Phosphorus-assisted biomass thermal conversion: reducing carbon loss and improving biochar stability

Preparation of phosphorylated rice husk for cadmium adsorption: Crucial role of phosphonyl group

Rice husk is a locally available biomass for preparation of adsorbents to deal with cadmium (Cd) contamination in paddy system. In this study, phosphorylation of rice husk using H3PO4 and NH4H2PO4 was carried out in the presence of urea at 165℃ to obtain APB-C and NPB-C, respectively. According to the material characterizations, phosphonyl groups were successfully grafted on the rice husk. Both APB-C and NPB-C had high performance for Cd(II) adsorption with the capacities of 146 and 129 mg/g, respectively. The main mechanism of Cd(II) adsorption was ion exchange with NH4+. The adsorption capacity was linearly corelated with phosphorus content (R2 = 0.9997), while the Langmuir constant had high correlation efficient (R2 = 0.996) with phosphonyl group percentage. Further quantum chemical calculation showed higher interaction energy between Cd(II) and phosphonyl group than other groups. These results indicated that phosphonyl group governed Cd(II) adsorption on phosphorylated biomass.

A critical review of biochar as an environmental functional material in soil ecosystems for migration and transformation mechanisms and ecological risk assessment

At present, biochar has a large application potential in soil amelioration, pollution remediation, carbon sequestration and emission reduction, and research on the effect of biochar on soil ecology and environment has made positive progress. However, under natural and anthropogenic perturbations, biochar may undergo a series of environmental behaviors such as migratory transformation, mineralization and decomposition, and synergistic transport, thus posing certain potential risks. This paper outlines the multi-interfacial migration pathway of biochar in "air-soil-plant-animal-water", and analyzes the migration process and mechanism at different interfaces during the preparation, transportation and application of biochar. The two stages of the biochar mineralization process (mineralization of easily degradable aliphatic carbon components in the early stage and mineralization of relatively stable aromatic carbon components in the later stage) were described, the self-influencing factors and external environmental factors of biochar mineralization were analyzed, and the mineral stabilization mechanism and positive/negative excitation effects of biochar into the soil were elucidated. The proximity between field natural and artificially simulated aging of biochar were analyzed, and the change of its properties showed a trend of biological aging > chemical aging > physical aging > natural aging, and in order to improve the simulation and prediction, the artificially simulated aging party needs to be changed from a qualitative method to a quantitative method. The technical advantages, application scope and potential drawbacks of different biochar modification methods were compared, and biological modification can create new materials with enhanced environmental application. The stability performance of modified biochar was compared, indicating that raw materials, pyrolysis temperature and modification method were the key factors affecting the stability of biochar. The potential risks to the soil environment from different pollutants carried by biochar were summarized, the levels of pollutants released from biochar in the soil environment were highlighted, and a comprehensive selection of ecological risk assessment methods was suggested in terms of evaluation requirements, data acquisition and operation difficulty. Dynamic tracing of migration decomposition behavior, long-term assessment of pollution remediation effects, and directional design of modified composite biochar materials were proposed as scientific issues worthy of focused attention. The results can provide a certain reference basis for the theoretical research and technological development of biochar.

Phosphoric acid activation of cow dung biochar for adsorbing enrofloxacin in water: Icing on the cake

In this study, we used different concentrations of H3PO4 to activate pristine biochar (BC) derived from cow dung (BC and BC modified with phosphoric acid at concentrations of 10% (10P-BC), 30% (30P-BC), and 50% (50P-BC)) in order to obtain cheap, high-performance adsorbents. Brunauer-Emmett-Teller analysis, scanning electron microscopy, X-ray diffraction, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, organic element composition determination, and other analyses showed that activation with H3PO4 increased the porosity and hydrophilicity compared with the original BC, thereby enhancing the adsorption properties. The Langmuir isotherm model obtained the best fit and the maximum capacities for adsorbing enrofloxacin by BCs were 12.66 mg/g for BC, 51.90 mg/g for 10P-BC, 63.61 mg/g for 30P-BC, and 26.79 mg/g for 50P-BC. The main mechanisms responsible for antibiotic loading on BC were hydrogen bonding, π-π electron donor-acceptor interactions, pore filling, and electrostatic interactions. Calculations of fixed carbon retention before and after pyrolysis, and adsorption showed that activated BC had a good carbon fixation capacity and it was more capable of adsorbing enrofloxacin compared with the original BC, thereby providing a new method for removing organic pollutants from the environment and reducing carbon emissions. The cost efficiency was analyzed using the improved fuzzy comprehensive evaluation model based on the entropy method. Removal efficiency and utilization efficiency indicators were calculated for the different phosphoric acid activated BCs. The pollutant removal efficiencies were better for 10P-BC and 30P-BC, and the optimal removal efficiency was determined for 30P-BC. Given the current global climate change situation, using 10P-BC and 30P-BC could also help to meet China's carbon neutrality goals by reducing emissions of pollutants containing carbon.

