Further reuse of phosphorus-laden biochar for lead sorption from aqueous solution: Isotherm, kinetics, and mechanism

Production, characterization and environmental remediation application of emerging phosphorus-rich biochar/hydrochar: a comprehensive review

Owing to the high carbon and phosphorus contents, large specific surface area and slow P release capacity of P-rich biochar/hydrochar (CHAR), its application in aquatic (or soil) environments and positive effects on heavy metal (HM) adsorption (or immobilization) have drawn global attention. To provide an overall picture of P-rich CHAR, this review includes a systematic analysis of the current knowledge on the preparation methods, characterization techniques, influencing factors and environmental applications of P-rich CHAR reported in the last ten years. The key findings and recommendations from this review are as follows: (1) there is still a knowledge gap concerning the regulatory mechanism of the key active components of P-rich CHAR at the molecular level. The dominant factors influencing these active components should be elucidated. (2) P-rich CHAR has a high capacity to immobilize most HMs (e.g., Cd, Cu, and Pb). However, it performs poorly with several HMs (e.g., As). Future studies should focus on the interactions between P-rich CHAR and HMs found in soil/water. (3) To meet the long-term requirements for plant growth, more attention should be given to improving the slow-release capacity and utilization efficiency of available P. (4) There is a potential risk of P loss (or eutrophication) due to rainfall and runoff, although P-rich CHAR exhibits excellent performance in terms of HM immobilization and carbon retention. Several reasonable suggestions are provided to solve these problems. In summary, P-rich CHAR has promising prospects in environmental remediation if these shortcomings are overcome.

Phosphorus-loaded coconut biochar: A novel strategy for cadmium remediation and soil fertility enhancement

The management of cadmium (Cd) contamination in soils poses a significant environmental challenge. This study investigates the effectiveness of phosphorus (P)-loaded coconut biochar, synthesized at various pyrolysis temperatures (450°C, 500°C, 550°C, and 600°C), in immobilizing Cd and enhancing P availability in soil environments. The biochar underwent a series of treatments including activation and P enrichment, followed by incubation trials to evaluate its performance in Cd immobilization and P bioavailability enhancement across varying soil concentrations (0.5 %, 1.0 %, and 2.0 %) over time periods of 15, 30, and 45 days. Remediation progress was monitored using phytotoxicity assessments with radish (Raphanus sativus) root length as a bioindicator, supplemented by urease activity analyses. Notably, the activation process increased the P loading capacity of biochar produced at 450°C, 500°C, and 550°C by 54.6 %, 72.4 %, and 51.8 %, respectively, while reducing the P retention capacity of biochar prepared at 600°C by 31.0 %. The biochar activated at 550°C presented the highest efficiency in remediating Cd-contaminated soils. Key findings indicate that the enhanced specific surface area and oxygenated functional group content of the activated biochar facilitated Cd adsorption and P uptake. The P-loaded biochar exhibited a substantial adsorption capacity for Cd, particularly effective at lower concentrations, rendering it highly suitable for soil remediation purposes. Additionally, the study revealed that the application of biochar led to an increase in soil pH, resulting in precipitation of Cd as hydroxide species and formation of insoluble complexes with phosphate ions, thereby reducing its bioavailability. In summary, incorporating P-loaded biochar into soil significantly improved soil quality and enhanced Cd passivation in contaminated soils. The utilization of biochar produced at 550°C, which exhibited optimal performance, suggests a practical and sustainable approach for soil remediation. Future research endeavors should prioritize the refinement of the biochar production process to enhance cost-effectiveness while maintaining high P loading efficiency.

