Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water

Enhanced selectivity and recovery of phosphate and nitrate ions onto coffee ground waste biochars via co-precipitation of Mg/Al layered double hydroxides: A potential slow-release fertilizer

In this study, the feasibility of Mg/Al layered double hydroxides (LDH) functionalized coffee ground waste biochars (LDHMgAl@CWGB) as a potential adsorbent to selectively recover phosphate (PO43-) and nitrate (NO3-) ions in aqueous phases and their consecutive uses as a slow-release fertilizer for stimulating the plant growth were identified. The higher adsorption capacity of PO43- and NO3- ions by LDHMgAl@CWGB (PO43- = 6.98 mgP/g, NO3- = 2.82 mgN/g) compared with pristine coffee ground waste biochars (CWGB; PO43- = 0.19 mgP/g, NO3- = 0.32 mgN/g) was mainly due to the incorporation of Mg/Al mixed oxides and Cl contents. Chemisorption and intra-particle mainly controlled the adsorptive recovery of PO43- and NO3- ions by CWGB and LDHMgAl@CWGB in aqueous phases and their adsorption toward CWGB and LDHMgAl@CWGB proceeded endothermically and spontaneously. The changes in the major adsorption mechanisms of PO43- and NO3- ions from ligand exchange (CWGB) to electrostatic surface complexation and anion-exchange (LDHMgAl@CWGB) supported the conclusion that the alternation of the surface features through Mg/Al LDH functionalization might improve selectivity and adsorption capacity of PO43- and NO3- ions onto CWGB under the co-existence of Cl-, SO42-, and HCO3- ions. Since PO43-- and NO3--loaded LDHMgAl@CWGB exhibited much higher seed germination, root and shoot growth rates of garden cress seeds (Lepidium sativum L) than other liquid and solid matrices, including 5 mgP/L PO43- and 5 mgN/L NO3-, 10 mgP/L PO43- and 10 mgN/L NO3-, and LDHMgAl@CWGB, it can be postulated that PO43-- and NO3--loaded LDHMgAl@CWGB could be practically applicable to the agricultural field as a slow-release fertilizer to facilitate the seed germination, root and shoot growth of the plants.

Simultaneous immobilization of heavy metals and nutrient elements in contaminated sediment using a novel composite agent product

In this research, an innovative type of sediment resource treatment agent (SRA) was synthesized successfully, which could immobilize ammonia nitrogen (NH₃-N), total phosphorus (TP), potassium (K), and simultaneously stabilize cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) in dredged sediment. The effects of SRA dosage on stabilizing the nutrient elements and heavy metals were investigated. The results demonstrated that the increase of SRA dosage significantly enhanced the stabilization of nutrients and heavy metals. The 14-day rainwater infiltration and rainwater scouring experiments were carried out. With the simulation test of rainwater infiltration, the stabilization ratios of Cr, Cu, Ni, Pb, Zn, Cd, NH₃-N, TP, and K with 2% SRA addition reached 80.8%, 76.8%, 80.3%, 77.5%, 78.0%, 72.7%, 64.3%, 73.9%, and 73.9%, respectively. Under the action of rainwater scouring, the stabilization ratios of Cr, Cu, Ni, Pb, Zn, Cd, NH3-N, TP, and K with 6.4% SRA addition reached 84.6%, 84.0%, 77.6%, 87.3%, 80.0%, 61.5%, 76.2%, 77.8%, and 91.7%, respectively. Therefore, the results demonstrate that SRA is an excellent composite material in stabilizing heavy metals while reserving the nutrients in dredged sediment, thus showing great potential in the application for dredged sediment resource treatment.

Detection and removal technologies for ammonium and antibiotics in agricultural wastewater: Recent advances and prospective

With the extensive development of industrial livestock and poultry production, a considerable part of agricultural wastewater containing tremendous ammonium and antibiotics have been indiscriminately released into the aquatic systems, causing serious harms to ecosystem and human health. In this review, ammonium detection technologies, including spectroscopy and fluorescence methods, and sensors were systematically summarized. Antibiotics analysis methodologies were critically reviewed, including chromatographic methods coupled with mass spectrometry, electrochemical sensors, fluorescence sensors, and biosensors. Current progress in remediation methods for ammonium removal were discussed and analyzed, including chemical precipitation, breakpoint chlorination, air stripping, reverse osmosis, adsorption, advanced oxidation processes (AOPs), and biological methods. Antibiotics removal approaches were comprehensively reviewed, including physical, AOPs, and biological processes. Furthermore, the simultaneous removal strategies for ammonium and antibiotics were reviewed and discussed, including physical adsorption processes, AOPs, biological processes. Finally, research gaps and the future perspectives were discussed. Through conducting comprehensive review, future research priorities include: (1) to improve the stabilities and adaptabilities of detection and analysis techniques for ammonium and antibiotics, (2) to develop innovative, efficient, and low cost approaches for simultaneous removal of ammonium and antibiotics, and (3) to explore the underlying mechanisms that governs the simultaneous removal of ammonium and antibiotics. This review could facilitate the evolution of innovative and efficient technologies for ammonium and antibiotics treatment in agricultural wastewater.

Dimorphism of Candida tropicalis and its effect on nitrogen and phosphorus removal and sludge settleability

Candida tropicalis PNY, a novel dimorphic strain with the capacity of simultaneous carbon, nitrogen and phosphorus removal in anaerobic and aerobic conditions, was isolated from activated sludge. Dimorphism of C. tropicalis PNY had effect on removing nitrogen and phosphorous and slightly affected COD removal under aerobic condition. Sample with high hypha formation rate (40 ± 5%) had more removal efficiencies of NH₄⁺-N (50 mg/L) and PO₄^3--P (10 mg/L), which could achieve 82.19% and 97.53%, respectively. High hypha cells dosage exhibited good settleability and filamentous overgrowth was not observed. According to label-free quantitative proteomics assays. Up-regulated proteins involved in the mitogen-activated protein kinase (MAPK) pathway indicated the active growth and metabolism process of sample with high hypha formation rate (40 ± 5%). And proteins concerning about glutamate synthetase and SPX domain-contain protein explain for the nutrient removal mechanism including assimilation of ammonia and polyphosphates synthesis.

