Capture and recover dissolved phosphorous from aqueous solutions by a designer biochar: Mechanism and performance insights

Enhanced adsorption of phosphate by rice straw-based biochar prepared via metal impregnation and bio-template technology

Phosphate removal from water through green, highly efficient technologies has received much attention. Biochar is an effective adsorbent for phosphate removal. However, adsorption capacity of phosphate on pristine rice straw-based biochar was not optimistic due to low anion exchange capacity. In this study, Fe-modified, Mg-modified and MgFe-modified rice straw-based biochar (Fe-BC, Mg-BC and MgFe-BC) were prepared by combining metal impregnation and biological template methods to improve the adsorption capacity of phosphate. The surface characteristics of biochar and the adsorption behavior of phosphate on biochar were investigated. The modified biochar had the specific surface area of 17.910-39.336 m2/g, and their surfaces were rich in a large number of functional groups and metal oxides. Phosphate release was observed on pristine rice straw-based biochar without metal impregnation. The maximum adsorption capacities of phosphate on MgFe-BC, Mg-BC and Fe-BC at 298 K were 6.93, 5.75 and 0.23 mg/g, respectively. Adsorption was a spontaneous endothermic process, while chemical adsorption dominated and electrostatic attraction and pores filling existed simultaneously. Based on the site energy distribution theory study, the standard deviation of MgFe-BC decreased from 6.96 to 4.64 kJ/mol with temperature increasing, which proved that the higher the temperature would cause the lower heterogeneity. Moreover, the effects of pH, humic acid, co-existing ions and ionic strength on phosphate adsorption of MgFe-BC were also discussed. MgFe-BC with fine pores and efficient adsorption sites is an ideal adsorbent for phosphate removal from water.

Efficiency and mechanism of phosphate adsorption and desorption of a novel Mg-loaded biochar material

Phosphate removal is complicated by the need for resource recovery. Biochar shows promise for efficient phosphate adsorption, but it must be modified to enhance its adsorption capacity. In this work, magnesium (Mg)-loaded biochar was synthesized through a two-step dipping and calcination process, and the MgBC600 product was used to adsorb phosphate from simulated water and biogas slurry wastewater. The phosphate adsorption capacity of Mg-loaded biochar was 109.35 mg/g, which was 12 times higher than that of unmodified biochar. The R2 of the Langmuir and pseudo-second-order kinetic models were 0.988 and 0.990, respectively, which fitted the phosphate adsorption process of MgBC600. Phosphate adsorption by MgBC600 was a spontaneous and endothermic process. The adsorption mechanism study showed that phosphate adsorption was controlled by the formation and electrostatic attraction of MgHPO4. In addition, 98% of chemically adsorbed phosphate was released after regeneration. Using phosphate-adsorbed MgBC600 as a soil amendment, Arabidopsis thaliana was 1.47 times higher than that in the biochar-only group, demonstrating that this is a promising strategy for enhancing phosphate adsorption efficiency and adsorbent recycling.

Enhancing dissolved inorganic phosphorous capture by gypsum-incorporated biochar: Synergic performance and mechanisms

Excess nutrients, such as phosphorus (P), in watersheds jeopardize water quality and trigger harmful algal blooms. Using phosphorus sorption material (PSM) to capture P from wastewater and agricultural runoff can help recover nutrients and prevent their water pollution. In this study, a novel designer biochar was generated by pyrolyzing woody biomass pretreated with a flue gas desulfurization gypsum. The removal of dissolved inorganic phosphorus (DIP) by the gypsum-incorporated designer biochar was more efficient than the gypsum, suggesting the pretreatment of biomass with the gypsum results in a synergic effect on enhancing DIP capture. The maximum P adsorption capacity of the designer biochar was more than 200 mg g-1 , which is one order of magnitude greater than that of the gypsum. This result clearly showed that the designer biochar is a better PSM to capture DIP from nutrient-contaminated water compared to the gypsum. Post-sorption characterization indicated that the sorption of DIP by the gypsum-incorporated biochar involves multiple mechanisms. The precipitation reactions of calcium (Ca) cations and P anions to form CaHPO4 and Ca3 (PO4 )2 precipitates on the highly alkaline surface of the designer biochar were identified as a main mechanism. By contrast, CaHPO4 ·2H2 O is the only precipitated product for DIP sorption by the gypsum. In addition, the initial solution pH and the coexisting bicarbonate had less effects on the DIP removal by the designer biochar in comparison with the gypsum, which further confirms that the former is an excellent PSM to capture DIP from a variety of aquatic media.

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

Removal of phosphorus using biochar derived from Fenton sludge: Mechanism and performance insights

A phosphorus removal biochar adsorbent was prepared from Fenton sludge. The adsorption process was optimized, and its phosphorus adsorption mechanism was discussed. It was found that the phosphorus adsorption performance of biochar prepared from single Fenton sludge (FBC-400) was better than that of co-pyrolysis of Fenton sludge and bamboo powder. The optimum condition was that Fenton sludge pyrolyzed at 400°C (FBC-400). FBC-400 had a larger specific surface area than that prepared by co-pyrolysis with bamboo powder. And the high content of iron element could provide a higher surface charge of the biochar, thereby increasing the electrostatic adsorption of phosphorus onto FBC-400. The phosphorus adsorption was highly pH dependent by FBC-400, which can enhance electrostatic adsorption and increase adsorption capacity in acidic conditions. The effect of coexisting anion on adsorption performance was mainly affected by CO₃ ^2- , reducing the adsorption capacity by at least 49%, whereas other anions had no obvious interference. The adsorption process of FBC-400 accorded with the pseudo-second-order kinetic model and the Langmuir model, which indicated that the adsorption process was monolayer adsorption and mainly chemical adsorption, and the maximum saturated phosphorus adsorption capacity was 8.77 mg g^-1 . The mechanisms for phosphorus adsorption were electrostatic adsorption and inner-sphere complexing. 1 M NaOH was used for desorption, and the adsorption capacity remained at 81% in the fifth cycle. PRACTITIONER POINTS: The Fenton sludge biochar usage as an adsorbent could be a win-win strategy to convert waste biomass to valuable - product. The adsorption process accorded with the Langmuir model, the maximum phosphorus adsorption capacity was 8.77 mg/g at 25°C. The adsorption mechanisms were electrostatic adsorption and inner-sphere complexing. 1M NaOH was used for desorption, and the adsorption capacity remained at 81% in the fifth cycle.

Solvent-Free Synthesis of MgO-Modified Biochars for Phosphorus Removal from Wastewater

Adsorption is an efficient technology for removing phosphorus from wastewater to control eutrophication. In this work, MgO-modified biochars were synthesized by a solvent-free ball milling method and used to remove phosphorus. The MgO-modified biochars had specific surface areas 20.50–212.65 m² g⁻¹ and pore volume 0.024–0.567 cm³ g⁻¹. The as-prepared 2MgO/BC-450-0.5 had phosphorus adsorption capacities of 171.54 mg g⁻¹ at 25 °C and could remove 100% of phosphorus from livestock wastewater containing 39.51 mg L⁻¹ phosphorus. The kinetic and isotherms studied show that the pseudo-second-order model (R² = 0.999) and Langmuir models (R² = 0.982) could describe the adoption process well. The thermodynamic analysis indicated that the adsorption of phosphorus on the MgO-modified biochars adsorbent was spontaneous and endothermic. The effect of pH, FTIR spectra and XPS spectra studies indicated that the phosphorus adsorption includes a protonation process, electrostatic attraction and precipitation process. This study provides a new strategy for biochar modification via a facile mechanochemical method.