Recovery of ammonia nitrogen and phosphate from livestock farm wastewater by iron-magnesium oxide coupled lignite and its potential for resource utilization

A new adsorbent called iron-magnesium oxide coupled lignite (CIMBC) was developed to address the challenges of recovering high concentrations of ammonia nitrogen and phosphate in livestock farm wastewater and improving the inefficient use of lignite (BC) with low calorific value. CIMBC was synthesized using the modified ferromagnesium salt double-coating method. The experiments demonstrated that Fe2O3 and MgO could be effectively loaded onto the surface of BC at a Fe/Mg molar ratio of 1:2 and pyrolysis temperature of 500 °C. The optimal conditions for adsorption were determined to be an N/P concentration ratio of 2:1, adsorbent dosage of 1 g/L, and pH of 7. The presence of coexisting cations (Ca2+ and Mg2+) inhibited the removal of ammonia nitrogen but enhanced the removal of phosphate. Likewise, the presence of coexisting anions (CO32- and SO42-) hindered the removal of both ammonia nitrogen and phosphate. The adsorption behavior followed the pseudo-second-order model and the Langmuir model, with a maximum adsorption capacity of 95.69 mg N/g for ammonia nitrogen and 101.32 mg P/g for phosphate. The adsorption process was a spontaneous endothermic process controlled by multiple levels. The main mechanisms of adsorption involved electrostatic attraction, intra-particle diffusion, ion exchange, chemical precipitation, and coordination exchange. After 5 times of adsorption-desorption, the recovery rate of CIMBC is less than 50%, and the removal rate of phosphate is less than 40%. Although the RCIMBC exhibited low reusability, but also it showed potential in removing heavy metals (Pb) from wastewater and for use as a slow-release fertilizer. CIMBC is a promising new adsorbent, which can realize resource utilization of lignite with low calorific value while removing nitrogen and phosphorus.

Multiple-functionalized biochar affects rice yield and quality via regulating arsenic and lead redistribution and bacterial community structure in soils under different hydrological conditions

Rice grown in soils contaminated with arsenic (As) and lead (Pb) can cause lower rice yield and quality due to the toxic stress. Herein, we examined the role of functionalized biochars (raw phosphorus (P)-rich (PBC) and iron (Fe)-modified P-rich (FePBC)) coupled with different irrigation regimes (continuously flooded (CF) and intermittently flooded (IF)) in affecting rice yield and accumulation of As and Pb in rice grain. Results showed that FePBC increased the rice yield under both CF (47.4%) and IF (19.6%) conditions, compared to the controls. Grain As concentration was higher under CF (1.94-2.42 mg kg^-1) than IF conditions (1.56-2.31 mg kg^-1), whereas the concentration of grain Pb was higher under IF (0.10-0.76 mg kg^-1) than CF (0.12-0.48 mg kg^-1) conditions. Application of PBC reduced grain Pb by 60.1% under CF conditions, while FePBC reduced grain As by 12.2% under IF conditions, and increased grain Pb by 2.9 and 6.6 times under CF and IF conditions, respectively, compared to the controls. Therefore, application of the multiple-functionalized biochar can be a promising strategy for increasing rice yield and reducing the accumulation of As in rice grain, particularly under IF conditions, whereas it is inapplicable for remediation of paddy soils contaminated with Pb.

Remediation of Pb-contaminated soil using biochar-based slow-release P fertilizer and biomonitoring employing bioindicators

Soil contamination by Pb can result from different anthropogenic sources such as lead-based paints, gasoline, pesticides, coal burning, mining, among others. This work aimed to evaluate the potential of P-loaded biochar (Biochar-based slow-release P fertilizer) to remediate a Pb-contaminated soil. In addition, we aim to propose a biomonitoring alternative after soil remediation. First, rice husk-derived biochar was obtained at different temperatures (450, 500, 550, and 600 °C) (raw biochars). Then, part of the resulting material was activated. Later, the raw biochars and activated biochars were immersed in a saturated KH₂PO₄ solution to produce P-loaded biochars. The ability of materials to immobilize Pb and increase the bioavailability of P in the soil was evaluated by an incubation test. The materials were incorporated into doses of 0.5, 1.0, and 2.0%. After 45 days, soil samples were taken to biomonitor the remediation process using two bioindicators: a phytotoxicity test and enzyme soil activity. Activated P-loaded biochar produced at 500 °C has been found to present the best conditions for soil Pb remediation. This material significantly reduced the bioavailability of Pb and increased the bioavailability of P. The phytotoxicity test and the soil enzymatic activity were significantly correlated with the decrease in bioavailable Pb but not with the increase in bioavailable P. Biomonitoring using the phytotoxicity test is a promising alternative for the evaluation of soils after remediation processes.