Fe-modified fly ash/cotton stalk biochar composites for efficient removal of phosphate in water: mechanisms and green-reuse potential

Excessive phosphate content input into natural water can lead to the waste of resource and eutrophication. Biochar is a kind of low-cost adsorbent. However, its adsorption capacity for phosphate is low. In order to solve this problem, Fe compound-modified fly ash/cotton stalk biochar composites (Fe-FBC) were prepared through co-pyrolyzed fly ash and cotton stalk at 800℃, followed by infiltration of FeSO₄ solution. The samples were characterized by scanning electron microscopy, Brunauer-Emmett-Teller, X-ray diffraction, Fourier transform infrared spectroscopy, and zeta potential. After modification, the hydrophilicity and polarity of Fe-FBC increased. In addition, the pore volume, specific surface area, and surface functional groups were significantly improved. The adsorption behavior of Fe-FBC for the removal of phosphate from water can be well fitted by the pseudo-second-order kinetic and Sips isotherm adsorption model, with a maximum adsorption capacity of 47.91 mg/g. Fe-FBC maintained a high adsorption capacity in the pH range of 3-10. The coexisting anions (NO₃^-, SO₄^2-, and Cl^-) had negligible effects on phosphate adsorption. The adsorption mechanisms of Fe-FBC include electrostatic attraction, ligand exchange, surface complexation, ion exchange, chemical precipitation, and hydrogen bonding. Moreover, the desorption process of phosphate was investigated, indicating that the phosphate-saturated Fe-FBC could use as slow-release phosphate fertilizer. This study proposed a potentially environmental protection and recycling economy approach, which consists of recycling resources and treating wastes with wastes.

Rapid simultaneous removal of nitrogen and phosphorous by a novel isolated Pseudomonas mendocina SCZ-2

Nitrogen (N) and phosphorous (P) removal by a single bacterium could improve the biological reaction efficiency and reduce the operating cost and complexity in wastewater treatment plants (WWTPs). Here, an isolated strain was identified as Pseudomonas mendocina SCZ-2 and showed high performance of heterotrophic nitrification (HN) and aerobic denitrification (AD) without intermediate accumulation. During the AD process, the nitrate removal efficiency and rate reached a maximum of 100% and 47.70 mg/L/h, respectively, under optimal conditions of sodium citrate as carbon source, a carbon-to-nitrogen ratio of 10, a temperature of 35 °C, and shaking a speed of 200 rpm. Most importantly, the strain SCZ-2 could rapidly and simultaneously eliminate N and P with maximum NH₄⁺-N, NO₃^--N, NO₂^--N, and PO₄^3--P removal rates of 14.38, 17.77, 20.13 mg N/L/h, and 2.93 mg P/L/h, respectively. Both the N and P degradation curves matched well with the modified Gompertz model. Moreover, the amplification results of functional genes, whole genome sequencing, and enzyme activity tests provided theoretical support for simultaneous N and P removal pathways. This study deepens our understanding of the role of HN-AD bacteria and provides more options for simultaneous N and P removal from actual sewage.

Insights into heavy metals shock on anammox systems: Cell structure-based mechanisms and new challenges

Anaerobic ammonium oxidation (anammox) as a low-carbon and energy-saving technology, has shown unique advantages in the treatment of high ammonia wastewater. However, wastewater usually contains complex heavy metals (HMs), which pose a potential risk to the stable operation of the anammox system. This review systematically re-evaluates the HMs toxicity level from the inhibition effects and the inhibition recovery process, which can provide a new reference for engineering. From the perspective of anammox cell structure (extracellular, anammoxosome membrane, anammoxosome), the mechanism of HMs effects on cellular substances and metabolism is expounded. Furthermore, the challenges and research gaps for HMs inhibition in anammox research are also discussed. The clarification of material flow, energy flow and community succession under HMs shock will help further reveal the inhibition mechanism. The development of new recovery strategies such as bio-accelerators and bio-augmentation is conductive to breaking through the engineered limitations of HMs on anammox. This review provides a new perspective on the recognition of toxicity and mechanism of HMs in the anammox process, as well as the promotion of engineering applicability.

Enhanced removal of metal-cyanide complexes from wastewater by Fe-impregnated biochar: Adsorption performance and removal mechanism

Metal-cyanide complexes are common contaminants in industrial wastewater. Removal of these refractory contaminants is essential before their discharge into the environment. This study investigated a biochar (BC)-based sorbent material that could be applied for the efficient removal of metal-cyanide complexes from wastewater. In consideration of the strong electrostatic repulsion of the pristine BC toward anions, iron-modified BC (Fe-BC) composites were fabricated by a one-step co-pyrolysis of corn straw and FeCl₃ at 600-800 °C. The adsorption performance and corresponding sorption mechanisms of representative metal-cyanide complexes (ferricyanide [Fe(CN)₆]^3- and tetracyanonickelate [Ni(CN)₄]^2-) onto the Fe-BC composites were investigated. The results indicated that the Fe-BC composites had significantly high affinity toward the metal-cyanide complexes, reaching a maximum sorption capacity of 580.96 mg/g for [Fe(CN)₆]^3- and 588.86 mg/g for [Ni (CN)₄]^2-. A mechanistic study revealed that Fe-impregnation during BC fabrication could effectively alter the negatively charged BC surface, forming more functional groups that could interact with the metal-cyanide complexes. Moreover, the transformation of carbon structure promoted the carbothermal reduction process, leading to the formation of various reductive-Fe minerals in the resulting Fe-BC composites. These modification-induced alterations to the surface and structural characteristics of BC were expected to facilitate the adsorption/precipitation of target contaminants. Different sorption mechanisms were proposed for the two metal-cyanide complexes that were the focus of this study. For [Fe(CN)₆]^3-, precipitation by Fe-bearing species in the Fe-BC composites was the major factor controlling [Fe(CN)₆]^3- removal, while for [Ni(CN)₄]^2- hydrogen bonding interactions between surface functional groups (especially hydroxyl (-OH) and carboxyl (-COOH)) and [Ni(CN)₄]^2- were the main factors controlling removal.

Pyrolysis of Ca/Fe-rich antibiotic fermentation residues into biochars for efficient phosphate removal/recovery from wastewater: Turning hazardous waste to phosphorous fertilizer

Ca/Fe-rich antibiotic fermentation residues (AFRs), a type of hazardous waste, can be regarded as recyclable biomass and metal resources. However, concurrent detoxification and reutilization of biomass and metals resources from AFRs have never been reported before. In this study, Ca/Fe-rich vancomycin fermentation residues were pyrolyzed into biochar to adsorb phosphate for the first time. The residual vancomycin and antibiotic resistance genes were completely decomposed during pyrolysis. The resultant Ca/Fe-rich biochar exhibited excellent performance at adsorbing phosphate without further modifications. The process had rapid kinetics and a maximum adsorption capacity of 102 mg P/g. Ca and Fe were the active sites, whereas different mechanisms were observed under acidic and alkaline conditions. Surprisingly, HCO₃^- enhanced phosphate adsorption with an increase of adsorption capacity from 43.9 to 71.0 mg/g when HCO₃^- concentration increased from 1 to 10 mM. Furthermore, actual wastewater could be effectively treated by the biochar. The phosphate-rich spent biochar significantly promoted seed germination (germination rate: 96.7 % vs. 80.0 % in control group, p < 0.01) and seedling growth (shoot length was increased by 57.9 %, p < 0.01) due to the slow release of bioavailable phosphate, and thus could be potentially used as a phosphorous fertilizer. Consequently, the hazardous waste was turned into phosphorous fertilizer, with the additional benefits of detoxifying AFRs, reutilizing biomass and metal resources from AFRs, controlling phosphate pollution, and recovering phosphate from wastewater.

