Initial biochar properties related to the removal of As, Se, Pb, Cd, Cu, Ni, and Zn from an acidic suspension

Occurrence, spatiotemporal trends, fate, and treatment technologies for microplastics and organic contaminants in biosolids: A review

This review provides a comprehensive overview of the occurrence, fate, treatment and multi-criteria analysis of microplastics (MPs) and organic contaminants (OCs) in biosolids. A meta-analysis was complementarily analysed through the literature to map out the occurrence and fate of MPs and 10 different groups of OCs. The data demonstrate that MPs (54.7% occurrence rate) and linear alkylbenzene sulfonate surfactants (44.2% occurrence rate) account for the highest prevalence of contaminants in biosolids. In turn, dioxin, polychlorinated biphenyls (PCBs) and phosphorus flame retardants (PFRs) have the lowest rates (<0.01%). The occurrence of several OCs (e.g., dioxin, per- and polyfluoroalkyl substances, polycyclic aromatic hydrocarbons, pharmaceutical and personal care products, ultraviolet filters, phosphate flame retardants) in Europe appear at higher rates than in Asia and the Americas. However, MP concentrations in biosolids from Australia are reported to be 10 times higher than in America and Europe, which required more measurement data for in-depth analysis. Amongst the OC groups, brominated flame retardants exhibited exceptional sorption to biosolids with partitioning coefficients (log Kd) higher than 4. To remove these contaminants from biosolids, a wide range of technologies have been developed. Our multicriteria analysis shows that anaerobic digestion is the most mature and practical. Thermal treatment is a viable option; however, it still requires additional improvements in infrastructure, legislation, and public acceptance.

Biomass based biochar production approaches and its applications in wastewater treatment, machine learning and microbial sensors

Biochar is a stable carbonaceous material derived from various biomass and can be utilized as adsorbents, catalysts and precursors in various environmental applications. This review discusses various feedstock materials and methods of biochar production via traditional as well as modern approaches. Additionally, the biochar characteristics, HTC process, and its modification by employing steam and gas purging, acidic, basic / alkaline and organo-solvent, electro- and magnetic fields have been discussed. The recent biochar applications for real water, wastewater and industrial wastewater for the abstraction of environmental contaminants also reviewed. Moreover, applications in machine learning and microbial sensors were discussed. In the meantime, analyses on commercial and environmental profit, current ecological concerns and the future directions of biochar application have been well presented.

Enhanced synergistic removal of Cu(II) and Cr(VI) with multifunctional biomass hydrogel from strong-acid media

Simultaneous recovery of heavy metal ions (HMIs) such as Cu(II) and Cr(VI) from strong-acid media was a great challenge due to the inhibition of protons. Herein, a novel biomass hydrogel (CMC/PEI-PD) containing various groups (bis-picolylamine, amino, and hydroxyl groups) was newly prepared by a facile two-step process. The static experiments relating pH, kinetics and isothermal co-adsorption confirmed the synergistic effect towards Cu(II) and Cr(VI) consistently. Specifically, the adsorption capacities of Cu(II) and Cr(VI) at pH 2.0 increased by 23.73% and 40.18% in comparison with the single systems. Moreover, coexistence of inorganic anions and cations could further increase the adsorption of Cu(II) and Cr(VI) by 59.90% and 43.39%, respectively. At the same time, the adsorption and desorption ratios for both HMIs remained stable. The superior performance came from the two dominant mechanisms of co-removal. On the one hand, Cu(II) chelated by bis-picolylamine group attracted Cr(VI) in the form of cation bridge, thus promoting Cr(VI) adsorption. On the other hand, the protonated amine group attracted Cr(VI) by electrostatic interaction and weakened the inter-cationic repulsion by electrostatic shielding, thus promoting Cu(II) adsorption. In addition, the dynamic column experiment towards simulated acidic electroplating wastewater involving Cu(II)-Cr(VI)-Ni(II) certified the high efficiency and feasibility of the co-removal. Therefore, CMC/PEI-PD owned great potential in the separation of typical HMIs even directly from strong-acid media.

