Insight into the mechanism of low molecular weight organic acids-mediated release of phosphorus and potassium from biochars

Low-molecular-weight organic acids-mediated transport of neonicotinoid pesticides through saturated soil porous media: Combined effects of the molecular structures of organic acids and the chemical properties of contaminants

Empirical information about the transport properties of neonicotinoid pesticides through the soil as affected by the ubiquitous low molecular weight organic acids (LMWOAs) is lacking. Herein, the impacts of three LMWOAs with different molecular structures, including citric acid, acetic acid, and malic acid, on the mobility characteristics of two typical neonicotinoid pesticides (Dinotefuran (DTF) and Nitenpyram (NTP)) were explored. Interestingly, under acidic conditions, different mechanisms were involved in transporting DTF and NTP by adding exogenous LMWOAs. Concretely, acetic acid and malic acid inhibited DTF transport, ascribed to the enhanced electrostatic attraction between DTF and porous media and the additional binding sites provided by the deposited LMWOAs. However, citric acid slightly enhanced DTF mobility due to the fact that the inhibitory effect was weakened by the steric hindrance effect induced by the deposited citric acid with a large molecular size. In comparison, all three LMWOAs promoted NTP transport at pH 5.0. Because the interaction between NTP with soil organic matter (e.g., via π-π stacking interaction) was masked by the LMWOAs coating on soil surfaces. Nevertheless, LMWOAs could promote the mobility of both neonicotinoid pesticides at pH 7.0 due to the steric hindrance effect caused by the deposited organic acids and the competitive retention between LMWOAs and pesticides for effective surface deposition sites of soil particles. Furthermore, the extent of the promotion effects of LMWOAs generally followed the order of citric acid > malic acid > acetic acid. This pattern was highly related to their molecular structures (e.g., number and type of functional groups and molecular size). Additionally, when the background solutions contained Ca2+, the bridging effect of cations also contributed to the transport-enhancement effects of LMWOAs. The findings provide valuable information about the mobility behaviors of neonicotinoid pesticides co-existing with LMWOAs in soil-water systems.

Potassium assisted pyrolysis of Chinese Baijiu distillers' grains to prepare biochar as controlled-release K fertilizer

A novel K-loaded biochar as controlled-release K fertilizer was prepared through K assisted pyrolysis of distillers' grains (DGs, typical solid-byproducts of Chinese Baijiu) under different atmospheres (N₂ and CO₂) and temperatures (400 and 800 °C). The fabricated DGs-based biochar exhibited high K loading (200.20-232.33 mg/g), and the release kinetics and column leaching experiments suggested that K-loaded biochar exhibited excellent controlled release performance in a long term. Compared with other biochar, the K-loaded biochar prepared at CO₂ and 400 °C has lower cumulative release ratio of 82.35 %, and could retain the durative K release at ~0.5 % for 25 d. The release kinetics suggested that the K release behavior was dominated by dissolution, electrostatic attraction, adsorption, confinement effect, and chemical interaction. Furthermore, pot experiments revealed that K-loaded biochar could promote the growth of Komatsuna, in which the fresh weight and chlorophyll relative content of Komatsuna cultivated with biochar prepared at CO₂ and 400 °C reached 0.146 g and 41.95 after 25 d growth, respectively. The above results suggested that the K-loaded biochar exhibited excellent utilization potential as a controlled-release K fertilizer, facilitating the sustainable development and resource valorization of Baijiu industry.

Spectral characteristics coupled with self-organizing maps analysis on different molecular size-fractionated water-soluble organic carbon from biochar

Biochar-derived water-soluble organic carbon (BWSOC) plays important roles in the environmental effect of biochar. The environmental behavior and fate of BWSOC are closely related to its size distribution and chemical components. However, the molecular size-dependent BWSOC components and properties remain little known. To evaluate molecular size-dependent BWSOC characteristics, BWSOC samples were prepared by pyrolyzing biomasses in air-limitation and N₂-flow atmospheres at 300-600 °C and fractionated through a series of membranes with different pore sizes including 0.7 μm, 0.45 μm, 100 kDa, 10 kDa, 3 kDa, and 1 kDa. In all BWSOCs, <1 kDa and 0.45-0.7 μm fractions had the maximum abundance (mean: 40.6 %) and the minimum abundance (mean: 4.4 %), respectively. The spectral characteristics of BWSOC including polarity index, spectral slope, and humification index varied significantly with molecular size. The fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) analysis indicated that BWSOC was mainly composed of three organic components (humic-like, fulvic-like, and aromatic protein/polyphenol-like substances). Humic-like and fulvic-like substances mainly existed in <1 kDa fraction, while aromatic protein/polyphenol-like substances mainly existed in medium-size fractions (3 kDa-0.45 μm). The different locations of <1 kDa, 1 kDa-0.45 μm, and 0.45-0.7 μm fractions in EEM and PARAFAC self-organizing maps indicated self-organizing maps could effectively distinguish 0.45-0.7 μm, 1 kDa-0.45 μm, and < 1 kDa fractions via the variations of fluorescence intensity and organic components. Additionally, the distribution ratio of different molecular size fractions as well as the abundances of organic components in different molecular size fractions were strongly controlled by pyrolysis atmospheres (air-limitation and N₂-flow). This study systematically clarified the organic components and properties of different molecular size fractions in BWSOC, and the results are helpful to understand the possible environmental behavior and fate of BWSOC.

