porosity, pores size distribution) and hydraulic properties (soil water holding capacity, hydraulic conductivity, plant-available water holding capacity) were measured after nine and eighteen months. The bulk density decreased slightly, and the porosity increased after nine months, for the biochar plus sugar cane filter cake (both 25 Mg ha−1). After 18 months, biochar plus filter cake interaction increase micropores, aggregate stability, and plant-available water content. Saturated hydraulic conductivity was not influenced by sugar cane filter cake. However, biochar significantly reduced saturated hydraulic conductivity when combined with sugar cane filter cake after 18 months. We concluded that sugar cane filter cake in combination with biochar modified the pore size distribution, slightly increased plant-available water holding capacity, and significantly decreased saturated hydraulic conductivity.
Lignin-rich recalcitrant biomass residues of coconut palms viz. (i) mature coconut husk, (ii) tender (immature or green) coconut husk (iii) coconut leaf petiole and (iv) coir-pith were successfully pyrolysed using a simple charring kiln into carbon-rich, black, light weight and porous biochars. High alkalinity and good ash content made them fit for remediating acid soils. High potassium content in these biochars could help reduce the use of inorganic K. Thermogravimetric analysis showed the mass loss phases of husk and coconut leaf petiole biochars to be similar. However, all four biochars gave smooth curves indicating thermal stability of the product. Positive seed germination and earthworm avoidance tests proved their potential as soil amendment. Soil incubation studies with coconut biochars in graded doses, alone or in combination with coconut leaf vermicompost, increased the pH, organic carbon and potassium contents, and promoted plant-beneficial microbiota and enzyme activities. Pot studies with tender coconut husk biochar and coconut leaf vermicompost enhanced the dry weight of cowpea plants accompanied with increased arbuscular mycorrhizal sporulation and root colonization, and root nodule dry weight. A field trial resulted in higher chilli yields with tender coconut husk biochar + coconut leaf vermicompost addition. The results from our studies highlight the potential of pyrolysis as an innovative technology for quick recycling of highly recalcitrant coconut palm biomass residues to biochars as a local source of soil amendment to aid regenerative agriculture in humid tropics.
Forest management practices in boreal peatlands increase nutrient export and suspended solids to watercourses calling for development of new water protection methods. One potential solution could be adsorption-based purification of runoff water using biochar. The aim of this study was to determine the adsorption rate and capacity for Norway spruce and silver birch biochars to design a biochar-filled reactor for a ditch drain. In a 10-day laboratory experiment, biochar was stirred with runoff water from a clear-cut peatland forest, and changes in water pH, total nitrogen, nitrate nitrogen, ammonium nitrogen, phosphorus, and total organic carbon concentrations were measured. Based on the concentration changes, adsorption was quantified and adsorption model containing the adsorption rate and capacity was fitted to the data. Our results indicate that biochar effectively adsorbs both inorganic and organic nitrogen from runoff water. Birch biochar had higher adsorption capacity of nitrogen than spruce biochar. This study demonstrates that the adsorption of nitrogen compounds onto biochar surfaces increases with increasing initial concentrations. Thus, aquatic ecosystems exposed to high nutrient loads from fertile peatlands would particularly benefit from biochar-based water purification.
Pennisetum purpureum is one of the most invasive perennial grasses of the Poaceae family, which are abundant in south-east Asia including Brunei Darussalam. The pyrolysis process at a slow heating rate proved to be highly promising for biochar production. The production and characterization of different Pennisetum purpureum biochars have been investigated at the pyrolysis temperatures of 400 °C, 500 °C and 600 °C with a heating and nitrogen flow rate of 5 °C/min and 0.5 L/min, respectively. The observed higher heating values were 22.18 MJ/kg, 23.02 MJ/kg, 23.75 MJ/kg, and the alkaline pH were 9.10, 9.86, 10.17 for the biochar at 400 °C, 500 °C, 600 °C temperatures, respectively. The water holding capacity was one hundred percent for all biochars and continued to increase for higher pyrolysis temperature. SEM images show that the porosity of the biochars has been enhanced with increased temperatures due to the rearrangement of crystallinity and aromaticity. On the other hand, the yields of biochar have been decreased from 35.13% to 23.02% for the increase of pyrolysis temperature from 400 °C to 600 °C. Energy dispersive X-ray analysis shows that the O/C atomic ratios were 0.15, 0.08 and 0.06 for the biochar of 400, 500 and 600 °C which validates the improvement in heating values. FT-IR analysis revealed that the available functional groups in the biochars were C–O, C=C, and C–H. Thermogravimetric analysis (TGA) under pyrolysis condition showed residue of 46.56%, 51.13% and 55.67% from the biochar at 400, 500, and 600 °C, respectively. The derivative thermogravimetry (DTG) graph indicates that the degradation rate is higher for 400 °C biochar than the 600 °C biochar.