Seasonal assessment of the distribution, source apportionment, and risk of water-contaminated polycyclic aromatic hydrocarbons (PAHs)

The research aims to evaluate the seasonal differences in the distribution, source, and risks of water-contaminated PAHs. The PAHs were extracted by the liquid-liquid method and analyzed with GC-MS, and a total of eight PAHs were detected. There was a percentage increase in the average concentration of the PAHs from the wet to the dry season in the range of 20 (Anthracene)-350 (Pyrene)%. Total PAHs (∑PAHs) range from 0.31 to 1.23 mg/l in the wet period and from 0.42 to 1.96 mg/l in the dry period. The distribution of the average PAHs in mg/l showed that Fluoranthene ≤ Pyrene < Acenaphthene < Fluorene < Phenanthrene < Acenaphthylene < Anthracene < Naphthalene in wet period and while Fluoranthene < Acenaphthene < Pyrene < Fluorene < Phenanthrene < Acenaphthylene < Anthracene < Naphthalene in the dry period. The children were exposed to non-carcinogenic risk through non-dietary ingestion due to the accumulative effect (HI) of the PAHs in the dry period. Furthermore, the naphthalene was responsible for ecological and carcinogenic risk in the wet period, while the fluorene, phenanthrene, and anthracene were responsible for ecological and carcinogenic risk in the dry period. However, while adults and children are both susceptible to carcinogenic risk through the oral channel during the dry period, only children are susceptible to non-carcinogenic risk through this pathway. The multivariate statistical analysis revealed the influence of physicochemical parameters on the detected PAHs and also showed the PAHs' sources to be mainly combustion, pyrolysis, and vehicular emission.

Activating soil nitrification by co-application of peanut straw biochar and organic fertilizer in a rare earth mining soil

The intensive mining activities to extract rare earth elements from ion-adsorption rare earth deposits have introduced massive amounts of ammonium into the tailing soils in southern China. Compared to the ubiquitous soil nitrification in cropland, forest, and grassland soils, however, there is no feasible strategy to alleviate the ammonium contamination in tailing soil. Herein, the feasibility to remove ammonium by adding ammonium adsorbents (e.g., biochar, activated carbon, and zeolite), alkaline materials, and organic fertilizer to the rare earth mining soil was explored. The amendment of rice straw biochar, activated carbon, or zeolite in combination with CaCO₃ and organic fertilizer showed no significant effect on ammonium removal due to their limited capacity to elevate soil pH. However, the co-application of peanut straw biochar (PSBC), CaCO₃, and organic fertilizer activated both the ammonia volatilization and soil nitrification processes. Specifically, the three components functioned as follows: organic fertilizer supplied active ammonia-oxidizing bacteria (AOB); PSBC stimulated AOB proliferation by elevating soil pH above 7.75; CaCO₃ ameliorated soil acidity and reduced the lag time for activating soil nitrification. The soil ammonium removal and nitrate accumulation rates were positively correlated to the acid neutralization capacity of PSBC prepared at 400 °C-800 °C. The qPCR and microbial community analysis results indicated that Nitrosomonas europaea was the dominant AOB that was responsible for enhanced soil nitrification. Our findings pave the way for developing cost-effective strategies to remediate ammonium contamination in rare earth mining soils.

Recyclable Magnesium-Modified Biochar Beads for Efficient Removal of Phosphate from Wastewater

Although ball milling is effective for biochar modification with metal oxides for efficient phosphate removal, the recyclability of the adsorbent as well as the precursors for modification, still need to be optimized. Herein, a magnesium-modified biochar was first prepared with the precursor of MgCl₂·6H₂O through the solvent-free ball milling method. After that, recyclable biochar beads were fabricated with the introduction of sodium alginate and Fe₃O_4. The beads were proved to have excellent adsorption performance for phosphate with a saturated capacity of 53.2 mg g^-1, which is over 12 times higher than that of pristine biochar beads. Although the particle size reduction, surface area, and O-containing group increments after milling are beneficial for adsorption, the remarkable promotion in performance should mainly result from the appropriate formation of magniferous crystals on biochar, which greatly accelerates the electrostatic interactions as well as precipitation for adsorption. The beads also exhibited excellent magnetism-driven recyclability, which greatly avoids secondary contamination and broadens the application field of the adsorbent.

Migration electric-field assisted electrocoagulation with sponge biochar capacitive electrode for advanced wastewater phosphorus removal

Migrating electric field-assisted electrocoagulation (MEAEC) is a three-electrode electrochemical system, including waste flour-derived sponge biochar (SBC) as an adsorption electrode for efficient phosphorus removal from wastewater. The SBC was applied in the MEAEC system as a pseudo capacitance electrode with low energy consumption and reached an excellent effluent level (0.12 mg/L) with a 200-s treatment time in 1 mg/L phosphate synthetic wastewater. The SBC adsorption electrode had a total charge capacitance of 1.14 F/g with abundant micropores. Continuous charging and discharging at a constant voltage over 100 cycles demonstrated the excellent durability of the biochar electrodes. The energy demand of SBC-MEAEC was only 0.0058 kWh/m³ for 90% phosphate removal, which was 65% less than that of the control. The use of SBC in the MEAEC system greatly enhanced phosphate removal at low concentrations. In the SBC-MEAEC system, the electro-desorption synchronous electrocoagulation process demonstrated efficient concentration and release of ions after electro-adsorption. These results indicate that MEAEC with an SBC electrode could achieve a high level of phosphate removal with a much lower energy consumption than in previous studies. The recovered concentrated phosphorus flocs also contained fewer metal impurities than those in previous electrochemical approaches. The proposed desorption synchronous electrocoagulation utilizing waste-derived SBC electrodes provides a cost-effective pathway to treat low phosphorous-containing wastewater.