The influence of biomass type on hydrothermal carbonization: Role of calcium oxalate in enhancing carbon sequestration of hydrochar

Addressing climate change through effective carbon sequestration strategies is critical. This study presents an investigation into the hydrothermal carbonization (HTC) and co-hydrothermal carbonization (Co-HTC) of invasive plants (IPs) to produce hydrochars to unveil the significant impact of biomass type and unique mineral on the stability of hydrochars. Nine hydrochars were produced from six IPs, utilizing both single and mixed biomass. A key finding is the observable that calcium oxalate forms as a surface mineral during HTC through different characterization techniques, the presence of which notably influenced the stability of hydrochars, resulting in enhanced thermal (highest R50 = 0.81) and chemical (lowest carbon loss rate = 4.02%) stability of hydrochars, possibly acting as a protective layer. Besides, a positive correlation was established between the yield of hydrochars and the lignin content of the original biomass. It is also observed that Co-HTC of plant materials rich in Ca2+ can enhance the formation of calcium oxalate minerals. This is likely due to their synergistic role in the HTC process, promoting the release of more C2O42- and Ca2+. Our results signify the crucial role of biomass composition in the HTC process and spotlight the potential of calcium oxalate in augmenting hydrochar stability. This study offers valuable insights that bolster the theoretical framework for employing hydrochar derived from IPs as a potent material for carbon sequestration.

NiCo2O4-loaded sunflower husk-derived biochar as efficient peroxymonosulfate activator for tetracycline removal in water

In this study, biochar produced from sunflower seeds husk was activated through ZnCl₂ to support the NiCo₂O₄ nanoparticles (NiCo₂O₄@ZSF) in catalytic activation of peroxymonosulfate (PMS) toward tetracycline (TC) removal from aqueous solution. The good dispersion of NiCo₂O₄ NPs on the ZSF surface provided sufficient active sites and abundant functional groups for the adsorption and catalytic reaction. The NiCo₂O₄@ZSF activating PMS showed high removal efficiency up to 99% after 30 min under optimal condition ([NiCo₂O₄@ZSF] = 25 mg L^-1, [PMS] = 0.04 mM, [TC] = 0.02 mM and pH = 7). The catalyst also exhibited good adsorption performance with a maximum adsorption capacity of 322.58 mg g^-1. Sulfate radicals (SO₄^•-), superoxide radical (O₂•-), and singlet oxygen (¹O₂) played a decisive role in the NiCo₂O₄@ZSF/PMS system. In conclusion, our research elucidated the production of highly efficient carbon-based catalysts for environmental remediation, and also emphasized the potential application of NiCo₂O₄ doped biochar.

Effect of Different Phosphates on Pyrolysis Temperature-Dependent Carbon Sequestration and Phosphorus Release Performance in Biochar

Carbon sequestration is the primary function of biochar. Hence, it is necessary to design biochar with high carbon (C) retention and low C loss. In this study, three P compounds, including KH₂PO₄, Ca(H₂PO₄)₂, and NH₄H₂PO₄, were premixed with corn stalk (1:4, w/w), aiming to produce biochars (CSB+K, CSB+Ca, and CSB+N) with high C sequestration and slow release of P at three temperatures (300, 500, and 700 °C). The addition of all P sources obviously increased C retention, with the order of NH₄H₂PO₄ (65.6-83.5%) > Ca(H₂PO₄)₂ (60.4-78.2%) > KH₂PO₄ (50.1-76.1%), compared with the pristine biochar (47.8-73.6%). The addition of Ca(H₂PO₄)₂ and KH₂PO₄ led to an increase in aromaticity and graphitization, as evidenced by H/C, FTIR, Raman and XPS analysis, whereas an opposite result occurred on CSB+N. Furthermore, all three phosphates reduced C loss of biochars with H₂O₂ oxidation, and CSB+Ca showed the best effect. Ca(H₂PO₄)₂ and KH₂PO₄ pretreated biochars had higher resistance to K₂Cr₂O₇ oxidation and thermal treatment. In contrast, the C loss of NH₄H₂PO₄-added biochar at 500 and 700 °C with K₂Cr₂O₇ oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H₂PO₄)₂ was transformed into insoluble Ca₂P₂O₇, leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H₂PO₄)₂ is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.

Improvement of the carbon yield from biomass carbonization through sulfuric acid pre-dehydration at room temperature

The pre-dehydration of a woody biomass waste (Douglas fir, DF) with 4.6-32 wt% of diluted sulfuric acid solutions was carried out mainly at room temperature aimed to improve the carbon yield from the thermal carbonization of pre-dehydrated biomass at 500 °C. By comparison (based on the raw DF), the pre-dehydration at room temperature increased the biochar yield and carbon retention up to about 32 wt% and 54%, respectively from that of about 22 wt% and 39% without pre-dehydration. When the pre-dehydration temperature increased to 90 °C, the biochar yield and carbon retention were sharply promoted to about 44 wt% and 76%, which was about two times higher than that of the biochar obtained without pre-treatment. This work for the first time proved the effectiveness of improving the carbon yield from lignocellulosic biomass via diluted sulfuric acid-assisted pre-dehydration at low or even room temperature.

Biochar-based slow-release of fertilizers for sustainable agriculture: A mini review

Increasing global population and decreasing arable land pose tremendous pressures to agricultural production. The application of conventional chemical fertilizers improves agricultural production, but causes serious environmental problems and significant economic burdens. Biochar gains increasing interest as a soil amendment. Recently, more and more attentions have been paid to biochar-based slow-release of fertilizers (SRFs) due to the unique properties of biochar. This review summarizes recent advances in the development, synthesis, application, and tentative mechanism of biochar-based SRFs. The development mainly undergoes three stages: (i) soil amendment using biochar, (ii) interactions between nutrients and biochar, and (iii) biochar-based SRFs. Various methods are proposed to improve the fertilizer efficiency of biochar, majorly including in-situ pyrolysis, co-pyrolysis, impregnation, encapsulation, and granulation. Considering the distinct features of different methods, the integrated methods are promising for fabricating effective biochar-based SRFs. The in-depth understanding of the mechanism of nutrient loading and slow release is discussed based on current knowledge. Additionally, the perspectives and challenges of the potential application of biochar-based SRFs are described. Knowledge surveyed from this review indicates that applying biochar-based SRFs is a viable way of promoting sustainable agriculture.

