Engineered biochar with anisotropic layered double hydroxide nanosheets to simultaneously and efficiently capture Pb2+ and CrO42- from electroplating wastewater

Cr(VI) Adsorption by Mg/Al Layered Double Hydroxide-Modified Sphagnum Moss Cellulose Gel: Performance and Mechanism

Hexavalent chromium (Cr(VI)), a highly toxic and carcinogenic contaminant, presents a significant hazard to aquatic ecosystems and human health. Developing environmentally friendly, cost-effective, biodegradable, and easily recyclable adsorbents is critical for efficient Cr(VI) removal. Here, we present an innovative solution using a Mg/Al layered double hydroxide (LDH)-modified sphagnum cellulose gel (MgAl/LDH@SMCG), prepared by pre-treating sphagnum cellulose, crosslinking with polyvinyl alcohol, and doping with LDH. The resulting porous composite gel features abundant -COOH and -OH chelating groups, significantly enhancing its adsorption capacity and structural stability. The material's structure and surface modifications were systematically characterized using SEM, TGA, FT-IR, and XPS. Batch adsorption experiments were conducted to assess the influence of adsorbent dosage, initial Cr(VI) concentration, pH, contact time, and temperature on performance. Adsorption kinetics, isotherms, and thermodynamics analyses revealed a primary mechanism of monolayer chemical adsorption, with experimental data closely fitting the Freundlich isotherm and pseudo-second-order kinetic models. The modified gel exhibits increased surface roughness and adsorption sites, resulting in markedly improved Cr(VI) removal efficiency. This study not only provides theoretical insights into Cr(VI) adsorption but also highlights the potential of LDH-functionalized cellulose gels for heavy metal wastewater treatment, offering a sustainable pathway for addressing global water contamination challenges.

Co-adsorption behaviors and mechanisms of Cd(Ⅱ), Pb(Ⅱ), and Cr(Ⅵ) on sodium dodecyl sulfate modified attapulgite clay-supported nano zero-valent iron: Competitive or synergistic effect?

Cadmium(Ⅱ), plumbum(Ⅱ), and chromium(Ⅵ) universally co-exist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Herein, a novel sodium dodecyl sulfate modified attapulgite clay-supported nano zero-valent iron (SDS@ATP/nZVI) was exploited as the synchronous adsorbent of Cd(Ⅱ), Pb(Ⅱ), and Cr(Ⅵ) composite pollution. With the aid of the carrier SDS@ATP, nZVI was highly dispersed and became the primary active ingredient. The adsorption kinetics results showed that there existed a strong competitive effect between Cd(Ⅱ) and Pb(Ⅱ), while the co-existence of Cr(Ⅵ) accelerated the removal of Cd(Ⅱ) on SDS@ATP/nZVI. The adsorption capacity (qmax) of Cd(Ⅱ) in their binary system was only 71.3 mg/g and 0.34 times that of single Cd(Ⅱ) adsorption under pH = 6.0 and 298 K, whereas that of Pb(Ⅱ) (qmax = 558.7 mg/g) was almost equivalent, supporting that Pb(Ⅱ) adsorption had the priority and quickly reduced to Pb0 by Fe0. Meanwhile, residue Pb(Ⅱ) and Cd(Ⅱ) mightily competed for active sites of surface chemical adsorption of iron hydroxide. Similarly, the qmax of Cd(Ⅱ) and Cr(Ⅵ) in their binary system were 238.7 and 70.9 mg/g, which was 1.12 and 0.94 times to those of individual Cd(Ⅱ) and Cr(Ⅵ) adsorption, implying a synergistic effect of Cr(Ⅵ). Cr(Ⅵ) species firstly was reduced and simultaneously facilitated the generation of iron hydroxide, of which both free Cd(Ⅱ) and newly reduced Cr(Ⅲ) with iron hydroxide co-precipitated to constitute the amorphous Fe-Cr(Ⅲ)-Cd(Ⅱ) sediment. These findings also provide exhaustive inspiration and strategy for the remediation of multiple composite pollution.

