X-ray diffraction (XRD) patterns of the unmodified and modified biochars.

<p>X-ray diffraction (XRD) patterns of the unmodified and modified biochars.</p

Carbon loss during pyrolysis for biochar production from wheat straw at 500°C with different modifications.

<p>Carbon loss during pyrolysis for biochar production from wheat straw at 500°C with different modifications.</p

TGA curves of the biochars in N₂ (a) and air (b) atmosphere.

<p>TGA curves of the biochars in N₂ (a) and air (b) atmosphere.</p

Emission rates of CO₂ from the biochars during aerobic incubation.

<p>Emission rates of CO₂ from the biochars during aerobic incubation.</p

XPS spectra of P 2p and C 1s electron for the unmodified and modified biochars.

<p>XPS spectra of P 2p and C 1s electron for the unmodified and modified biochars.</p

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)₈Cl_1.35 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 (<i>R</i>₅₀,_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

Role of Inherent Inorganic Constituents in SO₂ Sorption Ability of Biochars Derived from Three Biomass Wastes

Biochar is rich in both organic carbon and inorganic components. Extensive work has attributed the high sorption ability of biochar to the pore structure and surface chemical property related to its organic carbon fraction. In this study, three biochars derived from dairy manure (DM-biochar), sewage sludge (SS-biochar), and rice husk (RH-biochar), respectively, were evaluated for their SO₂ sorption behavior and the underlying mechanisms, especially the role of inherent inorganic constituents. The sorption capacities of SO₂ by the three biochars were 8.87–15.9 mg g^–1. With the moisture content increasing from 0% to 50%, the sorption capacities increased by up to about 3 times, mainly due to the formation of alkaline water membrane on the biochar surface which could promote the sorption and transformation of acidic SO₂. DM- and SS-biochar containing larger mineral constituents showed higher sorption capacity for SO₂ than RH-biochar containing less mineral components. CaCO₃ and Ca₃(PO₄)₂ in DM-biochar induced sorbed SO₂ transformation into K₂Ca­(SO₄)₂·H₂O and CaSO₄·2H₂O, while the sorbed SO₂ was converted to Fe₂(SO₄)₃·H₂SO₄·2H₂O, CaSO₄·2H₂O, and Ca₃(SO₃)₂SO₄·12H₂O in SS-biochar. For RH-biochar, K₃H­(SO₄)₂ might exist in the exhausted samples. Overall, the chemical transformation of SO₂ induced by biochar inherent mineral components occupied 44.6%–85.5% of the total SO₂ sorption. The results obtained from this study demonstrated that biochar as a unique carbonaceous material could distinctly be a promising sorbent for acidic SO₂ removal in which the inorganic components played an important role in the SO₂ sorption and transformation

Copyrolysis of Biomass with Phosphate Fertilizers To Improve Biochar Carbon Retention, Slow Nutrient Release, and Stabilize Heavy Metals in Soil

Two phosphate fertilizers, triple superphosphate (TSP) and bone meal (BM), were premixed with sawdust and switchgrass biomass for pyrolytic biochar formation. Carbon retention, P release kinetics, and capacity of biochar for stabilizing heavy metals in soil were evaluated. Results show that TSP and BM pretreatment increased carbon retention from 53.5–55.0% to 68.4–74.7% and 58.5–59.2%, respectively. The rate constants (<i>k</i>₂) of P release from the TSP- and BM-composite biochars are 0.0012–0.0024 and 0.89–0.91, respectively, being much lower than TSP and BM themselves (0.012 and 1.79, respectively). Copyrolysis with phosphate fertilizers enhanced biochar capability for stabilizing metals in soil significantly, especially the BM-composite biochar which increased Pb, Cu, and Cd stabilization rates by up to about 4, 2, and 1 times, compared to the pristine biochars. During the pyrolysis process, Ca­(H₂PO₄)₂ in TSP converted to Ca₂P₂O₇ and reacted with biomass carbon to form C–O–PO₃ or C–P, leading to greater carbon retention and lower P release. PO₄^3– in both composite biochars could precipitate with heavy metals, resulting heavy metal immobilization in soil. This study indicates that copyrolysis of biomass with P-containing fertilizers could obtain multiple environmental benefits

Kaolinite Enhances the Stability of the Dissolvable and Undissolvable Fractions of Biochar via Different Mechanisms

Input of biomass-derived biochar into soil is recognized as a promising method of carbon sequestration. The long-term sequestration effect of biochar depends on the stability of both its dissolvable and undissolvable fractions in soil, which could be affected by their interactions with soil minerals. Here, walnut shell-derived biochar was divided into dissolvable and undissolvable fractions and then interacted with kaolinite. Stability of kaolinite-biochar associations was evaluated by chemical oxidation and biological degradation. At low dissolvable biochar concentrations, the association was mainly attributed to “Ca²⁺ bridging” and “ligand exchange”, whereas “van der Waals attraction” was dominant at high concentrations. For the undissolvable biochar, kaolinite raised the activation energy of its surface by 22.1%, causing a reduction in biochar reactivity. By chemical oxidation, kaolinite reduced the C loss of total biochar by 42.5%, 33.1% resulting from undissolvable biochar and 9.4% from dissolvable biochar. Because of the presence of kaolinite, the loss of biodegradable C in total biochar was reduced by 49.4% and 48.2% from undissolvable fraction and 1.2% from dissolvable fraction. This study indicates that kaolinite can increase the stability of both dissolvable and undissolvable biochar, suggesting that kaolinite-rich soils could be a beneficial environment for biochar for long-term carbon sequestration

Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance

It has been reported that zerovalent iron can help biochar improve efficiency in heavy metal (HM) absorption, but the surface chemical behaviors and HM removal mechanisms remain unclear. We successfully synthesized the magnetic nanoscale zerovalent iron assisted biochar (nZVI-BC). The porosity, crystal structure, surface carbon/iron atom state, and element distribution were comprehensively investigated to understand nZVI-BC’s interfacial chemical behaviors and HM removal mechanisms. We clearly revealed the formation of a nanoscale Fe⁰ core–Fe₃O₄ shell on the surface/pores/channels of biochar. With the combination of iron nanoparticles and biochar, C–O/COOH groups were cracked with the formation of CO/CC, indicating the C–O–Fe acted as an electron acceptor during the reduction reaction. We also demonstrated that the stabilization was dramatically improved in the nZVI-BC, while more reduced iron and better homogeneity were observed. These results, showing the surface chemical behaviors of nZVI-BC, would help increase our understanding of the HM removal mechanisms. Moreover, our demonstration of the superior removal ability of multiple HM (Pb²⁺, Cd²⁺, Cr⁶⁺, Cu²⁺, Ni²⁺, Zn²⁺) from a solution can provide a breakthrough in making a feasible material for removing HM from polluted water resources