A critical review on metal-based catalysts used in the pyrolysis of lignocellulosic biomass materials

This review discusses the technical aspects of improving the efficiency of the pyrolysis of lignocellulosic materials to increase the yield of the main products, which are bio-oil, biochar, and syngas. The latest aspects of catalyst development in the biomass pyrolysis process are presented focusing on the various catalyst structures, the physical and chemical performance of the catalysts, and the mode of the catalytic reaction. In bio-oil upgrading, atmospheric catalytic cracking is shown to be more economical than catalytic hydrotreating. Catalysts help in the upgrading process by facilitating several reaction pathways such as polymerization, aromatization, and alkyl condensation. However, the grade of bio-oil must be similar to that of diesel fuel. Hence, the properties of the pyrolysis liquid such as viscosity, kinematic viscosity, density, and boiling point are important and have been highlighted. Switching between types of catalysts has a significant influence on the final product yields and exhibits different levels of durability. Various catalysts have been shown to enhance gas yield at the expense of the yields of bio-oil and biochar that shift the overall purpose of pyrolysis. Therefore, the catalytic activity as a function of temperature, pressure, and catalyst biomass ratio is discussed in detail. These operational parameters are crucial because they determine the overall yield as well as the ratio of the oil, char, and gas products. Although significant progress has been made in catalytic pyrolysis, the economic feasibility of the process and the catalyst cost remain the major obstacles. This review concludes that the catalytic process would be feasible when the fuel selling price is reduced to less than US $ 4 per gallon of gasoline-equivalent, and when the selectivity of catalysts is further enhanced.

New mechanistic insight into rapid adsorption of pharmaceuticals from water utilizing activated biochar

The presence of emerging pollutants especially hazardous chemicals and pharmaceuticals in aquatic environments is a matter of grave concern to human health and the environment. In this study, coffee bean waste (CBW) was utilized to synthesize pristine (CBW₅₅₀) and activated (CBW₅₅₀^HPO) biochars for the elimination of diclofenac (DF) and levofloxacin (LEV) from water. A facile two-step approach was used to synthesize CBW₅₅₀^HPO using chemical pretreatment and pyrolysis under N₂ purging. BET results of CBW₅₅₀^HPO revealed that chemical pretreatment increased surface area by approximately 160 times compared to CBW₅₅₀. The calculated I_D/I_G ratio from Raman spectra confirmed that CBW₅₅₀^HPO had a high functionalized surface. Different operational parameters such as contact time, pH, adsorbent dose, ionic strength, and adsorbate concentration were studied and optimized. Maximum Langmuir adsorption capacity of CBW₅₅₀^HPO was found to be 61.17 and 110.70 mg/g for DF and LVX, respectively. Experimental results demonstrated that presence of NaCl in solution enhanced DF removal efficiency due to the salting-out effect. Electrostatic attraction, π-π bonding, and hydrophobic interaction were prominently responsible mechanisms for the adsorption of DF and LVX. Furthermore, continuous-flow mode studies confirmed that CBW₅₅₀^HPO can be successfully utilized in large-scale treatment applications.

Pyrolysis temperature-dependent carbon retention and stability of biochar with participation of calcium: Implications to carbon sequestration

Converting biomass waste into biochar by slow pyrolysis with subsequent soil amendment is a prospective approach with multiple environmental benefits including soil contamination remediation, soil amelioration and carbon sequestration. This study selected cow manure as precursor to produce biochar under 300 °C, 400 °C, 500 °C and 600 °C, and a remarkable promotion of carbon (C) retention in biochar by incorporation of exogenous Ca was achieved at all investigated pyrolysis temperatures. The C retention was elevated from 49.2 to 68.3% of pristine biochars to 66.1-79.7% of Ca-composite biochars. It was interesting that extent of this improvement increased gradually with rising of pyrolysis temperature, i.e., doping Ca in biomass promoted pyrolytic C retention in biochar by 16.6%, 23.4%, 29.1% and 31.1% for 300 °C, 400 °C, 500 °C and 600 °C, respectively. Thermogravimetric-mass spectrometer (TG-MS) and X-ray photoelectron spectroscopy (XPS) showed that Ca catalyzed thermal-chemical reactions and simultaneously suppressed the release of small organic molecular substances (C₂-C₇) via physical blocking (CaO, CaCO₃, and CaClOH) and chemical bonding (CO and OC-O). The catalyzation mainly occurred at 200-400 °C, while the suppression was more prominent at higher temperatures. Raman spectra and 2D FTIR analysis on biochar microstructure showed that presence of Ca had negative influence on carbon aromatization and thus weakened biochar's stability, while increasing pyrolysis temperature enhanced the stability of carbon structure. Finally, with integrating "C retention" during pyrolysis and "C stability" in biochar, the maximum C sequestration (56.3%) was achieved at 600 °C with the participation of Ca. The study highlights the importance of both Ca and pyrolysis temperature in enhancing biochar's capacity of sequestrating C.