Synergistic effect of W(VI) and Ni(II) uptakes on an MgAl-layered double hydroxide

The coadsorption of anionic and cationic pollutants on adsorbents holds considerable importance in the development of relevant removal technologies and the understanding of pollutant transport in complex environmental media. Herein, tungsten (W), an emerging contaminant, and nickel (Ni) were chosen as two differently charged inorganic pollutants to investigate their removal characteristics on a magnesium-aluminum layered double hydroxide (LDH) prepared via microwave radiation. In the single systems, the amount of adsorbed W on LDH was initially increased and then decreased with increasing initial W concentration. In concentrated W solutions, LDH dissolution was intensified, accompanied by the intercalation and polymerization of W in the interlayer space. Among the various oxyanions, phosphate showed the most substantial inhibition on W uptake. In contrast, uptake of Ni was enhanced with the rapider adsorption rate at higher concentrations, due to the isomorphic substitution and precipitation. Coexisting cations of similar sizes competed with Ni to substitute with Mg, resulting in reduced uptake, except in the case of Fe3+ which disintegrated the LDH structure. In a binary system, the uptakes of W and Ni increased by 2.65 and 1.80 times, respectively, compared to their corresponding single systems, indicating an intriguing synergistic effect. Furthermore, the presence of Ni restored the LDH's ability to remove W in the presence of coexisting H2PO4-, SO42-, and CrO42-. However, due to the presence of W, the coexisting Co2+ and Zn2+ inhibited Ni uptake more significantly. The crystallinity decrease of LDH was induced and identified as the cause of the uptake synergy between W and Ni. These findings provide valuable insights for the development of efficient multifunctional adsorbents and enhance our understanding of the transfer dynamics of W in the presence of coexisting substances.

Recent advances in applications of animal biowaste-based activated carbon as biosorbents of water pollutants: a mini-review

Advances in green engineering and technology have revealed a number of environmentally acceptable alternatives for water purification. In line with this, recent advances in biosorption of pollutants from aqueous solutions using animal biowaste-based activated carbon (AC) are reported herein. Apart from the fish scale-derived AC which is extensively documented, animal bones, among the rest others, have been studied most widely, followed by hair and feathers. Out of the various target water pollutants, removal of heavy metals has been mostly studied. Majority of the reports showed the Freundlich isotherm and pseudo second order as the best fit. Few investigations on the thermodynamics of the adsorption studies and reports on the Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) have also been discussed in this report. It has been concluded that while plant-based AC has gained wide interest, the same is not true for the animal-based counterpart albeit the latter's potential for high sorption efficiency as seen in the present report.

Chromium immobilization from wastewater via iron-modified hydrochar: Different iron fabricants and practicality assessment

The recycling of industrial solid by-products such as red mud (RM) has become an urgent priority, due to their large quantities and lack of reutilization methods can lead to resource wastage. In this work, RM was employed to fabricate green hydrochar (HC) to prepare zero-valent iron (ZVI) modified carbonous materials, and conventional iron salts (IS, FeCl3) was applied as comparison, fabricated HC labeled as RM/HC and IS/HC, respectively. The physicochemical properties of these HC were comprehensively characterized. Further, hexavalent chromium (Cr(VI)) removal performance was assessed (375.66 and 337.19 mg/g for RM/HC and IS/HC, respectively). The influence of dosage and initial pH were evaluated, while isotherms, kinetics, and thermodynamics analysis were also conducted, to mimic the surface interactions. The stability and recyclability of adsorbents also verified, while the practical feasibility was assessed by bok choy-planting experiment. This work revealed that RM can be used as a high value and green fabricant for HC the effective removal of chromium contaminants from the wastewater.

Tosyl-carrageenan/alginate composite adsorbent for removal of Pb2+ ions from aqueous solutions

The current study effectively designed novel cross-linked tosyl-carrageenan/alginate (Ts-Car/Alg) beads to remove Pb2+ ions from their aqueous solutions. To confirm the structure of the produced matrix, characterization methods such as XRD, SEM, FTIR, and EDX were used. Batch experiments were employed in order to further evaluate the adsorption efficiency of Pb2+ ions. Additionally, various variables, including contact time, solution pH, adsorbent dosage, and initial concentration of Pb2+ ions were investigated using atomic absorption. The results of this study showed that the adsorption equilibrium increased as Pb2+ ions concentration increased at pH = 5.3 after a contact time of 120 min, with 0.3 g of Ts-Car/Alg that having the best adsorption capacity at 74 mg/g. The adsorption progression was further examined using the kinetic and isothermal models. With a correlation coefficient of 0.975, the Freundlich model was thought to better fit Pb2+ ions adsorption from the isotherm investigation. Also, the adsorption kinetics were investigated using a pseudo-second-order model with 1/n ratio of 0.683. This Ts-Car/Alg adsorbent is regarded as an effective candidate to be used for water treatment because the reusability process of produced beads was successfully completed twice, and the adsorbent maintained its ability to remove Pb2+ ions. The prepared Ts-Car/Alg beads are therefore excellent candidates to be used as potent Pb2+ ions adsorbents from their aqueous solutions. The Ts-Car/Alg beads' regeneration and reusability investigation for the removal of heavy metal ions was completed in at least two successful cycles.

