Biomass-based porous activated carbon is a carbon-based material with well-developed pore structure, good adsorption, and capacitive properties. To achieve the goals of peak carbon dioxide emission and carbon neutrality, biomass-based porous activated carbon is widely used in the fields of electrochemical energy storage, capacitive deionization, and wastewater treatment owing to its low cost, environmental friendliness, and cleanness. Biomass-based porous activated carbon is one of the commonly used electrode materials for supercapacitors, whose electrochemical performance is mainly influenced by its physicochemical structure. To address the problems of high energy consumption and difficult regulation of the performance of porous activated carbon electrode materials, some research has proposed the activation and oxidation gradient heat treatment technology. This technology adopts a chemical-physical co-activation synergistic control process and utilizes the waste heat of pyrolysis activation, which can reduce the consumption of chemical activator and reaction energy and alleviate environmental pollution. This technology can attain the dual objectives of efficient and clean conversion of agricultural and forestry waste and high-value utilization of porous activated carbon. In this study, porous activated carbon was prepared by using waste bamboo as the carbon source and KHCO₃ as the activator for medium- to high-temperature (500-800 ℃) activation and low-temperature (200-350 ℃) air oxidation using air for synergistic regulation. The study also investigated the physicochemical structure and electrochemical properties of porous activated carbon under the synergistic effect of different activation and oxidation temperatures. The results showed that porous activated carbon activated at 600 ℃ was oxidized by low-temperature air at 350 ℃ (PAC-600-350), and the specific surface area was increased from 154.361 to 264.235 m²/g, indicating that O₂ in the air activates the expansion of pores. An increase of oxidation temperature from 200 ℃ to 350 ℃ elevated the elemental oxygen content in the porous activated carbon and the content of surface oxygen-containing groups (such as -C=O-O and -C-OH), enhanced the wettability, and improved the defect structure and pore structure. Moreover, the porous activated carbon after activation at medium to low temperatures (600 ℃ and 700 ℃) is more sensitive and susceptible to oxidation by low temperature air. Among different activation temperatures from 500 to 800 ℃, the PAC-600-350 porous activated carbon had the highest oxygen content (25.54%) and degree of defects (2.53) at the activation temperature of 600 ℃; while the pore structure of PAC-800-350 is the most developed at the activation temperature of 800 ℃, with a specific surface area as high as 1096.18 m²/g. In addition, PAC-800-350 is less affected by the oxidation of low temperature air and its surface oxygen-containing groups change to a lesser extent. In the three-electrode test, the specific capacitance of PAC-600-350 at a current density of 1 A/g was 215.29 F/g, which is 1.47 times that of non-air oxidized porous activated carbon. At the current density of 5 A/g, PAC-600-350 exhibited higher cycling stability with a capacitance retention rate of 93.51% after 5000 cycles of charging and discharging. In the two-electrode test, the symmetrical supercapacitor with PAC-600-350 as the working electrode showed good electrochemical performance, with an energy density of 9.06 Wh/kg at a power density of 215 W/kg. Overall, PAC-600-350 porous activated carbon is considered to have potential for practical application, this study can provide a reference for high value utilization of agroforestry wastes.

Agricultural biomass waste can be expected to efficiently treat for environmental protection. Conventional agricultural waste disposal can often include direct incineration, direct landfilling, and simple composting. Among them, the direct incineration has caused air pollution; landfilling has also led to the waste of land resources and environmental contamination; while the simple composting can struggle to effectively handle the various microorganisms and pathogens, leading to pests and diseases after application. Agricultural waste, such as straw and sawdust, can be converted into the biomass activated carbon with an excellent pore structure rich in surface functional groups after heat treatment. The adsorption performance can be improved for agricultural production after the modification and preparation of multifunctional materials. Particularly, capacitive deionization is one of the main application directions of the biomass-activated carbon. Some challenges have remained in the structure of the biochar. However, the existing biochar cannot fully meet the demand for the high-performance electrodes of the capacitive deionization. It is generally required for sufficient surface functional groups and a better match between physical and chemical properties, rather than the single pore distribution in the electrodes. This study aims to systematically explore the effects of the one-step activation, two-step activation, element loading, and oxidation on the physicochemical properties, surface functional group composition, and electrochemical properties of the biochar materials. The peanut straw was taken as the raw material. The removal effect of the humic acid and Cu²⁺ was further explored in the capacitive deionization. The most suitable biochar preparation was achieved after optimization. The results showed that the two-step activation was more conducive to the synergistic effect of the oxidative