P-enriched hydrochar for soil remediation: Synthesis, characterization, and lead stabilization

One-step synthesis of multifunctional materials using biomass waste for environmental remediation is a current research hotspot. In this study, a novel P-enriched hydrochar was obtained by co-hydrothermal treatment of biomass (bamboo or hickory) with concentrated H₃PO₄ (biomass: H₃PO₄ = 1:4) at 200 °C for 7 h. The characteristics of the P-enriched hydrochar were determined and its effect on the stabilization of Pb in soils was investigated. Compared to pristine hydrochar, the weight yield of the P-enriched hydrochar was greater (by over 2 times). This was due to the enrichment of P (over 20% by weight), as the C, N, and H weight content was reduced. Moreover, the aromaticity, thermal stability, and surface functionality of P-enriched hydrochar were all higher than that of pristine hydrochar. Addition of the pristine hydrochar to a simulated 1300 mg·kg^-1 Pb-contaminated soil at 3% (w/w) resulted in a 20%-40% reduction in leached Pb only after 4 weeks, compared to the control without hydrochar amendment. However, addition of the P-enriched hydrochar to the spiked Pb-contaminated soil reduced Pb leaching by about 60% after only 1 week and about 90% after 3 weeks. Besides, using a real Pb-contaminated soil (149,000 mg·kg^-1 Pb), P-enriched hydrochar addition at 5% (w/w) resulted in a 100% decrease in Pb leaching in the first week and maintained leached Pb levels at <2 mg L^-1, meeting U.S.-E.P.A. standards. Thus, P-enriched hydrochar stabilized Pb in both simulated and real Pb-contaminated soil quickly and efficiently. Hence, the potential of one-step co-hydrothermal carbonization of biomass with H₃PO₄ to produce a novel and sustainable P-enriched hydrochar with properties suitable for environmental remediation of cationic metals.

Trace metal elements mediated co-pyrolysis of biomass and bentonite for the synthesis of biochar with high stability

The stability of biochar is a crucial parameter in determining the potential of biochar for carbon sequestration. Many studies have demonstrated that the addition of clay during the pyrolysis of biomass is beneficial for the production of biochar with a high stability, but finding a strategy for a further improvement of stability of clay-modified biochar is still highly desirable. Herein, the co-pyrolysis of biomass and clay mediated by trace metal elements is proposed as a new strategy for the production of biochar with exceptionally high stability. The results indicate that the biochar resistance index for biochar obtained from the trace metal elements mediated the co-pyrolysis of biomass and clay is ~0.75, which is much higher than that of biochars obtained from biomass pyrolysis or the co-pyrolysis of biomass and clay, demonstrating that the presence of metal ions during the co-pyrolytic process can significantly improve the oxidation resistance of biochar. Thermogravimetric analysis reveals that the carbon retention value is reduced when the addition of metal ions during the co-pyrolytic process, and the presence of metal ions can reduce the starting temperature of the pyrolysis reaction and catalyze the process of biomass pyrolysis. In addition, the percentages of CC, CC, and CH in all biochars obtained from trace metal elements mediated the co-pyrolysis of biomass and clay are greater than 41.82%, which are higher than that of original biochar. Besides, the percentage of oxygen-containing functional groups is found to be decreased after the presence of metal ions during the co-pyrolytic process. The presence of metal ions can form metal nano-sulfides and oxides on the surface, which plays a physical barrier to the anti-oxidation performance of biochar. Furthermore, compared to BBC or BC, MnBBC and ZnBBC have a better leaching resistance to P, while FeBBC has almost no leaching resistance to soil P. Overall, the study reveals that the addition of trace element metal ions during the co-prolysis of biomass and bentonite is an effective method to increase the stability of obtained biochar, and it is also beneficial for retarding the release of nutrients in the soil and thus increase the utilization of nutrients.

Effects of phosphorus modified nZVI-biochar composite on emission of greenhouse gases and changes of microbial community in soil

The effect of modified biochar on the greenhouse gas emission in soil is not clear until now. In this study, biochar (BC) was modified by phosphoric acid (P) and further combined with nano-zero-valent iron (nZVI) to form nZVI-P-BC composite. The P modified biochar could significantly increase the available phosphorus in soil. The release of CO₂ and N₂O in soil was inhibited during the initial stage of the experiment, with inhibition becoming more obvious over time. On the contrary, CH₄ and N₂O emission in soil was enhanced by nZVI-P-BC composite. The proportion of Sphingomonas and Gemmatimonas were the most abundant bacterial species, which were related to the metabolism and transformation of nitrogen. The community structure of the fungus was also affected by nZVI-P-BC composite with Fusarium as the main species. PCoA analysis result suggested that bacterial community was more affected by the incubation time while fungal community was more related to the addition of different biochar and modified biochars.

Long-term effect of biochar-based fertilizers application in tropical soil: Agronomic efficiency and phosphorus availability

Incorporation of phosphorus (P) into an organic matrix may be an effective strategy to increase plant P use efficiency in high P-fixing soils. The objective of this work was to evaluate the effect of biochar-based fertilizers (BBFs), produced from poultry litter (PLB) and coffee husk (CHB) enriched with phosphoric acid and magnesium oxide, in combination with triple superphosphate (TSP) on plant growth and soil P transformations. Treatments were prepared as: TSP, CHB, PLB, CHB + TSP [1:1], CHB + TSP [3:1], PLB + TSP [1:1] and PLB + TSP [3:1]; with numbers in brackets representing the proportion of BBF and TSP on a weight basis. Cultivations were: Mombasa grass, maize, and common bean interspersed with fallow periods. After cultivations, a sequential extraction procedure was employed to determine P distribution among different P pools. A kinetic study was performed and revealed that TSP released approximately 90% of total P, and BBFs less than 10% in the first hour. BBF alone or in combination with TSP presented higher or similar biomass yields, relative agronomic effectiveness, and P uptake when compared with TSP. As for the soil, BBFs increased non-labile P fractions, which can be due to pyrophosphate formed during pyrolysis. According to these results, BBFs could totally or partially replace conventional soluble P fertilizers without compromising crop yield either in the short and long-term.