Facile activation of natural calcium-rich sepiolite with oxalic acid for selective Pb(II) removal: Highly-efficient performance, mechanisms and site energy distribution

The design and development of adsorbents with high efficiency, selectivity, and economy for Pb(II) are essential to environmental governance and ecological safety. Herein, an oxalic acid (OA) activated natural sepiolite (nSEP) composite for highly efficient Pb(II) removal was prepared by a facile impregnation strategy. The OA activated nSEP nanocomposite (OA-nSEP) was characterized by various instrumental techniques and its adsorption performance towards Pb(II) was further evaluated through a series of static and dynamic experiments under various environmental conditions. Results revealed that OA reacted with the calcium impurities in nSEP to form calcium oxalate, causing mesoporous structure and larger specific surface area of OA-nSEP. The obtained OA-nSEP possessed super high Pb(II) adsorption capacities (858.4-1252 mg/g), which were much higher than that of most modified clays or conventional materials. The average adsorption site energy and the standard deviation of the site energy distribution were analyzed to investigate the strength of Pb(II) binding onto OA-nSEP and the adsorption site heterogeneity. Mechanism studies confirmed that oxalate groups exerted a primary role in the adsorption process. X-ray diffraction and X-ray photoelectron spectrometry (XPS) unveiled that the coordination of oxalate with Pb(II) and precipitation of lead oxalate was responsible for the high efficiency and selectivity. Distinguishing feature of high adsorption capacity, specific selective adsorption, abundant availability, and splendid reusability make the OA-nSEP a promising candidate for eliminating Pb(II) in practical scenarios.

Enhanced removal performance of magnesium-modified biochar for cadmium in wastewaters: Role of active functional groups, processes, and mechanisms

In this study, a series of biochar products with different active functional groups were developed by one-pot coprecipitation method, including magnesium-modified biochar (MgBC) and functional group-grafted MgBC (Cys@MgBC, Try@MgBC, and Glu@MgBC), for effective adsorption of cadmium (Cd(II)) from wastewaters. These biochars exhibited excellent removal performance for Cd(II), particularly Cys@MgBC, whose maximum Cd(II) adsorption capacity reached 223.7 mg g-1. The highly active and weakly crystalline Mg could adsorb Cd(II) through precipitation and ion exchange, which was further promoted by the introduced functional groups through complexation and precipitation. After 120 d of natural process, the immobilization efficiency of Cd(II) by Cys@MgBC, Try@MgBC, and Glu@MgBC was still maintained at 98.7%, 95.2%, and 82.7% respectively. This study proposes and clarifies the complexation mechanism of functional group-grafted Mg-modified biochar for heavy metals, providing new insights into the practical application of these biochars.

Biochar/layered double hydroxides composites as catalysts for treatment of organic wastewater by advanced oxidation processes: A review

Heterogeneous advanced oxidation process has been widely studied as an effective method for removing organic pollutants in wastewater, but the development of efficient catalysts is still challenging. This review summaries the present status of researches on biochar/layered double hydroxides composites (BLDHCs) as catalysts for treatment of organic wastewater. The synthesis methods of layered double hydroxides, the characterizations of BLDHCs, the impacts of process factors influencing catalytic performance, and research advances in various advanced oxidation processes are discussed in this work. The integration of layered double hydroxides and biochar provides synthetic effects for improving pollutant removal. The enhanced pollutant degradation in heterogeneous Fenton, sulfate radical-based, sono-assisted, and photo-assisted processes using BLDHCs have been verified. Pollutant degradation in heterogeneous advanced oxidation processes using BLDHCs is influenced by process factors such as catalyst dosage, oxidant addition, solution pH, reaction time, temperature, and co-existing substances. BLDHCs are promising catalysts due to the unique features including easy preparation, distinct structure, adjustable metal ions, and high stability. Currently, catalytic degradation of organic pollutants using BLDHCs is still in its infancy. More researches should be conducted on the controllable synthesis of BLDHCs, the in-depth understanding of catalytic mechanism, the improvement of catalytic performance, and large-scale application of treating real wastewater.

Magnetic hydrochar derived from waste lignin for thallium removal from wastewater: Performance and mechanisms

Waste lignin, such as black liquor (BL) from paper and pulping industries, is an agro-industrial biowaste while its reuse raised global concerns. In this work, a hydrothermal carbonization procedure was employed to convert BL into magnetic lignin-based hydrochar (MLHC) for thallium elimination from wastewater. The results exhibited water purification potential due to a wider working pH window (2-9) with the magnetization intensity of 11.12 emu/g. The maximum adsorption capacity for Tl(III) was 278.9 mg/g, while the contribution of various mechanisms was elucidated with the order: surface precipitation (31.3 %), complexation (20.6 %), physical adsorption (18.2 %), chemical reduction (15.0 %), and ion exchange (14.9 %). This study revealed that hydrothermal treatment could be a potential and promising method to convert waste lignin into magnetic bio-adsorbent to recycle pulping black liquor and apply it for thallium pollution control.

