We analyzed the effects of rice straw biochar (RSBC) and swine manure biochar (SMBC) on N₂O emission from paddy soil. The biochars were added to soil at the rates of 1% and 5% (w/w), and N₂O emission, soil properties and soil enzyme activities were determined at the elongation, heading and maturation stages of rice growth. The N₂O flux started within 2 h of adding the biochar, and decreased significantly thereafter during the three growth stages. The cumulative N₂O emission was suppressed by 45.14–73.96% following biochar application, and 5% SMBC resulted in the lowest cumulative emission. In addition, biochar application significantly increased soil pH, soil organic carbon (SOC), NO₃⁻ levels and urease activity, and decreased soil NH₄⁺ and nitrate reductase activity. Regression analysis indicated that cumulative N₂O emission was correlated positively to NH₄⁺, and negatively to soil pH, SOC and NO₃⁻. SEM further revealed that biochar application weakened the denitrification process, and the NH₄⁺ level had the most significant impact on N₂O emission. Taken together, RSBC and SMBC regulated the nitrogen cycle in paddy soil and mitigated N₂O emission by increasing soil pH, decreasing nitrate reductase activity and NH₄⁺ content.

We analyzed the effects of pyrolysis temperature and duration on the physiochemical properties and Cd(II) adsorption capacity of spent tea leaves (STL) biochar. The STL biochar was produced by pyrolysis at 300, 400, 500 and 600 °C for 1 and 2 h. The pyrolysis temperature was positively correlated to the ash content, pH, electrical conductivity, specific surface area (SBET), pore volume (PV) and C content, and negatively with the total yield, O, H and N content, and the O/C and H/C atomic ratios. Furthermore, the surface porosity of STL biochar increased, the density of oxygen-containing functional groups decreased, and the formation of aromatic structures was enhanced at higher pyrolysis temperatures. The adsorption of Cd(II) onto STL biochar fitted with the pseudo-second-order kinetics and Langmuir isotherms model. The STL biochar produced at 600 °C for 2 h showed the maximum Cd(II) adsorption capacity of 97.415 mg/g. In addition, Cd(II) adsorption was mainly physical and occurred in monolayers. Thus, STL biochar is a suitable low-cost adsorbent for wastewater treatment.

Iron-based catalysts with high load content of iron sulfide (FeS) were commonly peroxymonosulfate (PMS) and hydrogen peroxide (H₂O₂) activators to degrade organic pollutants but limited catalytic efficiency and increased risk of ferrous ion leaching restricted their use. Meanwhile, various biomass materials such as straw, peel, and branch have been extensively prepared into biochar for mechanical support for iron-based catalysts; however, the preparation process of biochar was energy-intensive. In this study, FeS nanoparticles modified rape straw composites (RS–FeS) encapsulated with ethylenediaminetetraacetic acid (RS–EDTA–FeS) were successfully presented by in-situ synthesis method for efficiently activating PMS and H₂O₂ to degrade oxytetracycline (OTC), which was economical and environmentally friendly. The results showed that the modified rape straw can remove OTC efficiently, and the addition of EDTA also significantly enhanced the stability and the reusability of the catalyst. In addition, EDTA also promoted the activation of H₂O₂ at neutral pH. The OTC degradation efficiency of the two catalysts by PMS was faster than that of H₂O₂, but H₂O₂ had a stronger ability to remove OTC than PMS. The highest OTC removal efficiency of RS–FeS and RS–EDTA–FeS were 87.51 and 81.15%. O₂^•– and ¹O₂ were the major reactive oxidative species (ROS) in the PMS system. Furthermore, compared with RS–FeS, the addition of EDTA inhabited the generation of O₂^•– in the PMS system. Instead, O₂^•– and ^•OH were the major ROS in the H₂O₂ system, but ¹O₂ was also identified in RS–FeS/H₂O₂ system. RS–EDTA–FeS showed a trend of rising first and then decreasing in recycle test. Instead, the removal rate of OTC by RS–FeS decreased significantly with the increase in reuse times. In the actual wastewater test, the TOC removal of two catalysts active by H₂O₂ was better than PMS, which was consistent with the test results of OTC, indicating that the two catalysts have application value in the removal of organic pollutants in actual wastewater. This study directly used plant materials as catalysts and omits the preparation process of biochar, greatly reduces the preparation cost and secondary pollution of catalysts, and provides theoretical support for the deepening of advanced oxidation technology.