A Biosorption-Pyrolysis Process for Removal of Pb from Aqueous Solution and Subsequent Immobilization of Pb in the Char

The application of biosorption in the removal of heavy metals from water faces a challenge of safe disposal of contaminated biomass. In this study, a potential solution for this problem was proposed by using a biosorption-pyrolysis process featured by pretreatment of biomass with phosphoric acid (PA). The PA pretreatment of biomass increased the removal efficiency of heavy metal Pb from water by sorption, and subsequent pyrolysis helped immobilize Pb in the residual char. The results indicate that most (>95%) of the Pb adsorbed by the PA-pretreated biomass was retained in the char, and that the lower pyrolysis temperature (350 °C) is more favorable for Pb immobilization. In this way, the bioavailable Pb in the char was hardly detected, while the Pb leachable in acidic solution decreased to <3% of total Pb in the char. However, higher pyrolysis temperature (450 °C) is unfavorable for Pb immobilization, as both the leachable and bioavailable Pb increased to >28%. The reason should be related to the formation of elemental Pb and unstable Pb compounds during pyrolysis at 450 °C, according to the X-ray diffraction study.

Effect of calcium dihydrogen phosphate addition on carbon retention and stability of biochars derived from cellulose, hemicellulose, and lignin

Pyrolysis of biomass with phosphate compound is a promising method to improve biochar characteristics. However, how phosphate compound affects the three components of biomass during the biochar formation is still unclear. In this study, a typical phosphate compound, calcium dihydrogen phosphate (Ca(H₂PO₄)₂), was premixed with cellulose, hemicellulose, and lignin reagent, at the ratio of 20% (w/w) for biochar production through pyrolysis, aiming to investigate the effects of Ca(H₂PO₄)₂ addition on biochar formation. Results show that, with Ca(H₂PO₄)₂ additions, carbon retention of biochars from cellulose (MCBC) and hemicellulose (MHBC) increased by 63.4% and 48.3%, respectively, but that of lignin (MLBC) decreased by 6.7% due to the reactions between lignin and Ca(H₂PO₄)₂. Moreover, the stable carbon proportion in the biochar decreased by 10.2% for MCBC, almost unchanged for MHBC, and increased by 6.15% for MLBC based on the potassium dichromate oxidation. During the pyrolysis process, Ca(H₂PO₄)₂ addition fixed more volatile and/or labile carbon in biochar, resulting in greater carbon retention. Declined carbon stability of biochar might be caused by the inhibited formation of aromatic-C, evidenced by the Fourier transform infrared spectroscopy analysis. This study highlights the importance and potential mechanisms of calcium dihydrogen phosphate influencing the carbon retention and stability of biochar derived from three biomass components.

Effect of phosphorus-modified biochars on immobilization of Cu (II), Cd (II), and As (V) in paddy soil

Novel phosphorus-modified biochars were produced by pyrolyzing biomass feedstocks (wood, bamboo, cornstalk and rice husk) pre-impregnated with potassium phosphate (K₃PO₄). The soil heavy metal immobilization performance and mechanisms of modified biochars were investigated. Incubation experiments showed that impregnation with phosphorous can decrease the extraction of Cu (II) and Cd (II) by 2 to 3 times. Phosphorus-modified biochars enhanced the transformation of Cu (II) and Cd (II) ions from acid soluble to more stable forms. Characterization results showed that phosphorus (P) compounds in modified biochar played a vital role to immobilize Cu (II) and Cd (II) by forming precipitates or complexes with them. Additionally, the modified rice husk and cornstalk biochars have in the average 14-24% and 19-33% higher immobilization efficiency for Cd (II) and Cu (II) than the other two P-assisted biochars. However, regardless of the feedstock, both the extraction and mobility of As (V) were increased by phosphorous. This study indicates that the P-modified biochar can serve as a novel remediation agent for heavy metal polluted soils.

Assessment of Agro-Environmental Impacts for Supplemented Methods to Biochar Manure Pellets during Rice (Oryza sativa L.) Cultivation

The agro-environmental impact of supplemented biochar manure pellet fertilizer (SBMPF) application was evaluated by exploring changes of the chemical properties of paddy water and soil, carbon sequestration, and grain yield during rice cultivation. The treatments consisted of (1) the control (no biochar), (2) pig manure compost pellet (PMCP), (3) biochar manure pellets (BMP) with urea solution heated at 60 °C (BMP-U60), (4) BMP with N, P, and K solutions at room temperature (BMP-NPK), and (5) BMP with urea and K solutions at room temperature (BMP-UK). The NO3−–N and PO4−–P concentrations in the control and PMCP in the paddy water were relatively higher compared to SBMPF applied plots. For paddy soil, NH4+–N concentration in the control was lower compared to the other SBMPFs treatments 41 days after rice transplant. Additionally, it is possible that the SBMPFs could decrease the phosphorus levels in agricultural ecosystems. Also, the highest carbon sequestration was 2.67 tonnes C ha−1 in the BMP-UK treatment, while the lowest was 1.14 tonnes C ha−1 in the BMP-U60 treatment. The grain yields from the SBMPFs treatments except for the BMP-UK were significantly higher than the control. Overall, it appeared that the supplemented BMP-NPK application was one of the best SBMPFs considered with respect to agro-environmental impacts during rice cultivation.