Potential Mechanism of Long-Term Immobilization of Pb/Cd by Layered Double Hydroxide Doped Chicken-Manure Biochar

Layered double hydroxide (LDH)-doped chicken-manure biochar (CMB) with long-term stability was synthesized to immobilize Pb/Cd. MgAl-Cl-LDH-doped CMB (MHs) showed prominent long-term oxidation resistance and the least biodegradation sensitivity. Efficient Pb/Cd adsorption was observed on MHs, and the maximum adsorption capacities of Pb(II)/Cd(II) reached 1.95 mmol/g and 0.65 mmol/g, respectively. Precipitation and isomorphous substitution were identified as the key adsorption mechanisms, which formed highly stable Pb/Cd species (PbAl-CO₃-LDH, Pb₃(OH)₂CO₃, CdAl-Cl-LDH and CdCO₃). Pb(II) and Cd(II) precipitated with CO₃^2- in MHs; meanwhile, Mg(II) and Ca(II) in LDH layers were substituted by Pb(II) and Cd(II) respectively. Therefore, MHs had the potential for long-term stability of Pb/Cd. Moreover, complexation and electrostatic adsorption also contributed to the Pb/Cd immobilization to a certain extent. When 5% MHs (w/w) was applied to Pb/Cd contaminated smelting site soils, the soil pH increased from 5.9 to 7.3. After applying MHs for 25 d, the content of bioavailable Pb(II) and Cd(II) decreased by 98.8% and 85.2%, respectively, and the content of soluble Pb and Cd dropped by 99.5% and 96.7%. This study paves the way for designing a novel LDH doped CMB as efficient Pb/Cd immobilizers for smelting site soils.

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

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

Synthesis of polydopamine modified MgAl-LDH for high efficient Cr(VI) removal from wastewater

A highly efficient absorbent was developed in this study by modifying polydopamine film on Mg-Al layered double hydroxide (PDA/MgAl-LDH) to remove Cr(VI) from wastewater. The characterization results showed that the polydopamine film was successfully coated on the MgAl-LDH surface. The preparation ratio, pH, and adsorbent dosage influencing absorption by PDA/MgAl-LDH were systematically investigated. The absorption capacity of Cr(VI) by PDA/MgAl-LDH was 87 mg/g. The equilibrium adsorption isotherm of PDA/MgAl-LDH was in good agreement with that of the Langmuir model. Therefore, the pseudo-second-order kinetic model is suitable for describing adsorption kinetics. The interaction between PDA and Cr(VI) and Cr(III) was investigated using density generalized function theory (DFT), which demonstrated that the PDA amino group could provide electrons for Cr(VI) reduction. Hydrogen and covalent bonding were dominant during the chemisorption process of PDA absorbing Cr(VI), the nitrogen of 5,6-dihydroxyindole was the primary active site for absorbing Cr(III), and electrostatic attraction was mainly responsible for Cr(III) absorption. Therefore, PDA/MgAl-LDH has the potential to adsorb and remove Cr(VI) from wastewater.

Valorization of agriculture waste biomass as biochar: As first-rate biosorbent for remediation of contaminated soil

Each year, Asia produces an estimated 350 million tonnes of agricultural residues. According to Ministry of Power projections, numerous tonnes of such waste are discarded each year, in addition to being used as green manure. The methodology used to convert agricultural waste into the most valuable biochar, as well as its critical physical and chemical properties, were described in this review. This review also investigates the beneficial effects of bio and phytoremediation on metal(lloid)-contaminated soil. Agriculture biomass-based biochar is an intriguing organic residue material with the potential to be used as a responsible solution for metal(lloid) polluted soil remediation and soil improvement. Plants with faster growth and higher biomass can meet massive remediation demands. Recent research shows significant progress in agricultural biomass-based biomass conversion as biochar, as well as understanding the frameworks of metal(lloid) accumulation and mobility in plants used for metal(lloid) polluted soil remediation. Biochar made from various agricultural biomass can promote native plant growth and improve phytoremediation efficiency in polluted soil with metal(lloid)s. This carbon-enriched biochar promotes native microbial activity by neutralising pH and providing adequate nutrition. Thus, this review critically examines the feasibility of converting agricultural waste biomass into biochar, as well as the impact on plant and microbe remediation potential in metal(lloid)s polluted soil.