Effect of Pyrolysis Temperature on the Sorption of Cd(II) and Se(IV) by Rice Husk Biochar

This study investigated the removal of metal cations (Cd(II)) and metalloid anions (Se(IV)) from their aqueous solution by using agricultural waste (rice husk biochar). Rice husk biochar samples were prepared under 300, 500, and 700 °C pyrolysis conditions and their physicochemical properties were characterized. Aqueous Cd(II) and Se(IV) sorption kinetics and isotherms of rice husk biochar were studied. The results showed that the yield of rice husk biochar decreased from 41.6% to 33.3%, the pH increased from 7.5 to 9.9, and the surface area increased from 64.8 m²/g to 330.0 m²/g as the pyrolysis temperature increased from 300 °C to 700 °C. Under the experimental conditions, at increasing preparation temperatures of rice husk biochar, the sorption performance of Cd(II) and Se(IV) was enhanced. The sorption capability and sorption rate were considerably higher and faster for Cd(II) ions than for Se(IV) ions. Cd(II) sorption was found to reach equilibrium faster, within 150 min, while Se(IV) sorption was slower and reached equilibrium within 750 min. The maximum sorption capacities of cadmium and selenium by rice husk biochar were 67.7 mg/g and 0.024 mg/g, respectively, according to Langmuir model fitting.

Effect of composting and amendment with biochar and woodchips on the fate and leachability of pharmaceuticals in biosolids destined for land application

Land application of biosolids can improve soil fertility and enhance crop production. However, the occurrence and persistence of pharmaceutical compounds in the biosolids may result in leaching of these contaminants to surface water and groundwater, causing environmental contamination. This study evaluated the effectiveness of two organic amendments [biochar (BC) and woodchips (WC)] for reducing the concentration and leachability (mobility) of four pharmaceuticals in biosolids derived from wastewater treatment plants in southern Ontario, Canada. The effect of 360-d composting on fate and leachabilities of target pharmaceuticals in biosolid mixtures was also investigated. Composting decreased total and leachable concentrations of pharmaceuticals in unamended and BC- and WC-amended biosolids to various degrees, from 10% up to 99% depending on the compound. Blending BC or WC into the biosolids greatly increased the removal rates of the target pharmaceuticals, while simultaneously decreasing their half-lives (t_0.5), compared to unamended biosolids. The t_0.5 of contaminants in this study followed the order: carbamazepine (304-3053 d) > gemfibrozil (42.3-92.4 d) > naproxen (15.3-104 d) > ibuprofen (12.5-19.0 d). Amendment with BC and(or) WC significantly reduced the leachability of carbamazepine, ibuprofen, and gemfibrozil to variable extents, but significantly enhanced the leachability of naproxen, compared to unamended biosolids (P < 0.05). Biochar and WC exhibited different (positive or negative) effects on the leachability of individual pharmaceuticals. Significantly lower concentrations of total and(or) leachable (mobile) pharmaceuticals were observed in amended biosolids than unamended biosolids (P < 0.05). Biochar and WC are effective amendments that can reduce the environmental impact of biosolid land applications with respect to pharmaceutical contamination.

Selenium removal from water using adsorbents: A critical review

Selenium (Se) has become an increasingly serious water contamination concern worldwide. It is an essential micronutrient for humans and animals, however, can be extremely toxic if taken in excess. Sorption can be an effective treatment for Se removal from a wide range of water matrices. However, despite the synthesis and application of numerous adsorbents for remediation of aqueous Se, there has been no comprehensive review of the sorption capacities of various natural and synthesized sorbents. Herein, literature from 2010 to 2021 considering Se remediation using 112 adsorbents has been critically reviewed and presented in several comprehensive tables including: clay minerals and waste materials (presented in Table 1); zero-valent iron, iron oxides, and binary iron-based adsorbents (Table 2); other metals-based adsorbents (Table 3); carbon-based adsorbents (Table 4); and other adsorbents (Table 5). Each of these tables, and their relevant sections, summarizes preparation/modification methods, sorption capacities of various Se adsorbents, and proposed model/mechanisms of adsorption. Furthermore, future perspectives have been provided to assist in filling noted research gaps for the development of efficient Se adsorbents for real-world applications. This review will help in preliminary screening of various sorbent media to set up Se treatment technologies for a variety of end-users worldwide.

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

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

Biosorbent derived from coffee husk for efficient removal of toxic heavy metals from wastewater

In this study, we investigated the feasibility on the utilization of coffee husk as biosorbents for the removal of heavy metal ions such as Pb²⁺ and Cd²⁺ from wastewater. Biochar was produced from coffee wastes and modified with sodium hydroxide to create functional groups on the surface and increase the specificic surface area. The resultant products were characterized by using various analytical methods such as scanning electron microscopy, specific area, surface functional group and zeta potential analysis. The as-prepared materials were employed as adsorbent to remove Pb²⁺ and Cd²⁺ ions from wastewater. The maximum sorption capacities of biosorbents were measured to be 116.3 and 139.5 mg g^-1 for Cd²⁺ and Pb²⁺, respectively, which is comparable to the conventional adsorbents. Kinetic studies showed that the sorption of Pb²⁺ and Cd²⁺ on biosorbents can be described by the Freundlich isotherm and second-order kinetic model. The coffee husk-derived biosorbent was capable of removing 89.6% of Pb²⁺ and 81.5% Cd²⁺ ions from wastewater, and therefore can be considered as low-cost and efficient adsorbent to remove heavy metal ions from wastewater.