Aging of biochar-based fertilizers in soil: Effects on phosphorus pools and availability to Urochloa brizantha grass

Water-soluble phosphate fertilizers release phosphorus (P) to soils promptly, causing P fixation and low plant availability in highly weathered tropical soils. Therefore, the development of strategies to improve P use efficiency is needed. We hypothesized that biochar-based fertilizers (BBFs) can provide available P to plants and improve P use efficiency when compared with soluble fertilizers. Thus, triple superphosphate (TSP) and phosphoric acid (H₃PO₄) were pyrolyzed with and without magnesium oxide (MgO) and poultry litter to produce slow-release P BBFs. A pot experiment under greenhouse conditions was performed to evaluate agronomic efficiency of BBFs compared with TSP in an Oxisol. The treatments were incubated over 100 days after the application of 25, 50, 100, and 200 mg kg^-1 of P. Three controls were used, including 200 mg kg^-1 of P as TSP incubated for 100 days (named TSP_incubation) and applied immediately before sowing (named TSP_planting) and a negative control (without P). Marandu grass (Urochloa brizantha cv. Marandu) was cultivated in pots for three cycles of 40 days each. After cultivation, a sequential extraction procedure was used to determine the P distribution among different P pools. The shoot dry matter yield in the first cropping cycle was higher at the highest P rate for TSP_planting. PLB-H₃PO₄-MgO showed 9% increase in the shoot dry matter when compared with TSP_incubation in the first cropping cycle. In subsequent cropping cycles, all BBFs promoted higher biomass yield when compared with TSP_planting. There was an increase in the labile and moderately labile P fractions in soil after cultivation with PLB-TSP. The results suggest that BBFs can enhance P use efficiency in tropical soils in the middle- to long-term run due to slow-release profile that prevent P fixation and promote higher residual effect of fertilization.

Selected bacterial strains enhance phosphorus availability from biochar-based rock phosphate fertilizer

Purpose

The co-pyrolysis of biomass and soluble phosphates generates biochar-based phosphate fertilizers (BBF), which may enhance phosphorus (P) input in soil and P uptake by plants. Conversely, pyrolysis of biomass impregnated with rock phosphate results in low P solubility and may not supplement plant requirement in short term. However, bacterial strains promoting rock phosphate solubilization increases P use efficiency and can be applied to BBFs.

Methods

An in vitro assay was conducted to investigate the solubilization profile of five bacterial strains (Pseudomonas sp.—UFPI-B5-8A, Burkholderia fungorum—UFLA 04-155, Acinetobacter sp.—UFLA 03-09, Paenebacillus kribbensis—UFLA 03-10, and Paenibacillus sp.—UFLA 03-116) isolated from common bean and cowpea nodules in a rock phosphate BBF. Additionally, a pot trial was carried out aiming to investigate the influence on maize growth by inoculation of three selected strains under a rock phosphate BBF fertilization.

Results

Inoculations with UFPI B5-8A, UFLA 04-155, and UFLA 03-09 were efficient in solubilizing P in vitro, being closely associated with pH decrease, likely due to the release of organic acids. As for the pot trial, the dose of 400 mg kg−1 of P in the BBF using UFPI B5-8A significantly increased maize shoot dry matter. All strains significantly enhanced P availability in the soil.

Conclusions

Bacterial inoculation in biochar-based rock phosphate aiming to improve its fertilizer value is an inexpensive and sustainable strategy to improve maize growth and enhance available P in soil and should be further explored.

Remediation of cadmium-contaminated soil with biochar simultaneously improves biochar's recalcitrance

Biochar sequesters cadmium (Cd) by immobilisation, but the process is often less effective in field trials than in the laboratory. Therefore, the involvement of soil components should be considered for predicting field conditions that could potentially improve this process. Here, we used biochar derived from Spartina alterniflora as the amendment for Cd-contaminated soil. In simulation trials, a mixture of kaolin, a representative soil model component, and S. alterniflora-derived biochar immobilised Cd by forming silicon-aluminium-Cd-containing complexes. Interestingly, the biochar recalcitrance index value increased from 48% to 53%-56% because of the formation of physical barriers consisting of kaolinite minerals and Cd complexes. Pot trials were performed using Brassica chinensis for evaluating the effect of S. alterniflora-derived biochar on plant growth in Cd-contaminated soil. The bio-concentration factor values in B. chinensis were 24%-31% after soil remediation with biochar than in control plants. In summary, these results indicated that soil minerals facilitated Cd sequestration by biochar, which reduced Cd bioavailability and improved the recalcitrance of this soil amendment. Thus, mechanisms for effective Cd remediation should include biochar-soil interactions.

Two years of aging influences the distribution and lability of metal(loid)s in a contaminated soil amended with different biochars

A two-year soil incubation experiment was performed to investigate the long-term impacts of biochars (kenaf core and sewage sludge biochar (KBC and SBC) pyrolyzed at 350 °C and 550 °C) on metal(loid)s immobilization. Both KBC and SBC can immobilize Pb and Cu in contaminated soil, whereas they showed little effects on the immobilization of Zn, Cd and As. Interactions between the biochar and soil during two-year aging changed the metal species on both soil and biochar particles. KB350 formed more biochar-mineral complexes and O-containing functional groups than KB550 and thus transferred more residual metal(loid)s to their reducible species. More metal(loid)s sorbed on the KB350 than KB550 after two-year aging. However, SBC changed the acid-soluble species of metal(loid)s into the residual species during the aging process, probably due to the release of phosphate. Upon aging, SB550 exhibited a more significant increase in the residual metal amount and more sorption of metal(loid)s on the biochar particles than SB350 due to sorption of organic carbon and formation of meta-kaolinite. A key finding of our study was that different biochars have contrasting impacts on metal speciation and lability upon 2-year aging. This should be considered in assessing the actual risk of biochar-amended soils.