Alfalfa biochar supported Mg-Fe layered double hydroxide as filter media to remove trace metal(loid)s from stormwater

Polluted stormwater (PSW) treatment is becoming increasingly important because of the existence of multiple pollutants from non-point pollution sources. Alfalfa biochar loaded with Mg/Fe layered double hydroxide (AF-LDH) was successfully synthesized to remove trace metal(loid)s from stormwater. The adsorption kinetics and isotherms of metal(loid)s in a mono-component system and the reusability of the composite materials was investigated in this study. The result showed that the maximum removal efficiency for Pb(II), Cu(II), Zn(II), Cd(II), As(V), and Cr(VI) were 98.98 %, 98.11 %, 97.88 %, 97.71 %, 98.81 %, and 50.89 %, respectively, when added calcined AF-LDH (AF-LDO) composite material to the multi-component solution. The AF-LDH and AF-LDO could efficiently remove trace pollutants (10-100 μg/L) from multi-component solution, especially for AF-LDO, which could completely remove the tested six trace metal(loid)s. Furthermore, Fourier transform infrared spectra and X-ray diffraction characterizations supported the Mg/Fe layered double hydroxide reconstruction. The main mechanisms of Pb(II), Cu(II), Zn(II), and Cd(II) (cationic metals) removal were ion exchange and surface precipitation, whereas As(V) and Cr(VI) (anionic metals) were mainly dislodged through the formation of surface complexation, electrostatic attraction, and interlayer anion exchange, concerning the -OH and -COOH of AF-LDH. Importantly, the results of the column experiment demonstrated that AF-LDO was superior to AF-LDH for anionic metal removal from stormwater. In this study, we synthesized AF-LDH and AF-LDO for trace metal(loid) removal and proposed a new and practical approach for stormwater purification.

Resource utilization of rice husk biomass: Preparation of MgO flake-modified biochar for simultaneous removal of heavy metals from aqueous solution and polluted soil

In general, the remediation performance of heavy metals can be further improved by metal-oxide modified biochar. This work used MgO-modified rice husk biochar (MgO-5%@RHB-450 and MgO-5%@RHB-600) with high surface activity for simultaneous remediation and removal of heavy metals in soil and wastewater. The adsorption of MgO-5%@RHB-450/MgO-5%@RHB-600 for Cd(II), Cu(II), Zn(II) and Cr(VI) followed the pseudo-second order, with the adsorption capacities reaching 91.13/104.68, 166.68/173.22, 80.12/104.38 and 38.88/47.02 mg g^-1, respectively. The addition of 1.0% MgO-5%@RHB-450 and MgO-5%@RHB-600 could effectively decrease the CaCl₂-extractable Cd concentration (CaCl₂-Cd) by 66.2% and 70.0%, respectively. Moreover, MgO-5%@RHB-450 and MgO-5%@RHB-600 facilitated the transformation of exchangeable fractions to carbonate-bound and residual fractions, and reduced the exchangeable fractions by 8.1% and 9.6%, respectively. The mechanisms for the removal of heavy metals from wastewater by MgO-5%@RHB-450 and MgO-5%@RHB-600 mainly included complexation, ion exchange and precipitation, and the immobilization mechanisms in soil may be precipitation, complexation and pore filling. In general, this study provides high-efficiency functional materials for the remediation of heavy metal pollution.

Application of functionalized layered double hydroxides for heavy metal removal: A review

Layered double hydroxides (LDHs) are ionic laminar composites composed of positively charged brucite-like layers with an interlayered region containing charged compensating anions and solvation molecules. Such functional LDHs materials present a strong potential for heavy metal treatment especially for wastewater and soil, due to the large surface area and layered structure. This paper started with the background of techniques for heavy metals treatment and then discussed the potential environmental toxic effects, feasibility, stability of LDH composites. The preparation strategies of LDHs composites, and their application were summarized, followed by main mechanisms involving chelation, complexation, surface precipitation, ion exchange. This work also presented the potential environmental toxic effects, feasibility, stability of LDHs composites, reuse of waste liquid and the ratio adjustment of M²⁺ and N³⁺ for LDHs synthesis. While most efforts focused on improving the absorption capacity of LDHs by composites construction, ignoring the toxicity effects and detailed mechanism investigation. Based on a thorough review of the latest development, the challenges and perspectives would be proposed, offering promising insights on environmental purification via LDHs based materials.

