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A machine learning model for microwave-assisted pyrolysis technology was developed in this study. The data set of 112 unique experiments was created by analysing the published literature based on biochar. The linear, interactive and quadratic regression models were trained with the selected data set. Out of three regressions models, the quadratic model $${\text{Biochar Yield}} = - 601.78 + 17.336 \times {\text{VM}} + 25.338 \times {\text{AC}} - 0.26367 \times T - 0.293 \times {\text{VM}} \times {\text{AC}} - 0.10305 \times {\text{VM}}^{2} - 0.1893 \times {\text{AC}}^{2} + 1.59 \times 10^{ - 3} \times T^{2}$$ was found to have highest R2 value of 0.894. The predicted model developed based on the data from literature was validated with laboratory experimental results. Prediction and validation results showed that data prediction can be a useful tool for the preview of selected feedstock and process parameters. Volatile matter, ash content and temperature were found to be the prominent factors in the trained model. Biochar yield was predicted with a minimum root mean square error of 7 when validated with experimental results. Thus, the predicted model can be used as empirical equation for future experiments to predict biochar yield.

Burning of agricultural waste like crop residues releases black carbon, a major source of air pollution in megacities like Delhi. Conversely, crop residues are a rich source of cellulose, hemicellulose, and lignin. The present study investigated the effect of heating time (HT) and MW power (P) on biochar yield from rice husk, an agricultural waste, using microwave pyrolysis. Biochar yield varied from 68% (HT 18 min and P 500 W) to 39% (at HT 15 min and P 900 W). A second-order polynomial model was obtained for predicting biochar yield, which signified that microwave power and interaction of heating time and microwave power positively affected the maximum yield of biochar. The high heating value and fuel ratio of biochar (25.46 MJ/kg and 2.82) showed improvement compared with that of rice husk (12.43 MJ/kg and 0.06). Also, the MW pyrolysis of rice husk resulted in increased fixed carbon content from 4.78% in rice husk to 52.2% in biochar. The property of rice husk and biochar indicated a decrease in O/C ratio from 1.5 to 0.66, improvement in surface area from 2.4 to 190 m2/g, and an increase in pH from 6.71 to 8.75. Also, energy density and energy yield in MW pyrolysis were found as 1.46 and 99.28% respectively. Hence, MW pyrolysis of rice husk resulted in a high quality of biochar that could be used as a potential source of energy, nutrient captive media, and soil amendment.

Microwave-assisted pyrolysis (MAP) is considered as one of the improved biochar production options given its cost efficiency, efficient heat transfer and better-quality control. Researchers have observed that biochar can be considered as an alternative erosion control material due to its high porosity, surface area, and surface functional groups. However, as per the available literature, contradictions exist regarding some erosion control mechanisms of biochar with respect to water holding capacity (WHC), bulk density, and aggregate formation. Further, there is a lack of exploration as to how biochar produced using MAP can be adopted for erosion control. The objective of this study is to critically review existing literature on the usage of biochar for erosion control in various types of soils. In addition, the use of the MAP technique for producing biochar for erosion control is explored. Based on the review, the available experimentation research is generally of short duration with the least consideration of the long-term ageing effects of biochar. The studies on erosion control using biochar have been mostly conducted in the shallower zone (i.e., within 20 cm). The effect of different types of biochars on various soils does not seem to have been investigated systematically. It has been observed that the mechanism of washing away biochar particles and nutrients is rarely taken into account while measuring erosion. Further, the comparative analysis of biochar types produced from various feedstocks, including their physiochemical properties, is rarely explored for their applications in soil erosion. This study aims to critically analyze available literature for the erosion suppressing mechanism of biochar, limitations and the feasibility of microwave-assisted biochar production for the purpose of soil erosion control.

The present study investigated the potential tertiary treatment of wastewater by adsorption using rice husk biochar (RHC) obtained from microwave pyrolysis of rice husk. Adsorption experiments with RHC indicated phosphate and nitrate adsorption of 65% and 75% respectively. The adsorption kinetics indicated pseudo-second-order adsorption model for adsorption of nutrients on biochar with an R2 value more than 0.98. In addition, the Langmuir-Freundlich isotherm was observed to represent the adsorption of phosphate on RHC, whereas adsorption data of nitrate depicted Langmuir isotherm followed by Temkin and Dubinin-Radushkevich isotherms. The maximum phosphate and nitrate adsorption capacity were 71 mg/kg and 497 mg/kg respectively using Langmuir isotherm model. The desorption study indicated a release of 96.6% and 44.4% of adsorbed phosphate and nitrate, respectively. Hence, from 1 kg of nutrient loaded RHC about 68 mg of phosphate and 220 mg of nitrate can be extracted for land application.

Biochar produced from various biomass has been widely used in environmental applications owing to its ability to immobilize or remove the contaminants from soil, water and air. The present work summarizes various pyrolysis variants used for biochar production from rice husk, modification of biochar and its environmental application. The high volatile matter content (70.2–78.5%) and carbon content (35.2–44.7%) favoured production of biochar from rice husk through pyrolysis. Microwave-assisted hydrothermal carbonization showed highest biochar yield from rice husk (57.9%) compared to other process variants, whereas wet pyrolysis produced biochar with the highest carbon content (71.2%). Steam activation of rice husk biochar resulted in a broader pore size distribution with the presence of significant micropores compared to CO2 activation. A substantial improvement in surface area and microporous volume was observed with alkali activation compared to that of acid activation, whereas metal impregnation caused a reduction in surface area. Rice husk biochar with/without modification has been employed for adsorption of pollutants such as cations, dyes, nutrients and tetracycline. The nutrient-loaded rice husk biochar improved the soil fertility and cation exchange capacity. The studies indicated that the choice of suitable pyrolysis variant and biochar modification method is vital to improve the adsorption capacity and nutrient release potential of the rice husk biochar.

In a sanitary landfill, the final cover plays an important role in reducing the landfill gas emission to the atmosphere and in preventing the ingress of rainwater into the dumped waste. The present study investigated the suitability of sugarcane bagasse biochar as an amendment to the cover soil to improve the required landfill liner properties. The amended cover soil sugarcane bagasse (SSB) was tested for its stability and effectiveness, in terms of both geotechnical properties and methane mitigation efficiency. The effects of amending 15%, 20%, and 25% of sugarcane bagasse biochar (passing through 300 micron Indian Standard sieve) on the geotechnical properties of the SSB indicated that the SSB with 25% biochar showed the required values as per the standard with maximum dry density of 1.57 grams per cubic centimeter (g/cm3), liquid limit, plasticity index, and percentage of fines 48.5%, 16.3%, and 74.7%, respectively, and permeability of 0.9 × 10−7. A column study that was conducted to determine the methane emission from the cover soil showed a 65.8% reduction in the methane emission compared to that of a column without SSB cover, with a cumulative methane emission of 410 milliliters (mL) at the end of 200 hours (h). On the other hand, the volume of methane emitted after 310 h from the column without cover and with the SSB cover was 1850 mL and 692 mL, respectively. The difference between these two values is found to be 22% of the total methane that the cover would have handled in its lifetime (5267 mL). Thus, there is an increase in the percentage of methane adsorption by soil cover from 15% to 22% when the soil was amended with 25% sugarcane bagasse biochar.