Inhibition of naphthalene leaching from municipal carbonaceous waste by a magnetic organophilic clay

Municipal solid waste conversion into biofuels via gasification is one of the latest technologies to divert waste from landfills. The byproduct of the process is a carbonaceous material, which is often tainted with polycyclic aromatic hydrocarbons (PAH) such as naphthalene that can leach into the environment and have toxic effects on aquatic organisms. In this paper, we present a novel method to address the issue of leachable naphthalene in a carbonaceous waste produced from a gasification process, using a magnetic sorbent. The sorbent was fabricated by the coprecipitation of iron oxide nanoparticles on an organophilic clay under atmospheric conditions. The characterization results show that the intercalated nanoparticles are predominantly magnetite with a diameter of 15-20 nm, and increase the clay specific surface area from 0.4 to 17 m² g^-1. Toxicity characteristic leaching procedure results indicate that the magnetic composite has a high naphthalene inhibition efficiency comparable to that of the original clay. As opposed to the clay alone, the magnetic hybrid can be separated from the carbonaceous waste with a magnet, regenerated by heat treatment, and reused without compromising its naphthalene removal efficiency. Thus, these composites may provide a cost-effective method to curtail leaching of PAH from contaminated carbonaceous waste.

A study of cadmium remediation and mechanisms: Improvements in the stability of walnut shell-derived biochar

Biochar has been recognized as an efficient soil amendment for cadmium remediation in recent years. In the present study, biochar was prepared using walnut shell, and it was incubated in Cd(NO₃)₂ and kaolin for 15 days. Different chemical forms of cadmium in kaolin and biochar were determined, and the stability of biochar was evaluated by R₅₀ using TGA analysis. It was found that walnut shell derived biochar could reduce the mobility of cadmium. After incubation, the R_{50, biochar} value increased from 61.31% to 69.57%-72.24%, indicating that the stability of biochar was improved. The mechanisms that initiated improvements in biochar stability were investigated by XPS, XRD and SEM-EDS analysis. The result showed that the enhanced biochar stability is likely due to physical isolation and the formation of precipitates and complexes, formed on the surface or interior of the biochar. The results suggested that walnut shell-derived biochar can be used as a cadmium sorbent for soil remediation.

Biochar produced from mineral salt-impregnated chicken manure: Fertility properties and potential for carbon sequestration

In this study, nutrient properties and carbon sequestration potential of biochars derived from chicken manure (CM) impregnated with mineral salts (calcium chloride, magnesium chloride, ferric chloride) were evaluated. Pretreatment with mineral salts reduced phosphorus (P) availability via the formation of insoluble metal phosphate minerals. Less carbon was lost during the pyrolysis of pretreated CM, and the produced biochars (BC_Ca, BC_Mg, and BC_Fe) were more stable (i.e., reduced C loss during chemical oxidation and less CO₂ release during incubation) than pristine biochars. Spectroscopic evidence indicated that enhanced biochar stability via metal salt pretreatment before pyrolysis was related to increased aromatization and enhanced physical protection due to the metal-oxygen interaction, together with the formation of metal mineral phases on biochar surfaces. Moreover, ferric chloride was the optimal additive, as it significantly decreased biochar P leachability and increased carbon sequestration potential.

Effects of copyrolysis of sludge with calcium carbonate and calcium hydrogen phosphate on chemical stability of carbon and release of toxic elements in the resultant biochars

The potential release of toxic elements and the stability of carbon in sludge-based biochars are important on their application in soil remediation and wastewater treatment. In this study, municipal sludge was co-pyrolyzed with calcium carbonate (CaCO₃) and calcium dihydrogen phosphate [Ca(H₂PO₄)₂] under 300 and 600 °C, respectively. The basic physicochemical properties of the resultant biochars were characterized and laboratory chemical oxidation and leaching experiments of toxic elements were conducted to evaluate the chemical stability of carbon in biochars and the potential release of toxic elements from biochars. Results show that the exogenous minerals changed the physico-chemical properties of the resultant biochars greatly. Biochars with exogenous minerals, especially Ca(H₂PO₄)₂, decreased the release of Zn, Cr, Ni, Cu, Pb, and As and the release ratios were less than 1%. Tessier's sequential extraction analysis revealed that labile toxic elements were transferred to residual fraction in the biochars with high pyrolysis temperature (600 °C) and exogenous minerals. Low risks for biochar-bound Pb, Zn, Cd, As, Cr, and Cu were confirmed according to risk assessment code (RAC) while the potential ecological risk index (PERI) revealed that the exogenous Ca(H₂PO₄)₂ significantly decreased the risks from considerable to moderate level. Moreover, the exogenous minerals significantly increased the chemical stability of carbon in 600 °C-pyrolyzed biochars by 10-20%. These results indicated that the copyrolysis of sludge with phosphate and carbonate, especially phosphate, were effective methods to prepare the sludge-based biochars with immobilized toxic elements and enhanced chemical stability of carbon.