Efficient adsorption of Pb(II) by sodium dodecyl benzene sulfonate intercalated calcium aluminum hydrotalcites: kinetic, isotherm, and mechanisms

Two novel adsorbents of CaAl-LDHs and sodium dodecyl benzene sulfonate (SDBS) intercalated CaAl-LDHs (SDBS-CaAl-LDHs) were successfully prepared by co-precipitation. The main composition and physical properties of two samples were characterized by XRD, XPS, FT-IR, TG, and SEM. Batch adsorption experiments were conducted to study the effect of pH, adsorption time, and initial concentration of Pb²⁺. The results showed that the prime adsorption conditions obtained were pH of 5.2 after 60 min with the initial concentration of 300 mg g^-1 for CaAl-LDHs and 350 mg g^-1 for SDBS-CaAl-LDHs. At 303 K, the adsorption capacities and removal rates of CaAl-LDHs and SDBS-CaAl-LDHs were found to be 456.05 mg g^-1, 91.21% and 682.26 mg g^-1, 97.47%, respectively. For CaAl-LDHs, the kinetic data for Pb²⁺ was best fitted with pseudo-2nd-order model, and the adsorption isotherms followed Langmuir and Freundlich isotherm model. The adsorption data of SDBS-CaAl-LDHs can be best described by the pseudo-second-order kinetic and Langmuir model. The Pb²⁺ adsorption mechanism on SDBS-CaAl-LDHs was explored by XRD, XPS, and SEM, and the important roles of the electrostatic attraction, precipitation, complexation, and ion exchange were demonstrated. The Langmuir adsorption capacities for SDBS-CaAl-LDHs were 797.63, 828.76, and 854.29 mg g^-1 at 293 k, 303 k, and 313 k, respectively. Thus, SDBS-CaAl-LDHs may be a highly economical adsorbent for the treatment of contaminated water.

Recovery, regeneration and sustainable management of spent adsorbents from wastewater treatment streams: A review

Adsorption is the most widely adopted, effective, and reliable treatment process for the removal of inorganic and organic contaminants from wastewater. One of the major issues with the adsorption-treatment process for the removal of contaminants from wastewater streams is the recovery and sustainable management of spent adsorbents. This review focuses on the effectiveness of emerging adsorbents and how the spent adsorbents could be recovered, regenerated, and further managed through reuse or safe disposal. The critical analysis of both conventional and emerging adsorbents on organic and inorganic contaminants in wastewater systems are evaluated. The various recovery and regeneration techniques of spent adsorbents including magnetic separation, filtration, thermal desorption and decomposition, chemical desorption, supercritical fluid desorption, advanced oxidation process and microbial assisted adsorbent regeneration are discussed in detail. The current challenges for the recovery and regeneration of adsorbents and the methodologies used for solving those problems are covered. The spent adsorbents are managed through regeneration for reuse (such as soil amendment, capacitor, catalyst/catalyst support) or safe disposal involving incineration and landfilling. Sustainable management of spent adsorbents, including processes involved in the recovery and regeneration of adsorbents for reuse, is examined in the context of resource recovery and circular economy. Finally, the review ends with the current drawbacks in the recovery and management of the spent adsorbents and the future directions for the economic and environmental feasibility of the system for industrial-scale application.

Simultaneous immobilization of arsenic, lead and cadmium by magnesium-aluminum modified biochar in mining soil

Owing to the human activities such as smelting and mining, arsenic (As), lead (Pb) and cadmium (Cd) seriously polluted the soil of non-ferrous metal mining areas, thus efficient methods for the simultaneous immobilization of the three heavy metals are urgently needed. In the present study, Mg-Al modified biochars (MABs) were synthesized through a simple one-pot pyrolysis method to immobilize the three heavy metals. According to the BET (Brunauer-Emmett-Teller) test method, MABs had larger specific surface areas than biochar. Compared to the materials obtained at 300 °C and 700 °C, MAB with a pyrolysis temperature of 500 °C (MAB 500) had a significant immobilization effect on As, Pb and Cd in the Gansu mining area. Compared with BC, the removal efficiencies of As, Pb and Cd increased from -62%, 17% and 5% to 52%, 100% and 66%, respectively. And the toxicity characteristic leaching procedure (TCLP) test showed that the leaching concentrations of the three heavy metals in the treated soil were all lower than the standard value. X-ray photoelectron spectroscopy and kinetic experiments showed that there were various mechanisms in the immobilization process of the three heavy metals, and the large specific surface area and the multi-Mg/Al-OH of MABs play an important role in this process. More charges were provided by larger specific surface for ion exchange with heavy metals. In addition, larger specific surface area also provided more adsorption sites. More complex sites were provided by Mg/Al-OH to form Mg/Al-O-M then immobilize the heavy metals. In summary, the immobilization mechanism may involve electrostatic attraction, precipitation/co-precipitation, and surface complexation.