Rapid and Effective Lead Elimination Using Cow Manure Derived Biochar: Balance between Inherent Phosphorus Release and Pollutants Immobilization

Cow manure derived biochar (CMBC) can serve as a promising functional material, and CMBC can be regarded as an ecofriendly approach compared to conventional ones. CM bioadsorbent can be employed for heavy metal immobilization (such as for lead) as well as an amendment to increase soil fertility (e.g., phosphorus). Few studies have examined the surface interactions between pollutants and bioadsorbents when inherent nutrient release is present. In this work, CMBC was prepared and applied for Pb(II) removal, and the vital roles of released phosphorus from CMBC were comprehensively disclosed. Furthermore, CMBC could immobilize part of the Pb(II) in soil and promote plant growth. CM400 was an effective adsorbent whose calculated Q_e reached 691.34 mg·g^-1, and it rapidly adsorbed 98.36 mg·g^-1 of Pb(II) within 1 min. The adsorption mechanisms of Pb(II) by CMBC include ion exchange, physical adsorption, electrostatic attraction, chemical precipitation, surface complexation, and cation-π bond interaction. Based on the residual phosphorus content and adsorption effect, complexation rather than the chemical precipitation had a greater contribution toward adsorption. Besides, as the concentration of Pb(II) increased, the main adsorption mechanisms likely transformed from chemical precipitation to ion exchange and complexation. CMBC not only had a good effect on Pb(II) removal in the solution, but also immobilized the Pb(II) in soil to restrain plant uptake as well as promote plant growth. The main novelty of this work is providing more insights to the cow manure bio adsorbent on Pb immobilization and phosphorus release. This study is expected to serve as a basis and reference for analyzing the release effects of inherent nutrients and the interfacial behaviors with heavy metals when using CMBC and other nutrient-rich carbon-based fertilizers for pollution control.

Preparation and characterization of MgO hybrid biochar and its mechanism for high efficient recovery of phosphorus from aqueous media

Conversion of organic waste into engineered metal-biochar composite is an effective way of enhancing biochar’s efficiency for adsorptive capture of phosphorus (P) from aqueous media. Thus, various strategies have been created for the production of metal-biochar composites; however, the complex preparation steps, high-cost metal salt reagent application, or extreme process equipment requirements involved in those strategies limited the large-scale production of metal-biochar composites. In this study, a novel biochar composite rich in magnesium oxides (MFBC) was directly produced through co-pyrolysis of magnesite with food waste; the product, MFBC was used to adsorptively capture P from solution and bio-liquid wastewater. The results showed that compared to the pristine food waste biochar, MFBC was a uniformly hybrid MgO biochar composite with a P capture capacity of 523.91 mg/g. The capture of P by MFBC was fitted using the Langmuir and pseudo-first-order kinetic models. The P adsorptive capture was controlled by MgHPO4 formation and electrostatic attraction, which was affected by the coexisting F− and CO32− ions. MFBC could recover more than 98% of P from the solution and bio-liquid wastewater. Although the P-adsorbed MFBC showed very limited reusability but it can be substituted for phosphate fertiliser in agricultural practices. This study provided an innovative technology for preparing MgO-biochar composite against P recovery from aqueous media, and also highlighted high-value-added approaches for resource utilization of bio-liquid wastewater and food waste.

Graphical Abstract

Eggshell based biochar for highly efficient adsorption and recovery of phosphorus from aqueous solution: Kinetics, mechanism and potential as phosphorus fertilizer

Development of an efficient and green adsorbent is of great significance for phosphorus removal and recovery from eutrophic water. This work prepared an eggshell modified biochar (ESBC) by co-pyrolysis of eggshells and corn stalk. ESBC exhibited an excellent performance for phosphorus adsorption over a wide pH range (5-13), and achieved a maximum adsorption of 557.0 mg P/g. The adsorption process was well fitted by pseudo-second-order model (R² > 0.962) and Sips model (R² > 0.965), and it was endothermic (ΔH⁰ > 0) and spontaneous (ΔG⁰ < 0) according to thermodynamic analysis. The column experiment confirmed the feasibility of ESBC as a filter media for phosphorus removal in flow condition, and obtained a P removal of 460.0 mg/g. Soil burial tests indicated P-laden ESBC has a good P slow-release performance (maintained for up to 25 days). Overall, ESBC has a promising application potential as an efficient adsorbent for phosphorus recovery and subsequently as a slow-release fertilizer.