Properties of biochar derived from wood and high-nutrient biomasses with the aim of agronomic and environmental benefits

Biochar production and use are part of the modern agenda to recycle wastes, and to retain nutrients, pollutants, and heavy metals in the soil and to offset some greenhouse gas emissions. Biochars from wood (eucalyptus sawdust, pine bark), sugarcane bagasse, and substances rich in nutrients (coffee husk, chicken manure) produced at 350, 450 and 750°C were characterized to identify agronomic and environmental benefits, which may enhance soil quality. Biochars derived from wood and sugarcane have greater potential for improving C storage in tropical soils due to a higher aromatic character, high C concentration, low H/C ratio, and FTIR spectra features as compared to nutrient-rich biochars. The high ash content associated with alkaline chemical species such as KHCO₃ and CaCO₃, verified by XRD analysis, made chicken manure and coffee husk biochars potential liming agents for remediating acidic soils. High Ca and K contents in chicken manure and coffee husk biomass can significantly replace conventional sources of K (mostly imported in Brazil) and Ca, suggesting a high agronomic value for these biochars. High-ash biochars, such as chicken manure and coffee husk, produced at low-temperatures (350 and 450°C) exhibited high CEC values, which can be considered as a potential applicable material to increase nutrient retention in soil. Therefore, the agronomic value of the biochars in this study is predominantly regulated by the nutrient richness of the biomass, but an increase in pyrolysis temperature to 750°C can strongly decrease the adsorptive capacities of chicken manure and coffee husk biochars. A diagram of the agronomic potential and environmental benefits is presented, along with some guidelines to relate biochar properties with potential agronomic and environmental uses. Based on biochar properties, research needs are identified and directions for future trials are delineated.

Roles of Phosphoric Acid in Biochar Formation: Synchronously Improving Carbon Retention and Sorption Capacity

Pretreatment of biomass with phosphoric acid (HPO) for biochar production was expected to improve carbon (C) retention, porosity structure, and the sorption ability of biochar. This study investigated the interaction of phosphorus with the C structure to elucidate the mechanisms by which HPO simultaneously captured C and created micropores. Sawdust was soaked in diluted HPO and dried for pyrolytic biochar generation at 350, 500, and 650°C. Results showed that HPO pretreatment resulted in 70 to 80% of biomass C retention in biochar, compared with only about 50% remaining without pretreatment. The specific surface area and total pore volume of the HPO-pretreated biochar were 930 m g and 0.558 cm g, respectively, compared with <51.0 m g and 0.046 cm g in the untreated biochar. The volume of micropores (<10 nm) increased from 59.0% to 78.4-81.9%. The presence of HPO shifted the decomposition temperature to a lower value and decreased the energy required for biomass decomposition. Micropore formation was via the insertion of P-O-P into the C lattice, leading to swelling and amplification of amorphous form and lattice defect of the C structure, as evidenced by Raman spectrum and small-angle X-ray scattering analysis. The crosslinking of P-O-P and C bonds resulted in greater biomass C retention in biochar. This biochar-phosphorus composite had a much higher sorption ability for Pb than the unmodified biochar, which was possibly dominated by phosphate precipitation and surface adsorption. This study provided a simple method to improve biochar properties and explored the multiple benefits of HPO in biomass pyrolysis.

Stabilization of Pb(II) accumulated in biomass through phosphate-pretreated pyrolysis at low temperatures

The remediation of heavy metal-contaminated soil and water using plant biomass is considered to be a green technological approach, although the harmless disposal of biomass accumulated with heavy metals remains a challenge. A potential solution to this problem explored in this work involves combining phosphate pretreatment with pyrolysis. Pb(II) was accumulated in celery biomass with superior sorption capacity and also in ordinary wood biomass through biosorption. The Pb(II)-impregnated biomass was then pretreated with phosphoric acid or calcium dihydrogen phosphate (CaP) and pyrolyzed at 350 or 450°C. Pb(II) from biomass was in turn almost totally retained in chars, and the percentage of DTPA-extractable Pb(II) was reduced to less than 5% of total Pb(II) in chars through CaP pretreatment. Pb(II) stabilization was further confirmed through a sequential extraction test, which showed that more than 95% of Pb(II) was converted into stable species composed mainly of lead phosphates according to X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) analyses. Overall, phosphate-pretreated pyrolysis can stabilize both Pb(II) and degradable biomass, so as to control efficiently the hazards of heavy metal-contaminated biomass.

The Interfacial Behavior between Biochar and Soil Minerals and Its Effect on Biochar Stability

In this study, FeCl3, AlCl3, CaCl2, and kaolinite were selected as model soil minerals and incubated with walnut shell derived biochar for 3 months and the incubated biochar was then separated for the investigation of biochar-mineral interfacial behavior using XRD and SEM-EDS. The XPS, TGA, and H2O2 oxidation were applied to evaluate effects of the interaction on the stability of biochar. Fe8O8(OH)8Cl1.35 and AlCl3·6H2O were newly formed on the biochar surface or inside of the biochar pores. At the biochar-mineral interface, organometallic complexes such as Fe-O-C were generated. All the 4 minerals enhanced the oxidation resistance of biochar surface by decreasing the relative contents of C-O, C═O, and COOH from 36.3% to 16.6-26.5%. Oxidation resistance of entire biochar particles was greatly increased with C losses in H2O2 oxidation decreasing by 13.4-79.6%, and the C recalcitrance index (R50,bicohar) in TGA analysis increasing from 44.6% to 45.9-49.6%. Enhanced oxidation resistance of biochar surface was likely due to the physical isolation from newly formed minerals, while organometallic complex formation was probably responsible for the increase in oxidation resistance of entire biochar particles. Results indicated that mineral-rich soils seemed to be a beneficial environment for biochar since soil minerals could increase biochar stability, which displays an important environmental significance of biochar for long-term carbon sequestration.

The effect of doped heteroatoms (nitrogen, boron, phosphorus) on inhibition thermal oxidation of reduced graphene oxide

The effect of doping different heteroatoms including nitrogen, boron and phosphorus on the thermal oxidation of RGO is investigated.