Synthesis of magnetic adsorbents from titanium gypsum and biomass wastes for enhanced phosphate removal

A novel scheme was proposed to prepare magnetic adsorbents by co-pyrolysis of titanium gypsum (TiG) and agricultural biomass wastes for phosphate (P) recovery. Co-presence of biomass wastes could improve TiG decomposition in inert atmosphere to generate magnetic centers and active sites, and P adsorption correlated well with organic volatiles of biomass wastes. The adsorption process evolved from a biomass-controlled process to a TiG-controlled process when increasing the mass ratio of corncob above 10 %. The optimal adsorbent (i.e. GC₁₀) exhibited higher P adsorption capacity (Q_m 183 mg/g) than many previous adsorbents; moreover, it can be magnetically separated from water after P adsorption. Active sites including CaO, CaS and Fe₃O₄ were deemed as the main factors for P chemisorption and surface precipitation. Most of adsorbed P could be released continuously and slowly by dilute NaHCO₃. These results highlight potential applications of TiG and biomass waste derived adsorbents in P purification and recovery.

Recent Advances in the Investigation of Poly(lactic acid) (PLA) Nanocomposites: Incorporation of Various Nanofillers and their Properties and Applications

Poly(lactic acid) (PLA) is considered the most promising biobased substitute for fossil-derived polymers due to its compostability, biocompatibility, renewability, and good thermomechanical properties. However, PLA suffers from several shortcomings, such as low heat distortion temperature, thermal resistance, and rate of crystallization, whereas some other specific properties, i.e., flame retardancy, anti-UV, antibacterial or barrier properties, antistatic to conductive electrical characteristics, etc., are required by different end-use sectors. The addition of different nanofillers represents an attractive way to develop and enhance the properties of neat PLA. Numerous nanofillers with different architectures and properties have been investigated, with satisfactory achievements, in the design of PLA nanocomposites. This review paper overviews the current advances in the synthetic routes of PLA nanocomposites, the imparted properties of each nano-additive, as well as the numerous applications of PLA nanocomposites in various industrial fields.

Recovery of nitrogen and phosphorus in wastewater by red mud-modified biochar and its potential application

A large amount of wastewater containing nitrogen, phosphorus, and fluorine produces in the production of phosphate fertilizer. In this study, to simultaneously recover nitrogen and phosphorus from phosphorus-containing wastewater and realize the resource utilization of red mud and rape straw, red mud-modified rape straw biochar (RM/RSBC) was prepared by facile one step, and the physicochemical properties were characterized by Zeta potential, SEM-EDS, BET specific surface area (SSA), FTIR, XRD, and XPS. The adsorption performance and mechanisms of ammonium and phosphate onto RM/RSBC were explored through static, fixed-bed column adsorption, and practical wastewater experiments. The results showed that pH = 3.0 and 8.0 were favorable for the removal of phosphate and ammonium, respectively. The main adsorption mechanisms of ammonium and phosphate were the interaction between ammonium and surface functional groups and surface precipitation, respectively. The removal efficiencies of ammonium and phosphate by fixed-bed column adsorption mainly depended on the addition amount of RM/RSBC, the concentration of ammonium and phosphate, and the flow rate. The results of the germination experiment showed that adding > 0.5 wt% of RM/RSBC loaded with ammonium and phosphate promoted the growth of mung beans. This study shows that RM/RSBC can not only recover ammonium and phosphate in wastewater, but also realize the resource utilization of red mud and rape straw.

Ecotoxicological characteristics and properties of zinc-modified biochar produced by different methods

Despite the dynamic progress of BC engineering, there is a lack of knowledge on the toxicity and environmental impact of modified BC. The aim of this study was the ecotoxicological evaluation of BC modified with zinc (Zn) using different methods: impregnation of feedstock with Zn before pyrolysis (PR), impregnation with Zn after pyrolysis (PS) and impregnation with Zn after pyrolysis with an additional calcination step (PST). The ecotoxicological assessment was based on tests with invertebrates (Folsomia candida, Daphnia magna) and bacteria (Aliivibrio fischeri). The post-treated and calcined composites had a higher content of total (C_tot) PAHs (144-276 μg kg^-1) than pre-treated BC-Zn (68-157 μg kg^-1). All BC-Zn treatments stimulated the reproduction of F. candida at the lowest BC dose (0.5%) by 4-24%. Increasing the biochar dose to 1% and 3% retained the stimulating effect of the pre-modified biochars (from 19 to 41%). Pre-modified BC-Zn reduced the luminescence of A. fischeri from 40% to 80%. Post-treated BCs reduced bacterial luminescence by 99%, but the calcination step limited the toxic effects to the level observed for the control. Post-treated BCs had a toxic effect on D. magna, with EC50 values ranging from 433 to 783 mg L^-1. The ecotoxicity of composites depends on modification methods, BC dose and pyrolysis temperature. The application of limiting conditions for HM leaching (i.e., pre-modification, calcination) increased the safety of using Zn-biochar composites.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Development of iron-based biochar for enhancing nitrate adsorption: Effects of specific surface area, electrostatic force, and functional groups

The problem of nitrate contamination in water has attracted widespread attention. Original biochar has a poor adsorption capacity for nitrate adsorption. Iron impregnation and acid protonation (base deprotonation) are common modification methods for biochar. In order to develop iron-mediated biochar containing multi-functional groups for enhancing nitrate adsorption, Fe-BC@H and Fe-BC@OH were prepared using a two-stage development process, including an iron-based carbon pyrolysis followed by acid protonation (or base deprotonation). The pseudo-second-order kinetic and Langmuir models can well describe the adsorption process which is a physicochemical complex monolayer adsorption. The data proved that Fe-BC@H (9.35 mg/g NO₃^--N) had a stronger adsorption capacity than Fe-BC@OH (2.95 mg/g NO₃^--N). Surface morphologies, functional groups, and mineral compositions of Fe-BC@H and Fe-BC@OH were analyzed through Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Characterization results showed that acid protonation can further improve the specific surface area (SSA), pore volume, and Zeta potential of Fe-based biochar, providing more adsorption sites for nitrate and enhancing the electrostatic force between nitrate and biochar. However, these effects were suppressed through base deprotonation. In addition, acid protonation can significantly increase the type and number of functional groups of biochar to enhance the chemisorption of nitrate. Such results suggested that the acid protonation can further improve the adsorption capacity of Fe-based biochar for nitrate, while base deprotonation had an inhibitory effect on that of Fe-based biochar. Overall, this study reveals that specific surface area, electrostatic force, and functional groups are crucial effects of the nitrate adsorption on acid/base modified biochar.