Behaviors of Organic Ligands and Phosphate during Biochar-Driven Nitrate Adsorption in the Presence of Low-Molecular-Weight Organic Acids

A batch experiment was conducted to examine the behavior of nitrate, organic ligands, and phosphate in the co-presence of biochar and three common low-molecular-weight organic acids (LMWOAs). The results show that citrate, oxalate, and malate ions competed with nitrate ion for the available adsorption sites on the biochar surfaces. The removal rate of LMWOA ligands by the biochar via adsorption grew with increasing solution pH. The adsorbed divalent organic ligands created negatively charged sites to allow binding of cationic metal nitrate complexes. A higher degree of biochar surface protonation does not necessarily enhance nitrate adsorption. More acidic conditions formed under a higher dose of LMWOAs tended to make organic ligands predominantly in monovalent forms and failed to create negatively charged sites to bind cationic metal nitrate complexes. This could adversely affect nitrate removal efficiency in the investigated systems. LMWOAs caused significant release of phosphate from the biochar. The phosphate in the malic acid treatment tended to decrease over time, while the opposite was observed in the citric- and oxalic-acid treatments. This was caused by re-immobilization of phosphate in the former due to the marked increase in solution pH over time.

Influence of pyrolysis atmosphere and temperature co-regulation on the sorption of tetracycline onto biochar: structure-performance relationship variation

Presently, as the prevalent pyrolysis atmospheres, N₂ is widely used, while air-limitation and CO₂ are rarely considered, to produce biochar to adsorb tetracycline. This study thus used N₂, CO₂, and air-limitation to produce various biochars at 300 ∼ 750 °C, and explored their structure-performance relationship for tetracycline sorption. The maximum sorption capacities of biochars produced in CO₂ and air-limitation were 55.36 mg/g and 71.11 mg/g (at 750 °C), respectively, being 2.34 and 3.01 times that of biochars produced in N₂ (23.60 mg/g at 750 °C). Interestingly, except for high pore volume and specific surface area supported pore filling and sites providing effect, ash (containing metal cations, P-O, and S=O) induced complexing effect was the primary mechanism for tetracycline sorption, rather than hydrophobic effect, π-π interaction, and hydrogen bond caused by C composition. This study provides important information about adjusting the pyrolysis atmosphere to improve the sorption performance of biochar toward tetracycline.

Effects of low molecular weight organic acids with different functional groups on arsenate adsorption on birnessite

In an aquatic ecosystem, especially constructed wetlands receiving arsenic (As)-containing wastewater, the fate and mobility of As is influenced by manganese (Mn) oxides and organic matter. Although Mn oxides have been extensively investigated for As(V) adsorption, effects of low molecular weight organic acids (LMWOAs) with different functional groups on As(V) adsorption onto birnessite and underlying mechanisms remain elusive. In this study, LMWOAs with two carboxyl groups (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively) were used. Results showed that more As(V) was adsorbed on birnessite with the presence of LMWOA, indicating that the LMWOA promoted As(V) adsorption via birnessite-carboxyl-As(V) ternary complex. Before birnessite dissolution, TA and MA facilitated As(V) adsorption more efficiently than SA, indicating that hydroxyl group enhanced the coordination among carboxyl groups, As(V) and birnessite. However, within high TA/MA batches, As(V) concentrations decreased sharply and then gradually increased, but Mn(II) concentrations continuously increased, showing the initial reductive dissolution of birnessite promoted As adsorption, while further dissolution was conducive to As mobilization. This study identifies the mechanisms of As adsorption in the presence of LMWOAs and highlights the importance of functional groups in As fate and mobility in aqueous environments.