Biochar for Wastewater Treatment—Conversion Technologies and Applications

Biochar as a stable carbon-rich material shows incredible potential to handle water/wastewater contaminants. Its application is gaining increasing interest due to the availability of feedstock, the simplicity of the preparation methods, and their enhanced physico-chemical properties. The efficacy of biochar to remove organic and inorganic pollutants depends on its surface area, pore size distribution, surface functional groups, and the size of the molecules to be removed, while the physical architecture and surface properties of biochar depend on the nature of feedstock and the preparation method/conditions. For instance, pyrolysis at high temperatures generally produces hydrophobic biochars with higher surface area and micropore volume, allowing it to be more suitable for organic contaminants sorption, whereas biochars produced at low temperatures own smaller pore size, lower surface area, and higher oxygen-containing functional groups and are more suitable to remove inorganic contaminants. In the field of water/wastewater treatment, biochar can have extensive application prospects. Biochar have been widely used as an additive/support media during anaerobic digestion and as filter media for the removal of suspended matter, heavy metals and pathogens. Biochar was also tested for its efficiency as a support-based catalyst for the degradation of dyes and recalcitrant contaminants. The current review discusses on the different methods for biochar production and provides an overview of current applications of biochar in wastewater treatment.

Enhanced synergistic removal of Cr(VI) and Cd(II) with bi-functional biomass-based composites

Cr(VI) and Cd(II) are typical heavy metal ions and their co-removal is significant. Nitrogen-doped, bi-functional, and biomass-based composites (CP-BA) were successfully applied for efficient and synergistic removal of Cr(VI) and Cd(II). The mutual promotion ratios of adsorption capacity by Cr(VI) and Cd(II) were 161.32 % and 14.13 %, respectively. In addition, all coexisting inorganic anions could extremely promote the removal of Cd(II) with the best promotion ratio upon phosphate ions as high as 80.00 %. The following co-removal mechanisms were deeply revealed: (1) Cr(VI) combined with protonated imine groups could weaken the electrostatic repulsion between CP-BA and Cd(II) by electrostatic shielding, and further promoted the coordination of Cd(II) with hydroxyl, carboxyl and neutral imine groups. (2) Cd(II) which were coordinated to neutral imine groups could form cation bridges, and thus promoted the interaction with Cr(VI) because of the electrostatic attraction. Moreover, the removal capacity for Cr(VI) and Cd(II) did not display obvious reduction even after four cycles. Therefore, CP-BA showed a great potential in the co-removal of inorganic anion-cation complexes from wastewater.

Wheat straw biochar reduces environmental cadmium bioavailability

Cadmium contamination in waters and soils can lead to food chain accumulation and ultimately deterioration in human health; means for reducing bioavailable Cd are desperately required, and biochars may play a role. Long-term (240 d) lab incubation experiments were utilized to explain wheat straw-derived biochar effects on Cd sorption and decreasing Cd bioavailability in soils and solutions (0, 5, and 15% biochar as wt:wt or wt:vol, respectively), and to identify Cd forms present using both the European Community Bureau of Reference (BCR) chemical sequential extraction procedure and synchrotron-based X-ray absorption spectroscopy (XAS). Biochar Cd removal was up to ~90% from Cd-containing solutions and contaminated soil as compared to the control. Based on the wet chemical sequential extraction procedure in conjunction with XAS, biochar application promoted the formation of (oxy)hydroxide, carbonate, and organically bound Cd phases. As a material, biochar may be promoted as a tool for reducing and removing bioavailable Cd from contaminated waters and soils. Thus, biochar may play a role in reducing Cd bioaccumulation, trophic transfer, and improving environmental quality and human health.