Engineered biochar effects on soil physicochemical properties and biota communities: A critical review

Biochar can be effectively used in soil amendment, environmental remediation as well as carbon sequestration. However, some inherent characteristics of pristine biochars (PBCs) may limit their environmental applications. To improve the physicochemical properties of PBCs and their effects on soil amendment and pollution remediation, appropriate modification methods are needed. Engineered biochars (EBCs) inevitably have a series of effects on soil physicochemical properties and soil biota after being applied to the soil. Currently, most studies focus on the effects of PBCs on soil physicochemical properties and their amendment and remediation effects, while relatively limited studies are available on the impacts of EBCs on soil properties and biota communities. Due to the differences of biochars modified by various methods on soil physicochemical properties and biota communities, the impact mechanisms are different. For a better understanding of the recent advances in the effects of EBCs on soil physicochemical properties and biota communities, a systematic review is highly needed. In this review, the development of EBCs is firstly introduced, and the effects of EBCs on soil physicochemical properties and biota communities are then systematically explored. Finally, the suggestions and perspectives for future research on EBCs are put forward.

Effects of biochar on anaerobic treatment systems: Some perspectives

Many anaerobic activities involve carbon, nitrogen, iron, and sulfur cycles. As a well-developed porous material with abundant functional groups, pyrolytic biochar has been widely researched in efforts to promote microbial activities. However, the lack of consensus on the biochar mechanism has limited its practical application. This review summarizes the effects of different pyrolysis temperatures, particle sizes, and dosages of biochar on microbial activities and community in Fe(III) reduction, anaerobic digestion, nitrogen removal, and sulfate reduction systems. It was found that biochar could promote anaerobic activities by stimulating electron transfer, alleviating toxicity, and providing suitable habitats for microbes. However, it inhibits microbial activities by releasing heavy metal ions or persistent free radicals and adsorbing signaling molecules. Finding a balance between the promotion and inhibition of biochar is therefore essential. This review provides valuable perspectives on how to achieve efficient and stable use of biochar in anaerobic systems.

Screening tests for N sorption allow to select and engineer biochars for N mitigation during biomass processing

Biochar amendment during biomass processing can improve those processes and products, and reduce the emissions of greenhouse gases and NH₃, resulting in ecologic and economic benefits. The potential positive effects of biochar are related to NH₄⁺-N and NH₃ sorption, which in turn are depending on different biochar characteristics. By knowing the relationship between biochar characteristics and NH₄⁺-N and NH₃ sorption, biochar production can be steered towards a higher N sorption or existing biochars can be selected for targeted applications for high N sorption. Therefore, this study aims to develop fast screening tests to estimate the potential for both NH₄⁺-N and NH₃ sorption of biochar, before application in biomass processing. Further, the effects of feedstock, pyrolysis temperature, biochar characteristics and biochar treatments on N sorption are studied. The results show that NH₄⁺-N sorption varied between 0 and 1.54 mg NH₄⁺-N/g fresh biochar and was highest for manure-based biochars with a high nutrient content and cation exchange capacity, produced at lower temperatures (300-450 °C). For some biochars, the feedstock itself had a higher NH₄⁺-N sorption than the biochar. Grinding and washing increased the NH₄⁺-N sorption. In addition, a general linear model was proposed to predict the NH₄⁺-N sorption based on three chemical characteristics_· NH₃ sorption varied between 0 and 100 % of the negative control and showed a linear positive relationship with the NH₄⁺-N sorption, moisture retention factor and cation exchange capacity. Pyrolysis temperature and feedstock type did not significantly affect NH₃ sorption. NH₄⁺-N and NH₃ desorption varied amongst biochar type.

Phosphate removal from aqueous solutions with a zirconium-loaded magnetic biochar composite: performance, recyclability, and mechanism

Phosphate (P) removal is significant for water pollution control. In this paper, a novel penicillin biochar modified with zirconium (ZMBC) was synthesized and used to adsorb P in water. The results showed that ZMBC had a porous structure and magnetic properties, and the zirconium (Zr) was mainly present in the form of an amorphous oxide. P adsorption displayed strong pH dependence. The Freundlich model described the adsorption process well, and the saturated adsorption capacity was 27.97 mg/g (25 ℃, pH = 7). The adsorption kinetics were consistent with the pseudo-second-order model, and the adsorption rates were jointly controlled by the surface adsorption stage and intraparticle diffusion stage. Coexisting anion experiments showed that CO₃^2- inhibited P adsorption, reducing the adsorption capacity by 62.63%. The adsorbed P was easily desorbed by washing with a 1 M NaOH solution, and after 5 cycles, the adsorbent had almost the same capacity. The mechanism for P adsorption was inner-sphere complexation and electrostatic adsorption.

Insights into the characteristics, adsorption and desorption behaviors of microplastics aged with or without fulvic acid

Many aging experiments on microplastics (MPs) have been carried out using UV radiation or strong oxidants. Little attention has been paid to the role of water environmental factors such as dissolved organic matter (DOM). In this study, the role of fulvic acid (FA), the main component of DOM, in the UV-aging process of MPs was explored. MPs aged under UV, and UV along with 0.5 mg/L and 2 mg/L FA, were selected as subjects. The results showed that (1) FA accelerated the aging process of polyethylene (PE). PE aged with FA had a larger specific area (S_BET), with more holes and cracks on the surface. (2) FA enhanced the adsorption capacity of PE. The TC adsorption quantities of 0, 0.5, and 2 mg/L FA-aged PE were 1.100, 1.447, and 1.812 mg/L, respectively. (3) The quantity of TC desorbed by PE increased, whereas the desorption rate decreased as the FA concentration increased. The desorption rates of TC at 0, 0.5, and 2 mg/L FA-aged PE were 25.16%, 22.05%, and 19.52% in water, and 72.10%, 70.36%, and 59.51% in simulated intestinal fluid. This study explored the role of FA in the aging process of MPs. Moreover, research on the aging mechanism of MPs is enriched.