Layered double hydroxides functionalized by carbonaceous materials: from preparation to energy and environmental applications

Along with the exponential demand for energy and pollution-free-environment, layered double hydroxides (LDHs) have gained extensive explorations because of their diverse nanostructures and tunable elemental compositions. However, the applications of LDHs are hindered by their poor activity, sluggish mass transfer, and aggregation. LDHs functionalized by carbonaceous materials (CMs) (LDH-CM) are expected to overcome the above disadvantages and even generate more excellent performance. This review first analyzes the research evolvement of LDH-CM composites during the past 25 years. Next, the advantages of LDH-CM composites are highlighted, such as morphology optimization, high electrical conductivity, more stable, good heat, and mass transfer performance. Following the synthetic strategies, including chemical assembly of LDHs and CMs, direct growth of LDH on CMs (two-step nucleation and growth and surface-confined growth) and direct CM formation on LDHs are fully discussed. Then, the recent progress achieved in LDH-CM composites for the application of energy storage and environmental protection is summarized in detail. In particular, the review illustrates the reasons why these constructing strategies can improve the performance of LDH-CM composites. Finally, challenges and future research prospects of LDH-CM composites are highlighted.

Adsorption behaviors and mechanisms of Fe/Mg layered double hydroxide loaded on bentonite on Cd (II) and Pb (II) removal

In this study, FeMg-LDH loaded with bentonite (FeMg-LDH@bentonite) was prepared using the facile co-precipitating method in situ to remove heavy metals from water and then characterized using XRD, SEM, TEM, FTIR, BET, TGA, and XPS. Pb (II) and Cd (II) were selected as the representative heavy metals to evaluate the adsorption capability of the FeMg-LDH@bentonite. The batch adsorption method was adopted to test the effects of the contact time, pH, initial concentration, different cations, and temperatures. The kinetic study indicated that the adsorption of heavy metals onto FeMg-LDH@bentonite was well fitted by the pseudo-second-order method. Isotherms were effectively simulated based on the Langmuir model. The maximal adsorption capability of Cd (II) and Pb (II) can reach 510.2 mg/g and 1397.62 mg/g, exceeding those of conventional adsorbents. The adsorption mechanisms of FeMg-LDH@bentonite demonstrating that there may exist surface complexation, ion exchange, and chemical deposition between FeMg-LDH@bentonite and heavy metals. Moreover, FeMg-LDH@bentonite was found to have a promising application for practically treating wastewater with heavy metals and can be used for various environmental water pollution treatments. The material may be used for heavy metal contaminated soil in the future.

Removal of heavy metals from wastewaters with biochar pyrolyzed from MgAl-layered double hydroxide-coated rice husk: Mechanism and application

This study reports a MgAl-LDH rice husk biochar composite (MgAl-LDH@RHB) with a regular hydrotalcite structure synthesized by a simple hydrothermal method, which was then used to remove Cd(II) and Cu(II) from water. The influencing factors on the adsorption performance were determined through batch adsorption experiments, and the adsorption characteristics and cycling capacity were evaluated with eight models and adsorption-desorption experiments. The results showed that the adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB conformed to the Langmuir-Freundlich model and PSO kinetics model, indicating single-layer chemical adsorption. In addition, the experimental maximum adsorption capacities for Cd(II) and Cu(II) were 125.34 and 104.34 mg g^-1, respectively. The adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB was dominated by surface precipitation and ion exchange. The findings reveal the mechanism for the heavy metal removal by MgAl-LDH@RHB and provide a theoretical reference for agricultural waste disposal and water pollution control.

Pb(II)-mediated precipitate transformation promotes Cr(VI) immobilization by biogenic hydroxyapatite

In this work, the mechanism of Pb(II)-mediated precipitation transformation to improve the removal of Cr(VI)-oxyanion on biogenic hydroxyapatite (BHAp) were investigated. The Pb(II)-preloading formed pyromorphite [Pb₅(PO₄)₃Cl] precipitate on the BHAp surface (Pb@BHAp), thus causing an increase of 2.2 times in the uptake of Cr(VI) by Pb@BHAp at pH of 2.4. It was primarily due to the dissolution of Pb₅(PO₄)₃Cl accompanied with the release of Pb(II), resulting in the rapid formation of crocoite (PbCrO₄). Although the K_sp of Pb₅(PO₄)₃Cl was approximately 23 orders of magnitude lower than that of PbCrO₄, Pb(II)-mediated precipitation transformation could still occur. XRD and SEM-EDX analyses demonstrated that the process was a time-dependent that included rapid crystal precipitation in the initial 10 min and subsequent precipitate accumulation for several hours. The Pb(II) released from the dissolution of Pb₅(PO₄)₃Cl was immediately immobilized by Cr(VI); therefore, it did not cause any retention risk of Pb(II) in the solution. Furthermore, a small quantity of Cr(VI) could be reduced to Cr(III) by BHAp, and Cr(III) could enter into the BHAp lattice for the exchange of Ca(II). This study provides a new insight into the resource utilization of Pb-bearing BHAp and a potential method for the successive removal of Pb(II) and Cr(VI).