Differentiating Inorganics in Biochars Produced at Commercial Scale Using Principal Component Analysis

Characterizing the inorganic phase of biochar, beyond determining element concentration, is needed for appropriate application of these materials because mineral forms also influence element availability and behavior. Inorganics in 13 biochars (produced from Poultry litter, switchgrass, and different types of wood) were characterized by proximate analysis, chemical analysis, powder X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) spectroscopy. Principal component analysis (PCA) was used to compare biochars and characterize associations between elements. The biochars were produced using commercial-scale reactors and represent materials with properties relevant to field application. Bulk inorganic concentration and composition were responsible for differentiating biochars after PCA of chemical data. In comparison, differentiation based on PCA of diffractogram fingerprints was more nuanced. Here, contributions from cellulose and turbostratic crystalline C influenced separation between samples. It was also sensitive to mineral forms of Ca (whewellite and calcite). Differences in crystalline C and Ca minerals separated two biochars generated from the same willow feedstock using the same pyrolysis conditions at different temperatures. PCA of 606 SEM-EDX point scans revealed that inorganics belong to four main clusters containing Ca, Fe, [Al, Si], and [Cl, K, Mg, Na, P, S] consistent with XRD identification of calcite, magnetic Fe-oxide, silicates, and sylvite. It further suggested that amorphous P-containing minerals associated with Ca (not identified through XRD) were constituents of willow and poultry litter-derived biochars. However, unlike PCA of XRD, it was not able to differentiate the two biochars derived from willow. The three analysis methods provided different perspectives on the properties of the biochar inorganic phase. Combining information from multiple methods is needed to better understand the inorganic composition of biochars.

Sandwich-like Nanosystem for Simultaneous Removal of Cr(VI) and Cd(II) from Water and Soil

In this work, a novel nanosystem with a sandwich-like structure was synthesized via face-to-face combination of two pieces of waste cotton fabrics (CFs) carrying ferrous sulfide (FeS) and carboxyl-functionalized ferroferric oxide microsphere (CFFM), respectively, and the obtained nanosystem was named as FeS/CFFM/CF. Therein, FeS has high reduction and adsorption capabilities for hexavalent chromium (Cr(VI)), CFFM possesses a high adsorption ability on cadmium ion (Cd(II)) through electrostatic attraction and chelation, and CF displays high immobilization ability for FeS and CFFM and adsorption performance on Cd(II). FeS/CFFM/CF could simultaneously remove Cr(VI) and Cd(II) from water and inhibit the uptake of Cr and Cd by fish and water spinach, ensuring the food safety. Besides, this technology could efficiently control the migration of Cr(VI) and Cd(II) in the sand-soil mixture, which was favorable to prevent their wide diffusion. Importantly, FeS/CFFM/CF possessed a high flexibility and could be conveniently produced with needed scale and shape and easily separated from water and soil, displaying a promising approach to remediate Cr(VI)-/Cd(II)-contaminated water and soil and a huge application potential.

Fabrication of magnetic biochar as a treatment medium for As(V) via pyrolysis of FeCl3-pretreated spent coffee ground

This study investigated the preparation of magnetic biochar from N₂- and CO₂-assisted pyrolysis of spent coffee ground (SCG) for use as an adsorption medium for As(V), and the effects of FeCl₃ pretreatment of SCG on the material properties and adsorption capability of the produced biochar. Pyrolysis of FeCl₃-pretreated SCG in CO₂ atmosphere produced highly porous biochar with its surface area ∼70 times greater than that produced in N₂ condition. However, despite the small surface area, biochar produced in N₂ showed greater As(V) adsorption capability. X-ray diffraction and X-ray photoelectron spectrometer analyses identified Fe₃C and Fe₃O₄ as dominant mineral phases in N₂ and CO₂ conditions, with the former being much more adsorptive toward As(V). The overall results suggest functional biochar can be facilely fabricated by necessary pretreatment to expand the applicability of biochar for specific purposes.

Metal Immobilization on Wood-Derived Biochars: Distribution and Reactivity of Carbonate Phases

Metals can be immobilized on biochars by precipitation with carbonate. The distribution of metal-carbonate phases at the surface of biochars and the conditions of their formation, however, are unknown. Electron microscopy and X-photon spectroscopy were used to characterize carbonate phases in various morphological groups of particles of a wood-derived biochar, both before and after a metal-sorption experiment. Our results showed that the distribution of metals at the surface of biochar particles depended on the corresponding wood tissues and the presence of carbonate phases. Metals were particularly concentrated (i) within calcium carbonate crystals in bark-derived particles, which originated from calcium oxalate crystals formed prior to pyrolysis, and (ii) as new phases formed by the reprecipitation of carbonate on specific tissues of biochar. The formation of biochar carbonate phases and their redistribution by dissolution-precipitation mechanisms may primarily control the localization of metals on biochar particles and the durability of metals immobilization.