Arsenic release from arsenopyrite weathering in acid mine drainage: Kinetics, transformation, and effect of biochar

Arsenopyrite (FeAsS) oxidative dissolution provides an important source for the occurrence of high arsenic in acid mine drainage (AMD). Biochar is a potent material that can dramatically sequestrate an array of heavy metals in water. However, little is known about the role of biochar on the fate of As from arsenopyrite in AMD. This study investigates the effects of biochar concentrations, AMD acidities, and temperatures on the release of As from arsenopyrite in a simulated AMD over a range of environmentally relevant conditions. Results show that biochar inhibits As release and further acidification without changing the arsenopyrite weathering mechanism. Arsenopyrite is first oxidized to Fe(II), As(III) and S⁰ and ultimately oxidized to Fe(III), As(V) and SO₄^2-, respectively. Higher concentration, temperature or higher acidity promotes the arsenic release rate. Electrochemical studies showed that biochar inhibited As release and acidification for reduced the charge transfer resistance at the double layer and film resistance at the passivation layer, which was mainly attributed to Fe(III) ions in AMD being adsorbed, oxidized, and As complexed to biochar-Fe-As(V). This study reveals the release mechanism of As from arsenopyrite weathering in AMD and suggests the applicability of biochar in mitigating arsenic pollution and further acidification in sulfide mineral mine drainage.

Batch adsorption of herbicides from aqueous solution onto diverse reusable materials and granulated activated carbon

This study reports the kinetics and isotherms of the adsorption of five herbicides, MCPA, mecoprop-P, 2,4-D, fluroxypyr and triclopyr, from aqueous solutions onto a range of raw and pyrolysed waste materials originating from an industrial setting. The raw waste materials investigated demonstrated little capability for any herbicide adsorption. Granulated activated carbon (GAC) was capable of the best removal of the herbicides, with >95% removal observed. A first order kinetic model fitted the data best for GAC adsorption of 2,4-D, while a pseudo-first order model fitted the data best for GAC adsorption of fluroxypyr and triclopyr, indicating that adsorption was via physisorption. A pseudo-second order kinetic model fitted the GAC adsorption of MCPA and mecoprop-P, which is indicative of chemisorption. The adsorption of the herbicides in all cases was best described by the Freundlich model, indicating that adsorption occurred onto heterogeneous surfaces.

Removal of Tetracycline Hydrochloride from Water by Visible-Light Photocatalysis Using BiFeO3/BC Materials

It is widely considered that photocatalysis is an effective and eco-friendly method of dealing with organic pollutants dissolved in water. Nonetheless, photocatalysts still have some drawbacks, such as poor visible-light absorption, easy recombination of photogenerated charge carriers, and limited active sites. In this study, bismuth ferrite coupled with biochar material (BiFeO3/BC) was simply synthesized, and its photocatalysis reactivity was systemically examined under an irradiation of λ > 400 nm. The experimental results showed that under a relatively acidic environment, the removal rate of tetracycline hydrochloride reached 95%. Using a variety of characterization investigations, we analyzed the morphology structure and chemical composition of BiFeO3/BC. In consideration of simple preparation and high respondence toward visible light, further explorations of BiFeO3/BC and its properties and optimized degradation conditions are worthwhile.

Mineral-hydrogel composites for mitigating harmful algal bloom and supplying phosphorous for photo-biorefineries

Harmful algal blooms (HAB) are a major environmental concern in eutrophic aquatic systems. To mitigate HABs and recover the phosphorus that drives algal growth, this study developed hydrogel composites seeded with calcium phosphate and wollastonite particles, which first adsorb phosphate (P) and then precipitate it as calcium phosphate. Using a fast-growing cyanobacterium, Synechococcus elongatus 2973, as a model microalga, we found that the mineral-hydrogel composites reduced dissolved P in BG11 media from 5.1 mg/L to 0.31 mg/L, initially reducing the biomass growth rate by up to 73 % and ultimately reducing the total biomass concentration by 75 %. When applied to municipal wastewater and agricultural run-off, the composites removed 96 % and 91 % of the dissolved P, respectively. Moreover, when the recovered P-enriched composites were reused as a slow-release bio-compatible fertilizer in a photobioreactor, they effectively supported algal growth without blocking light and interfering with photosynthesis. The P-enriched composites could tune the P concentration in the culture medium and significantly promote algal lipid accumulation. This study demonstrates the mineral-hydrogel composites' potential to treat point sources of P pollution and subsequently facilitate photoautotrophic biofuel production as a nutrient, effectively recycling the captured P.

Reduced cadmium (Cd) accumulation in lettuce plants by applying KMnO4 modified water hyacinth biochar

In this study, water hyacinth was adopted to prepare biochar followed by modification using KMnO₄. And the modified biochars were applied in Cd contaminated soil, exploring the effects of water hyacinth biochar on lettuce growth, Cd enrichment, soil enzyme activities and microbial changes by pot experiments. Modified biochar application significantly reduced the Cd accumulation in lettuce shoots and roots. Compared to the control, the application of water hyacinth biochar at 1% rate resulted in significant reduction of Cd contents by 40.7% and 33.7% in the shoots and roots of lettuce. Also, the reduction was 33.3% and 20.8% compared with the application rate of unmodified biochar. With the increase of biochar application, the amount of Cd was absorbed by lettuce shoots and roots showing significant reduction of plant Cd accumulation in response to the biochar application rate. Additionally, the lowest available Cd concentration in soil (1.34 mg kg⁻¹) was obtained with the application of modified biochar at 1% rate, which might be the main reason for the lower Cd concentration in lettuce shoot and root parts. Furthermore, structural analysis showed that Cd was fixed on the modified biochar, in a passivated state, by larger specific surface area, more active sites and more stable covalent binding complexes leading to a strong decrease of the available Cd in the soil. Moreover, it was concluded that the increment of the enzyme activities in the soil was up to 2.51 times significantly following the application of modified water hyacinth biochar with 3% amount. Lastly, 16sRNA sequencing showed that biochar addition may lead to changes of microbial structure and abundance in soil.

Removal of ofloxacin from water by natural ilmenite-biochar composite: A study on the synergistic adsorption mechanism of multiple effects

The preparation cost is one of the major constraints for adsorbent applied to practical situations. Here, a novel, economical and eco-friendly ilmenite biochar composite (ILM-BC) was successfully prepared by co-cracking of natural ilmenite and corn stover for the removal ofloxacin from water. The adsorption experiments indicated that the removal ofloxacin by ILM-BC was chemisorption and belonged to a spontaneous and entropy-increasing heat absorption process. Among composites, ILM-BC5 had superior adsorption capacity and stability, with a removal rate 1.6 times higher than that of biochar, and it could remove more than 90% ofloxacin in the pH range of 2-10. Multiple characterization results indicated that the adsorption of ILM-BC was the result of the synergistic effect of pore filling, hydrogen bonding, and π-π interactions. The introduction of ilmenite promoted hydrogen bonding formation and π-π interactions by enriching -OH and -COO on the surface of ILM-BC, which could enhance the adsorption capacity of ILM-BC.

