The stability and decomposition of biochar are fundamental to understand its persistence in soil, its contribution to carbon (C) sequestration, and thus its role in the global C cycle. Our current knowledge about the degradability of biochar, however, is limited. Using 128 observations of biochar‐derived CO2 from 24 studies with stable (13C) and radioactive (14C) carbon isotopes, we meta‐analyzed the biochar decomposition in soil and estimated its mean residence time (MRT). The decomposed amount of biochar increased logarithmically with experimental duration, and the decomposition rate decreased with time. The biochar decomposition rate varied significantly with experimental duration, feedstock, pyrolysis temperature, and soil clay content. The MRTs of labile and recalcitrant biochar C pools were estimated to be about 108 days and 556 years with pool sizes of 3% and 97%, respectively. These results show that only a small part of biochar is bioavailable and that the remaining 97% contribute directly to long‐term C sequestration in soil. The second database (116 observations from 21 studies) was used to evaluate the priming effects after biochar addition. Biochar slightly retarded the mineralization of soil organic matter (SOM; overall mean: −3.8%, 95% CI = −8.1–0.8%) compared to the soil without biochar addition. Significant negative priming was common for studies with a duration shorter than half a year (−8.6%), crop‐derived biochar (−20.3%), fast pyrolysis (−18.9%), the lowest pyrolysis temperature (−18.5%), and small application amounts (−11.9%). In contrast, biochar addition to sandy soils strongly stimulated SOM mineralization by 20.8%. This indicates that biochar stimulates microbial activities especially in soils with low fertility. Furthermore, abiotic and biotic processes, as well as the characteristics of biochar and soils, affecting biochar decomposition are discussed. We conclude that biochar can persist in soils on a centennial scale and that it has a positive effect on SOM dynamics and thus on C sequestration.

Biochar stability in soil: Meta-analysis of decomposition and priming effects

Field Crops Research

A review on halloysite-based adsorbents to remove pollutants in water and wastewater

Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.

https://www.mdpi.com/1996-1073/13/16/4098/pdf

Hydrothermal Carbonization as a Valuable Tool for Energy and Environmental Applications: A Review

Presence of dyes in the aquatic systems has become a serious environmental issue. There are > 100,000 commercially available dyes with over 7 × 105 metric tons of dyestuff produced annually. Among them, 10%–15% of the dye used during the manufacturing of textile products are released into the environment worldwide annually. The main aim of this work is to study potential adsorbent for textile industry wastewater treatment. The investigation includes the mechanism, reaction, types, comparative study with other synthetic adsorbents, mode of operation, importance, and application. Bioadsorbents are coming under green chemistry. It’s economical, environmentally friendly, and nonhazardous in nature. As compared to other physicochemical treatment techniques, bioadsorbents process generates less sludge and is comparable, easy to operate, and highly efficient. This study brings a new idea for those working in waste management, dye industry wastewater treatment, mechanism of adsorption, and sustainable treatment.

Significance of bioadsorption process on textile industry wastewater

Fabrication of hydrochar based on food waste (FWHTC) and its application in aqueous solution rare earth ions adsorptive removal: Process, mechanisms and disposal methodology

The invention discloses a metal element improved biochar-based nitrate radical adsorbent and a preparation method thereof. Straws are used as raw materials, and metal ions are impregnated, pyrolyzed and washed under certain conditions, so a metal ion improved cbiochar is prepared. According to the metal element improved biochar-based nitrate radical adsorbent and the preparation method thereof disclosed by the invention, by using adsorption research on nitrate radical waste water and comparing the adsorption effect of the biochar before and after improvement, the improved biochar prepared by the preparation method disclosed by the invention has good adsorption effect for potassium nitrate, wherein Mg improved wheat straw biochar has optimal adsorption effect for nitrate nitrogen; compared with the unmodified biochar, the Mg improved biochar prepared under the optimizing conditions has the advantages that the adsorption performance is improved by 10 times, and the adsorption equilibrium can be realized in a shorter time. The modified biochar adsorbent is a green adsorbent with application potential and can be used for effectively removing the nitrate nitrogen in a water body; besides, the effects of fixing carbon, reducing discharge and relieving the non-point source pollution are achieved.

Metal element improved biochar-based nitrate radical adsorbent and preparation method thereof

Surface soil mixing is more beneficial than the plough layer mixing mode of biochar application for nitrogen retention in a paddy system.

The aim of this study was to develop an environmental-benign bio-measure that could be used to purify dye-contaminated wastewater. Herein, a hybrid adsorption-periphyton reactor (HAPR), combining a bioadsorbent based adsorption unit and a periphyton-based photo-bioreactor (PPBR), was built and applied for the first time. Firstly, an efficient bioadsorbent, i.e, microwave-activated swede rape hull (MSRH), was produced, characterized and applied in an adsorption column, to pretreat methylene blue (MB) wastewater with high concentration (~100 mg L−1 or higher). Thereafter, the effluent of adsorption column, with lower dye concentration (~0.5 mg L−1), was collected and further purified by PPBR. Results showed that dye removal efficiency by HAPR was 99.95% (from 200 mg L−1 to 0.1 mg L−1 or lower). Decolorization mechanisms by PPBR, included adsorption process by extracellular polymeric substances (EPS) on the surface of periphyton and degradation process. The study showed that HAPR was a novel, environmental friendly, efficient and promising dye-purification method and deserved further attention in future investigation.

Purification of Dye-stuff Contained Wastewater by a Hybrid Adsorption-Periphyton Reactor (HAPR): Performance and Mechanisms.

The invention discloses a preparation method of hydrothermal charcoal for relieving ammonia volatilization emission in a rice field and application thereof. The preparation method comprises: preparing an original material of hydrothermal charcoal, i.e., carrying out crushing on land crops and/or aquatic plants, and preparing a mixed material of the original material and a solvent, i.e., mixing the original material and the solvent to obtain the mixed material; preparing hydrothermal charcoal, i.e., placing the mixed material into a hydrothermal reaction kettle, carrying out heating pyrolysis, then cooling to room temperature, finally, taking out a prepared carbon-based material, draining, drying, then naturally air-drying and ageing to obtain the hydrothermal charcoal; mixing the hydrothermal charcoal and a citric acid solution, and performing an esterification reaction under a certain condition to obtain a final product. The product is used for controlling ammonia volatilization in the rice field. According to the invention, in the rice field, by applying the improved hydrothermal charcoal according to an application method provided by the invention, ammonia volatilization in the rice field can be relieved, a soil charcoal library is increased, a utilization rate of biomass resources is improved, and a reasonable use quantity can also increase rice yield.

Chen Yudong

Papers

Fabrication of hydrochar based on food waste (FWHTC) and its application in aqueous solution rare earth ions adsorptive removal: Process, mechanisms and disposal methodology

Metal element improved biochar-based nitrate radical adsorbent and preparation method thereof

Surface soil mixing is more beneficial than the plough layer mixing mode of biochar application for nitrogen retention in a paddy system.

Purification of Dye-stuff Contained Wastewater by a Hybrid Adsorption-Periphyton Reactor (HAPR): Performance and Mechanisms.

Preparation method of improved hydrothermal charcoal for relieving ammonia volatilization emission in rice field and application thereof