Effects of phosphorus-modified biochar as a soil amendment on the growth and quality of Pseudostellaria heterophylla

Phosphorus (P) deficiency in agricultural soil is a worldwide concern. P modification of biochar, a common soil conditioner produced by pyrolysis of wastes and residues, can increase P availability and improve soil quality. This study aims to investigate the effects of P-modified biochar as a soil amendment on the growth and quality of a medicinal plant (Pseudostellaria heterophylla). P. heterophylla were grown for 4 months in lateritic soil amended with P-modified and unmodified biochar (peanut shell) at dosages of 0, 3% and 5% (by mass). Compared with unmodified biochar, P-modified biochar reduced available heavy metal Cd in soil by up to 73.0% and osmotic suction in the root zone by up to 49.3%. P-modified biochar application at 3% and 5% promoted the tuber yield of P. heterophylla significantly by 68.6% and 136.0% respectively. This was different from that in unmodified biochar treatment, where tuber yield was stimulated at 3% dosage but inhibited at 5% dosage. The concentrations of active ingredients (i.e., polysaccharides, saponins) in tuber were increased by 2.9–78.8% under P-modified biochar amendment compared with control, indicating the better tuber quality. This study recommended the application of 5% P-modified biochar for promoting the yield and quality of P. heterophylla.

Mixed bacteria-loaded biochar for the immobilization of arsenic, lead, and cadmium in a polluted soil system: Effects and mechanisms

The present study explored the immobilization of mixed bacteria-loaded biochar on As, Pb, and Cd was explored. Physisorption and sodium alginate encapsulation were used to synthesize two kinds of mixed bacteria-loaded biochars, referred to as BCM and BCB. The observations of Scanning electron microscope, X-ray diffraction, and Fourier transform infrared spectroscopy distinctly demonstrated the colonization of mixed bacteria on biochar. Besides, the addition of BCM and BCB could increase soil pH with increasing incubation time. The residual fraction of heavy metals and soil dehydrogenase activities were also enhanced after 28 days of incubation. Pb was mainly immobilized by co-precipitation, which meant that Pb could be converted into a consistent crystalline form such as Pb₅(PO₄)₃OH. The X-ray photoelectron spectroscopy and X-ray diffraction analyses of materials identified the formation of Ca₂As₂O₇ and the presence of oxidation from trivalent arsenic to pentavalent arsenic. Cd was adsorbed by forming precipitations (CdCO₃) and exchanging ions with the BCM and BCB. Synergistic reactions between anions and cations also contributed to the immobilization of heavy metals, such as the formation of PbAs₂O₆ and Cd₃(AsO₄)₂. These results confirmed that mixed bacteria-loaded biochar was a feasible technology for the remediation of heavy metals contamination in site soils.

Sediment metals adhering to biochar enhanced phosphorus adsorption in sediment capping

Metal ions in sediment are inherent Ca and Fe sources for biochar modification. In this work, the effect of Ca²⁺ and Fe²⁺ released from sediment on biochar for phosphorus adsorption was evaluated. Results showed that raw peanut shell biochar (PSB) was poor in phosphorus adsorption (0.48 mg/g); sediment-triggered biochar (S-PSB) exhibited a P adsorption capacity of 1.32 mg/g in capping reactor and maximum adsorption capacity of 10.72 mg/g in the Langmuir model. Sediment released Ca²⁺ of 2.2-4.1 mg/L and Fe²⁺/Fe³⁺ of 0.2-9.0 mg/L. The metals loaded onto the biochar surface in the forms of Ca-O and Fe-O, with Ca and Fe content of 1.47 and 0.29%, respectively. Sediment metals made point of zero charge (pHpzc) of biochar shifted from 5.39 to 6.46. The mechanisms of enhanced P adsorption by S-PSB were surface complexation of CaHPO₄ followed by precipitation of Ca₃(PO₄)₂ and Ca₅(PO₄)₃(OH). Sediment metals induced the modification of biochar and improvement of P adsorption, which was feasible to overcome the shortcomings of biochar on phosphorus control in sediment capping.