From waste to fertilizer: Nutrient recovery from wastewater by pristine and engineered biochars

Biochar application for the recovery of nutrients from wastewater is a sustainable method based on a circular economy. Wastewater, food wastewater, and stormwater are valuable sources of nutrients (i.e., PO₄^3-, NO₃^-, and NH₄⁺). The unique properties of biochar, such as its large specific surface area, pH buffering capacity, and ion-exchange ability, make it a cost-effective and environmentally friendly adsorbent. Biochar engineering improves biochar properties and provide targeted adsorbents. The biochar-based fertilizers can be a sustainable alternative to traditional fertilization. The aim of the study was to compare the potential of pristine and engineered biochars to recover nutrients from wastewater and to determine the factors which may affect this process. Engineered biochar can be used as a selective adsorbent from multicomponent solutions. Adsorption on engineered biochar can be also regulated by additional mechanisms: surface precipitation and ligand/ion exchange. Metal modification (e.g. Mg, Fe) enhances PO₄^3- and NO₃^- adsorption capacity, and thus may provide the extra plant macro-/micronutrients. The desorption mechanism, which is the basis for nutrient release are strongly pH depended. The use of biochar-based fertilizer can have economic and agricultural benefits when using waste materials and reducing pyrolysis energy costs.

Investigation on the evolution of hydrothermal biochar

The objective of this study was to visualize trends and current research status of hydrothermal biochar research through a bibliometric analysis by using CiteSpace software. The original article data were collected from the Web of Science core database published between 2009 and 2020. A visual analysis network of national co-authored, institutional co-authored and author co-authored articles was created, countries, institutions and authors were classified accordingly. By visualizing the cited literature and journal co-citation networks, the main subject distribution and core journals were identified respectively. By visualizing journal co-citations, the main research content was identified. Further the cluster analysis revealed the key research directions of knowledge structure. Keyword co-occurrence analysis and key occurrence analysis demonstrate current research hotspots and new research frontiers. Through the above analysis, the cooperation and contributions of hydrothermal biochar research at different levels, from researchers to institutions to countries to macro levels, were explored, the disciplinary areas of knowledge and major knowledge sources of hydrothermal biochar were discovered, and the development lineage, current status, hotspots and trends of hydrothermal biochar were clarified. The results obtained from the study can provide a reference for scholars to gain a deeper understanding of hydrothermal biochar.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

Comparison of pyrochar, hydrochar and lignite as additive in anaerobic digestion and NH4+ adsorbent

The impact of pyrochar, hydrochar and lignite addition on anaerobic digestion of food waste was investigated with and without ammonia inhibition under batch conditions. Furthermore, ammonium adsorption capacities of the chars were investigated. To determine anaerobic degradation of char, reference samples containing inoculum and char were analyzed, indicating a significant degradation of hydrochar. Depending on the evaluation method, the increase in methane yield due to hydrochar addition varied between no statistically significant difference and +14 %. No significant impact due to the addition of 5 g/l pyrochar and lignite on AD was found. NH₄⁺ adsorption capacities showed a significantly higher net adsorption capacity of lignite (1.58mg_NH4⁺/g_L), compared to pyrochar (0.63mg_NH4⁺/g_PC). A negative NH₄⁺ adsorption capacity (-0.51 mg_NH4⁺/g_HC) was found for hydrochar. A high H/C-ratio, O/C-ratio and cation exchange capacity of hydrochar and lignite indicate many functional groups and low chemical stability, enabling an increased interaction between NH₄⁺ and char.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.

Impact of sulfur-impregnated biochar amendment on microbial communities and mercury methylation in contaminated sediment

S-impregnation of biochar through elemental S streaming is known to increase its sorption performance against Hg and methyl mercury (MeHg). However, the effects of %S-loading on biochar's mechanism and sorption capacities for MeHg, and its consequent impact when used as an amendment material for Hg-contaminated sediments, are poorly understood, and thus, were investigated in this work. Our results showed that a minimum sulfur loading of 1% was the most effective in reducing MeHg levels in sediments. At higher %S-loading (3-20%), the reduction in surface area, pore blockage due to unreacted sulfur particles, and presence of poorly bound sulfur species resulted in lowered effectiveness for MeHg control. Increasing S-functionalization during impregnation shifted the sorption process of MeHg from Hg-O to Hg-S in S-impregnated biochar (BCS). Our 60-day slurry experiment showed a significant reduction in pore water THg (40-70%) and MeHg (30-55%), as well as sediment MeHg (50-60%) in biochar-amended sediments. The reduction in the bioavailable Hg resulted in lowered Hg methylation, as supported by the suppression of both the Fe- and SO₄^2--reduction activities in the amended sediments. The microbial community structure in BCS-amended sediments showed a shift towards sulfur-consuming, iron-reducing, thiosulfate-oxidizing, and sulfate-reducing bacterial populations. At the genus level, the overall relative abundance of principal Hg methylators was also lower in the BCS treatment than in the unamended sediments. This study highlights the application of BCS as a promising strategy for remediation of Hg-contaminated sediments.

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.

Bamboo Chopstick Biochar Electrodes and Enhanced Nitrate Removal from Groundwater

The nitrate pollution of groundwater can cause serious harm to human health. Biochar electrodes, combined with adsorption and electroreduction, have great potential in nitrate removal from groundwater. In this study, bamboo chopsticks were used as feedstocks for biochar preparation. The bamboo chopstick biochar (BCBC), prepared by pyrolysis at 600 °C for 2 h, had a specific surface area of 179.2 m2/g and an electrical conductivity of 8869.2 μS/cm, which was an ideal biochar electrode material. The maximum nitrate adsorption capacity of BCBC-600-2 reached 16.39 mg/g. With an applied voltage of 4 V and hydraulic retention time of 4 h, the nitrate removal efficiency (NRE) reached 75.8%. In comparison, the NRE was only 32.9% without voltage and 25.7% with graphite cathode. Meanwhile, the average nitrate removal rate of biochar electrode was also higher than that of graphite cathode under the same conditions. Therefore, biochar electrode can provide full play to the coupling effect of adsorption and electroreduction processes and obtain more powerful nitrate removal ability. Moreover, the biochar electrode could inhibit the accumulation of nitrite and improve the selectivity of electrochemical reduction. This study not only provides a high-quality biochar electrode material, but also provides a new idea for nitrate removal in groundwater.