Mechanisms of straw biochar's improvement of phosphorus bioavailability in soda saline-alkali soil

High pH and exchangeable sodium percentage, structural deterioration due to alkalinity, and nutrient deficiencies are typical characteristics of soda saline-alkali soil. In addition, phosphorus is typically the main limiting nutrient. Thus, there have been intense efforts to counter the salinity and improve the phosphorus availability of these soils (which cover large and growing areas). A promising approach is long-term application of straw biochar, which can significantly reduce soil salinity and promote the transformation of soil phosphorus. However, the mechanisms involved remain unclear. Thus, major aims of this review are to systematically address the mechanisms whereby biochar improves phosphorus bioavailability in soda saline-alkali soil through changes in the soil's physico-chemical properties, aggregate stability, contents of organic acids, enzyme activities, key functional genes, and microbial community structure. Another is to provide theoretical foundations for establishing effective methods for applying straw biochar to improve soda saline-alkali land and optimize phosphorus fertilizer applications.

Release characteristics of phosphate from ball-milled biochar and its potential effects on plant growth

Ball-milled biochar could potentially supply phosphorus, an essential element for plant growth. To realize resource reuse and phosphorus recovery, three feedstocks (rice straw, distillers grains, and Eupatorium adenophorum) were used to prepare ball-milled biochar to evaluate its release characteristics of phosphorus and potential effects on germination and growth. The results showed that the phosphate release performance of ball-milled distillers grains biochar (DM) at 300 and 600 °C was better than that of other biochars ball-milled for 12 h. The DM prepared at 600 °C and incubated for 12 (DM-12) or 24 h (DM-24) had the best phosphate release capacity. The solution with pH 3.0 was beneficial to the release of phosphate from DM-12. The pseudo-second-order model could better fit the phosphate release of DM-12. A germination and seedling growth experiment suggested that adding 2.5 wt% DM-12 was beneficial to the height of mung beans. This study shows that DM-12 can be used as a slow-release fertilizer for the growth of mung beans, which provides a new way for resource utilization of distillers grains and phosphorus-rich biochar.

Effects of biochar on transport and retention of phosphorus in porous media: Laboratory test and modeling

Given the complexity of soil components, a detailed understanding of the effects of single factors on phosphorus transport and retention will play a key role in understanding the environmental effects of phosphorus. In this work, quartz sand columns (considering five factors: doping rate, pH, particle size, ionic strength and cation type), combined with a two-site nonequilibrium transport model (TSM), were used to investigate phosphate (P) transport behavior. The results show that changes in doping ratio (0.4%-1.6%) and pH (5-9) have a notable effect on the transport of P, while, particle size of quartz sand hardly impacts the transport. When biochar was added at 1.6%, the surface of biochar increased the P fixation rate by about 37% through direct interaction with phosphate and bridging action with metal ions. As the morphology of P changed under different pH conditions, a part of P was immobilized in the form of precipitation. The immobilization of P was further enhanced with the increase of ionic strength. Compared with the direct interaction of P with biochar in Na⁺ solution, Ca²⁺ and Mg²⁺ solutions are more likely to adsorb P. Meanwhile, the TSM model also fits the transport behavior well. This study provides a perspective for evaluating the environmental behavior of P in the porous media interaction with biochar.

The composition, energy, and carbon stability characteristics of biochars derived from thermo-conversion of biomass in air-limitation, CO2, and N2 at different temperatures

This study systemically investigated the characteristics of biochars derived from thermo-conversion of pine sawdust and wheat straw in air-limitation, CO2, and N2 atmospheres at the temperatures of 300-750 °C. Meanwhile, their energy and C stability parameters were also evaluated here. The results showed that biochar produced in air-limitation had less yield, fixed C and bulk C, as well as more volatile matter and inorganic elements than that produced in CO2 and N2. Biochars derived from thermo-conversion of pine sawdust in CO2 and N2 at 450-750 °C had the greatest energy densification ratios (EDR) (range: 1.40-1.61), because pine sawdust contained more lignin than wheat straw, and the thermo-conversion of lignin in N2 and CO2 at 450-750 °C benefited for the formation of fixed C. Recalcitrance potential (R50) results showed that the biochars produced in CO2 and N2 at 600-750 °C had the highest carbon stability (R50: 0.54-0.64) for given biomass, owing to the thermo-conversion of biomass in CO2 and N2 at 600-750 °C preferring to form the organic C with high aromaticity and low polarity. Nonetheless, thermo-conversion of biomass in CO2 and N2 at 300 °C presented the greatest C sequestration potential, owing to high biochar yields under these conditions. Generally, the temperature-variability for the composition, EDR, and C sequestration potential followed the order: air-limitation > CO2 > N2, whereas carbon stability presented an opposite order. Our results contributed to selecting the appropriate atmosphere to optimize the properties and performances of biochars.