modification and phosphorus doping than the one-step activation, indicating the longer reaction depth and the richer P-O functional groups. Compared with the impregnation oxidation, the HNO₃ hydrothermal and air oxidation shared the milder modification strength. The oxidation reaction was completed without damage to the carbon skeleton. The N, P, and O elements were introduced to improve the pore structure of the biochar for the high adsorption; Compared with the unmodified biochar, the specific surface area of the N-P-O co-doped biochar increased by up to 10.61 times; The N-P-O co-doped biochar (APCNW) exhibited an increase in the mesopore volume from 0.013 to 0.078 cm³/g after the two-step activation with the HNO₃ hydrothermal oxidation, compared with the phosphorus-loaded biochar (APC); The one-step activation N-P-O co-doped biochar (PCNW) showed an increase in the mesopore volume from 0.033 to 0.042 cm³/g, compared with the phosphorus-loaded biochar (PC) after the one-step activation; Better mesoporous structures of the materials were obtained for the ion diffusion paths; Specific capacitance magnitude was one of the most important indicators to measure the capacitive deionization of the materials; After the introduction of N, P, and O elements under the current density at 1 A/g, the specific capacitance of the APC increased from 93 to 190 F/g for the APCNW, with an increment reaching 104.3%; The HA deionization capacities of the APCNW and PCNW were 10.92 and 10.06 mg/g, respectively; There was the increase by 110% and 147%, respectively, compared with the 5.2 mg/g for APC and 4.07 mg/g for PC; The Cu²⁺ deionization capacities of the APCNW and PCNW were 53.83 and 36.02 mg/g, respectively; There was some increase by 99% and 223%, respectively, compared with the 27.01 mg/g for APC and 11.15 mg/g for PC; The capacitive deionization of the biochar electrodes were attributed to the complexation of the phosphorus-containing functional groups with the humic acid, and the coordination between O and P atoms on the biochar surface and Cu²⁺. Among them, the APCNW shared an intact carbon skeleton structure and abundant surface P-O groups; The combined proportion of the -COOH/P-O-P and C=O/P=O functional groups reached 53.1% in the O 1s deconvolution peaks; The optimal capacitive deionization was achieved among all the materials.

Excessive use of chemical fertilizers has posed a serious threat to soil degradation and environmental pollution during agricultural production. It is urgent to recycle resources for high soil fertility and crop yield. This study aims to explore the effects of biochar and biogas slurry treatment on the decomposition rate of straw and the nutrient release rate after straw decomposition. A one-year rice-wheat rotation was carried out in the field. 8 treatment groups (T1-T8) were also set. A systematic investigation was made to compare the effects of the single and combined application of straw, biochar, and biogas slurry on the growth and yield, soil microbial biomass, soil nutrient content, soil pH, soil conductivity, and soil aggregates. The results showed that the decomposition rate of the straw returned was about 75%. The decomposition rate and nutrient release rate of deep straw returning in the paddy field were slightly higher than those of the shallow straw. While in the dry field, the abundance of soil microorganisms was significantly improved before and after returning to the field. The abundance of actinomycetes increased to 36.04% at the classification level suitable for the high straw decomposition. While the abundance of Proteobacteria was reduced to 10.24%, this was beneficial to balance the soil trophic flora. The dominant flora of straw decomposition was expanded to accelerate the straw decomposition. The combined application of straw, biochar, and biogas slurry significantly improved the soil quality and crop yield. Among them, the total nitrogen, phosphorus, potassium, and organic carbon content of soil increased respectively. The average content of the total organic carbon in the wheat-paddy field was higher than that in the paddy field. But the range of the paddy field was greater than that of the wheat paddy field. The biochar and biogas slurry were applied to hold the carbon and nitrogen ratio caused by pure straw returning to the paddy field. As such, the overall C/N ratio was within the range of suitable soil for cultivation. The combined application of straw, biochar, and biogas slurry increased the proportion of soil aggregates greater than 0.053 mm. The soil aggregate structure was improved by the combined application of biochar and biogas slurry, which was higher than that of straw returning. There was some increase with the increase in biogas slurry returning. At the same time, the crop yield shared an upward trend with the increase in the amount of straw returning to the field. The yield of rice and wheat in the total straw returning treatment group was higher than that in the half straw returning treatment. The biogas slurry (T8) performed the best in the total nitrogen content of biochar, and twice the amount of nitrogen. The soil fertility, the excellent physical and chemical properties of the soil were maintained to significantly improve the crop yield. The yield of rice reached 8 748.33 kg/hm², which was 17.98% higher than that of the biochar and the nitrogen return treatment group (T6). The yield of wheat was 3 182.20 kg/hm², which was 25.02% higher than that of the T6 treatment group. Moderate application of the straw, biochar, and biogas slurry can be expected to improve the soil quality of farmland for high crop yield.