Volatilisations of ammonia from the soils amended with modified and nitrogen-enriched biochars

Biochar's capacity to abate NH₃ emissions from fertilised agricultural soils may be enhanced through both modifications and formulation of slow-release biochar-based N fertilisers but there is a dearth of data in this area. Sulphuric acid (H₂SO₄), hydrogen peroxide (H₂O₂) and potassium hydroxide (KOH) were used to modify biochars which are denoted as BSAD, BHPO and BKOH, respectively. Nitrogen (N) enrichment was performed using urea and ammonium nitrate and the enriched biochars are denoted as BUR and BAN, respectively. The biochars were characterised by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The ammonia abatement potentials of both the modified and N-enriched biochars were assessed in the incubation experiments which lasted for 30 days. Urea was used as a control while non-modified biochar (PrBC) was included for comparison. Compared to the control, PrBC, BKOH, BHPO, BSAD, BUR and BAN attenuated gaseous NH₃ emissions by 57.62%, 63.06%, 73.23% and 74.85%, 79.93% and 82.88%, respectively. Biochar modifications increased the content of oxygen containing surface groups especially carboxyl and sulphoxide in the case of BSAD as depicted from the instrumental analysis data, which most probably increased the sorption of NH₃ and its transformation to nitrates thus, resulting in a higher NH₃ abatement capacity than that of PrBC. XPS data indicated that N-enrichment resulted in reactions of N with the surface groups of biochar which slowed its release, concomitantly lowering NH₃ volatilisation better than even the modified biochars.

Synergistic role of inherent calcium and iron minerals in paper mill sludge biochar for phosphate adsorption

Phosphate adsorption using metal-based biochar has awakened much attention and triggered extensive research. In this study, novel Ca/Fe-rich biochars were prepared via a one-step process of pyrolyzing paper mill sludge (PMS) at various temperatures (300, 500, 700, and 800 °C) under a CO₂ atmosphere for phosphate removal. Batch adsorption experiments showed that the biochar obtained at 800 °C (PB-800), which could be easily separated magnetically, exhibited the best phosphate adsorption capacity in a wide range of solution pH (5-11). Based on the Langmuir model, the maximum phosphate adsorption capacity for PB-800 was 17.33 mg/g. Besides, the effects of ambient temperature as well as coexisting ions on phosphate removal were also investigated. Kinetic and thermodynamic analysis revealed that chemisorption dominated the adsorption process. The calcium carbonate and ferric salts in the sludge were converted into CaO and Fe₃O₄ through pyrolysis at 800 °C. The CaO inherent in PB-800 was proved to serve as active sites for the chemical precipitation, showing its synergistic effect with iron oxide compounds (i.e., Fe₃O₄, α-Fe₂O₃) on phosphate removal through chemical precipitation, ligand exchange, and complexation. This study not only provides a feasible waste-to-wealth strategy for converting PMS into a Ca/Fe-rich magnetic biochar that can be used as an effective phosphate adsorbent, but also offers new insights into the synergistic effect of calcium and iron species for the adsorption of phosphate using biochar.

A critical review of exogenous additives for improving the anammox process

Anammox achieves chemoautotrophic nitrogen removal under anaerobic and anoxic conditions and is a low-carbon wastewater biological nitrogen removal process with broad application potential. However, the physiological limitations of AnAOB often cause problems in engineering applications, such as a long start-up time, unstable operation, easily inhibited reactions, and difficulty in long-term strain preservation. Exogenous additives have been considered an alternative strategy to address these issues by retaining microbes, shortening the doubling time of AnAOB and improving functional enzyme activity. This paper reviews the role of carriers, biochar, intermediates, metal ions, reaction substrates, redox buffers, cryoprotectants and organics in optimizing anammox. The pathways and mechanisms of exogenous additives, which are explored to solve problems, are systematically summarized and analyzed in this article according to operational performance, functional enzyme activity, and microbial abundance to provide helpful information for the engineering application of anammox.

A re-analysis of NH4+ sorption on biochar: Have expectations been too high?

Sorption of nutrients such as NH₄⁺ is often quoted as a critical property of biochar, explaining its value as a soil amendment and a filter material. However, published values for NH₄⁺ sorption to biochar vary by more than 3 orders of magnitude, without consensus as to the source of this variability. This lack of understanding greatly limits our ability to use quantitative sorption measurements towards product design. Here, our objective was to conduct a quantitative analysis of the sources of variability, and infer which biochar traits are more favourable to high sorption capacity. To do so, we conducted a standardized remodelling exercise of published batch sorption studies using Langmuir sorption isotherm. We excluded studies presenting datasets that either could not be reconciled with the standard Langmuir sorption isotherm or generated clear outliers. Our analysis indicates that the magnitude of sorption capacity of unmodified biochar for NH₄⁺ is lower than previously reported, with a median of 4.2 mg NH₄⁺ g^-1 and a maximum reported sorption capacity of 22.8 mg NH₄⁺ g^-1. Activation resulted in a significant relative improvement in sorption capacity, but absolute improvements remain modest, with a maximum reported sorption of 27.56 mg NH₄⁺ g^-1 for an activated biochar. Methodology appeared to substantially impact sorption estimates, especially practices such as pH control of batch sorption solution and ash removal. Our results highlight some significant challenges in the quantification of NH₄⁺ sorption by biochar and our curated data set provides a potentially valuable scale against which future estimates can be assessed.

Metallic iron (Fe0)-based materials for aqueous phosphate removal: A critical review

The discharge of excessive phosphate from wastewater sources into the aquatic environment has been identified as a major environmental threat responsible for eutrophication. It has become essential to develop efficient but affordable techniques to remove excess phosphate from wastewater before discharging into freshwater bodies. The use of metallic iron (Fe⁰) as a reactive agent for aqueous phosphate removal has received a wide attention. Fe⁰ in-situ generates positively charged iron corrosion products (FeCPs) at pH > 4.5, with high binding affinity for anionic phosphate. This study critically reviews the literature that focuses on the utilization of Fe⁰-based materials for aqueous phosphate removal. The fundamental science of aqueous iron corrosion and historical background of the application of Fe⁰ for phosphate removal are elucidated. The main mechanisms for phosphate removal are identified and extensively discussed based on the chemistry of the Fe⁰/H₂O system. This critical evaluation confirms that the removal process is highly influenced by several operational factors including contact time, Fe⁰ type, influent geochemistry, initial phosphate concentration, mixing conditions, and pH value. The difficulty in comparing independent results owing to diverse experimental conditions is highlighted. Moreover, contemporary research in progress including Fe⁰/oxidant systems, nano-Fe⁰ application, Fe⁰ material selection, desorption studies, and proper design of Fe⁰-based systems for improved phosphate removal have been discussed. Finally, potential strategies to close the loop in Fe⁰-based phosphate remediation systems are discussed. This review presents a science-based guide to optimize the efficient design of Fe⁰-based systems for phosphate removal.