Ameliorative effects of biochar on persistency, dissipation, and toxicity of atrazine in three contrasting soils

The presence of atrazine a persistent herbicide in soil poses a serious threat to the ecosystem. The biochar amendment in soil altered the fate of this herbicide by modifying the soil properties. The present study examines the dissipation and toxicity of atrazine in three contrasting soils (silty clay, sandy loam, and sandy clay) without and with biochar amendment (4%). The experiment was performed for 150 days with three application rates of atrazine (4, 8, and 10 mg kg^-1). The speciation and degradation of atrazine, metabolite content, microbial biomass, and enzymatic activities were evaluated in all treatments. Three kinetic models and soil enzyme index were calculated to scrutinize the degradation of atrazine and its toxicity on soil biota, respectively. The goodness of fit statistical indices suggested that the first-order double exponential decay (FODE) model best described the degradation of atrazine in silty clay soil. However, a single first order with plateau (SFOP) was best fitted for atrazine degradation in sandy loam and sandy clay soils. The half-life of atrazine was higher in sandy clay soil (27-106 day^-1) than silty clay (28-77 day^-1) and sandy loam soil (27-83 day^-1). The variations in the dissipation kinetics and half-life of the atrazine in three soil were associated with atrazine partitioning, availability of mineral content (silica, aluminum, and iron), and soil microbial biomass carbon. Biochar amendment significantly reduced the plateau in the kinetic curve and also reduced the atrazine toxicity on soil microbiota. Overall, biochar was more effective in sandy clay soil for the restoration of soil microbial activities under atrazine stress due to modulation in the pH and more improved soil quality.

Nutrient alterations following biochar application to a Cd-contaminated solution and soil

Biochars, when applied to contaminated solutions or soils, may sequester potentially toxic elements while releasing necessary plant nutrients. This purpose of this study focused on quantifying both phenomenon following wheat straw (Triticum aestivum L.) biochar application (0, 5, and 15% by wt) to a Cd containing solution and a Cd-contaminated paddy soil using 240-day laboratory batch experiments. Following both experiments, solid phases were analyzed for elemental associations using a combination of wet chemical sequential extractions and synchrotron-based X-ray absorption spectroscopy (XAS). When wheat straw biochar was applied at 15% to Cd containing solutions, Cd and Zn concentrations decreased to below detection in some instances, Ca and Mg concentrations increased by up to 290%, and solution pH increased as compared to the 5% biochar application rate. Similar responses were observed when biochar was added to the Cd-contaminated paddy soil, suggesting that this particular biochar has the ability to sequester potentially toxic elements while releasing necessary plant nutrients to the soil solution. When significant, positive correlations existed between nutrient release over time, while negative correlations were present between biochar application rate, potentially toxic element sorption and pH. The latter suggests that potentially toxic elements were sorbed by a combination of organic functional groups or mineral precipitation based on whether pH was above or below ~ 7. In support of this contention, the wet chemical sequential extraction procedure in conjunction with previously observed Cd or current Zn XAS showed that biochar application promoted the formation of layered double hydroxides, sorption to (oxy)hydroxides, and organically bound to biochar as Zn species. As a multifunctional material, biochar appears to play an important role in sequestering Cd while releasing essential plant nutrients. These findings suggest that biochar may be a 'win-win' for improving environmental quality in potentially toxic element contaminated agroecosystems.

The residual effect of sewage sludge biochar on soil availability and bioaccumulation of heavy metals: Evidence from a three-year field experiment

Conversion of sewage sludge (SS) into biochar through pyrolysis is an alternative to make this residue useful for agricultural purposes. Despite advances in interpreting the functions of SS biochar (SSB) for improving soil quality, it is still necessary to understand its residual effect on the dynamics of heavy metals (HM), especially under field conditions in tropical soils. Therefore, the objective of this study was to evaluate the residual effect of the application of SSB obtained at different pyrolysis temperatures on the accumulation, availability and bioaccumulation of HMs by corn cultivated in a tropical soil. For this purpose, a field experiment was conducted for three years to assess the total and available levels of HMs in the soil and the leaf concentration after suspending the application of 30 t ha^-1 of SSB produced at 300 °C (BC300) and 500 °C (BC500). In general, the HM contents were below the maximum allowed by environmental legislation in several countries. SSB, regardless of temperature, was effective in immobilizing non-essential HMs for plants, such as Cd, Co, Cr and Pb, in the soil. On the other hand, SSB was able to supply micronutrients to corn plants after amendment ceased. Thus, the lack of negative long-term effects confirms the feasibility and safety of using SSB in agricultural areas with regards to contamination by HM, and makes it an alternative for the disposal of domestic SS.