In order to solve problems such as low pollutant concentrations, large treatment volumes and high purification costs in tailwater treatment of recirculating aquaculture systems, a new renewable biomass ash filter material was developed to remove nitrogen and phosphorus from tailwater, which is environment-friendly, economical and highly effective. Properties of biomass ash filter materials produced under different conditions were studied through orthogonal experiments with sintering temperature, keeping time and material trapped time as factors and the main control factors affecting the properties of filter materials were obtained. The pore evolution and forming principle of the filter material during the sintering process can be inferred by characterizing and analyzing the structural characteristics and microscopic morphology of biomass ash filter materials produced at different sintering temperatures. This helps to clarify the performance control mechanism of these filter materials via their different structures. Based on this, the biomass ash filter material that has the best performance could be selected as a test group. And commercial filter materials including biochar, ceramic granules and natural zeolite were used as control groups to conduct phosphate and nitrite adsorption tests to evaluate the adsorption performance of the biomass ash filter material. The results show that sintering temperature is the main control factor affecting the properties of biomass ash filter materials. As the sintering temperature increases, wollastonite, melilite and anorthite phases are generated sequentially in the filter materials, which leads to a series of microstructure transformations, such as crystallization, vitrification and precipitation. This is the reason for the formation of the different pore structures. In this process, water pores in the filter materials are gradually converted into closed pores. This causes positive effects of high sintering temperature on stomatal porosity, total porosity, and compressive strength. It also brings negative effects of high sintering temperature on permeable porosity, water pore rate, water absorption and permeability, pH and EC values. The performance control mechanism of these filter materials exists because of the two contrary effects. Without aging, the biomass ash filter material produced by sintering at 1170 ℃ for 40 minutes has better performance. This kind of filter material has a high saturation water absorption and average permeability rate, short water holding time, good formability and pore structure with a compressive strength of 0.74 MPa and total porosity of 65.49%. Its pH and EC values meet the demands of water purification. The unit adsorption capacity and removal rate of this filter material on phosphate can reach 0.996 mg/g and 2.767%, respectively. The phosphate adsorption effect is significantly better than that of biochar, ceramic granules and natural zeolite. The unit adsorption capacity and removal rate of this filter material on nitrite are 0.317 mg/g and 0.881%, respectively, which is equivalent to that of ceramic granules, significantly higher than natural zeolite and slightly inferior to biochar. The new renewable biomass ash filter material improves the defects of current mainstream filter materials for water purification. It is expected to meet the demand for nitrogen and phosphorus removal from circulating aquaculture tail water and can be used widely.

Flow electrode capacitive deionization (FCDI) is one of the most promising technologies for the continuous desalination and energy-efficient water purification. However, the performance of FCDI is confined to the electrode materials and the electrolyte composition. In this study, the porous carbon was synthesized to activate the biochar with different mass ratios of ZnCl₂. A systematic investigation was implemented to evaluate the physicochemical properties and capacitive deionization performance of the resulting porous carbon materials. An assessment was then performed on the deionization efficiency under varying electrolytes and initial ion concentrations. The capacitive deionization was analyzed using kinetic models. The key factors were determined on by the FCDI process. The potential application was given in various environments with the high ion concentrations. The results demonstrated that the ZnCl₂ activation was enhanced the physicochemical and capacitive deionization properties of the porous carbon. When the mass ratio of ZnCl₂ to porous carbon was 1 (PC-1Zn-600), the specific surface area of the material increased to 1 137.23 m²/g, and the pore diameter was reduced to 1.70 nm. Additionally, there was the an increase in the concentration of oxygen-containing functional groups (e.g., C=O, O=C-O). The specific capacitance values of PC-1Zn-600 and PC-2Zn-600 were improved to 72.30 F/g and 169.98 F/g, respectively, compared with the PC-600 (25.74 F/g), indicating the 1.81 and 5.60 fold increase, respectively. Furthermore, the electrical resistance of PC-1Zn-600 was reduced to 2.53 Ω. The removal efficiencies of ammonia nitrogen, Phosphorus (P), Potassium (K), and Calcium (Ca) ions using the PC-1Zn-600 flow electrode reached 84.98%, 79.9%, 72.79%, and 91.21%, respectively, after 210 min, compared with the PC-600 and PC-2Zn-600. Among the three electrolytes (feed solution, K₂SO₄, and H₂O), the H₂O electrolyte was the most effective to remove ammonia nitrogen and phosphorus, with the removal rates of 91.69% and 75.55%, respectively. The H₂O showed the lower energy consumption for the ion recovery, compared with the K₂SO₄ electrolyte. No addition of chemicals was required to offer both low cost and high performance in practical applications. The FCDI shared the low removal rate in the short term, as the concentrations of initial ion increased. However, the removal efficiencies reached 99.33%, 98.15%, 98.50%, and 98.22%, respectively, even with the high initial concentrations of ammonia nitrogen (1 000 mg/L), phosphorus (150 mg/L), potassium (1500 mg/L), and calcium (150 mg/L), after 6-9 h of operation. The removal rates were comparable to those observed at the lower ion concentrations (e.g., 100 mg/L ammonia nitrogen, 50 mg/L phosphorus, 500 mg/L KCl, and 50 mg/L CaCl₂). As such, the FCDI can be expected to effectively treat the high-concentration solutions. Therefore, it is recommended that the operation time for the FCDI treatment of high-ion concentration solutions can be extended to at least 6-9 hours. Additionally, the first-stage kinetic model (R² 0.96) was found to be well-suited for the FCDI deionization. The process was primarily governed by electrostatic interactions. The migration of ammonia nitrogen and potassium ions included three stages: double-layer membrane, ion-exchange membrane, and equilibrium stage. In phosphorus and calcium ions, the process consisted of a double-layer membrane followed by an ion-exchange membrane stage, with the ion-exchange membrane stage acting as the rate-limiting step. Thus, the properties of the ion-exchange membrane were improved to optimize the contact area between the influent solution and the ion-exchange membrane. The efficiency of FCDI was then enhanced to treat the highly concentrated solutions.