Nanomaterials for the Treatment of Heavy Metal Contaminated Water

Nanotechnology finds its application almost in every field of science and technology. At the same time, it also helps to find the solution to various environment-related problems, especially water contamination. Nanomaterials have many advantages over conventional materials, such as high surface area, both polar and non-polar chemistries, controlled and size-tunable, easier biodegradation, which made them ideal candidates for water and environmental remediation as well. Herein, applications of non-carbon nanomaterials, such as layered double hydroxides, iron oxide magnetite nanoparticles, nano-polymer composites, metal oxide nanomaterials and nanomembranes/fibers in heavy metal contaminated water and environmental remediation are reviewed. These non-carbon nanomaterials, due to their tunable unique chemistry and small size have greater potentials for water and environmental remediation applications.

Artificial intelligence (AI) applications in adsorption of heavy metals using modified biochar

The process of removal of heavy metals is important due to their toxic effects on living organisms and undesirable anthropogenic effects. Conventional methods possess many irreconcilable disadvantages pertaining to cost and efficiency. As a result, the usage of biochar, which is produced as a by-product of biomass pyrolysis, has gained sizable traction in recent times for the removal of heavy metals. This review elucidates some widely recognized harmful heavy metals and their removal using biochar. It also highlights and compares the variety of feedstock available for preparation of biochar, pyrolysis variables involved and efficiency of biochar. Various adsorption kinetics and isotherms are also discussed along with the process of desorption to recycle biochar for reuse as adsorbent. Furthermore, this review elucidates the advancements in remediation of heavy metals using biochar by emphasizing the importance and advantages in the usage of machine learning (ML) and artificial intelligence (AI) for the optimization of adsorption variables and biochar feedstock properties. The usage of AI and ML is cost and time-effective and allows an interdisciplinary approach to remove heavy metals by biochar.

Recent Advances in Layered Double Hydroxide-Based Electrochemical and Optical Sensors

Layered double hydroxides (LDHs) have attracted considerable attention as promising materials for electrochemical and optical sensors owing to their excellent catalytic properties, facile synthesis strategies, highly tunable morphology, and versatile hosting ability. LDH-based electrochemical sensors are affordable alternatives to traditional precious-metal-based sensors, as LDHs can be synthesized from abundant inorganic precursors. LDH-modified probes can directly catalyze or host catalytic compounds that facilitate analyte redox reactions, detected as changes in the probe's current, voltage, or resistance. The porous and lamellar structure of LDHs allows rapid analyte diffusion and abundant active sites for enhanced sensor sensitivity. LDHs can be composed of conductive materials such as reduced graphene oxide (rGO) or metal nanoparticles for improved catalytic activity and analyte selectivity. As optical sensors, LDHs provide a spacious, stable structure for synergistic guest-host interactions. LDHs can immobilize fluorophores, chemiluminescence reactants, and other spectroscopically active materials to reduce the aggregation and dissolution of the embedded sensor molecules, yielding enhanced optical responses and increased probe reusability. This review discusses standard LDH synthesis methods and overviews the different electrochemical and optical analysis techniques. Furthermore, the designs and modifications of exemplary LDHs and LDH composite materials are analyzed, focusing on the analytical performance of LDH-based sensors for key biomarkers and pollutants, including glucose, dopamine (DA), H2O2, metal ions, nitrogen-based toxins, and other organic compounds.

Application of layered double hydroxide-biochar composites in wastewater treatment: Recent trends, modification strategies, and outlook

In recent years, layered double hydroxide-biochar (LDH-BC) composites as adsorbents and catalysts for contaminants removal (inorganic anions, heavy metals, and organics) have received increasing attention and became a new research point. It is because of the good chemical stability, abundant surface functional groups, excellent anion exchange ability, and good electronic properties of LDH-BC composites. Hence, we offer an overall review on the developments and processes in the synthesis of LDH-BC composites as adsorbents and catalysts. Special attention is devoted to the strategies for enhancing the properties of LDH-BC composites, including (1) magnetic treatment, (2) acid treatment, (3) alkali treatment, (4) controlling metal ion ratios, (5) LDHs intercalation, and (6) calcination. In addition, further studies are called for LDH-BC composites and potential areas for future application of LDH-BC composites are also proposed.

Application of biochars in the remediation of chromium contamination: Fabrication, mechanisms, and interfering species

Chromium (Cr) is one of the most toxic pollutants that has accumulated in terrestrial and aqueous systems, posing serious risks towards living beings on a worldwide scale. The immobilization, removal, and detoxification of active Cr from natural environment can be accomplished using multiple advanced materials. Biochar, a carbonaceous pyrolytic product made from biomass waste, is considered as a promising material for the elimination of Cr contamination. The preparation and properties of biochar as well as its remediation process for Cr ions have been well investigated. However, the distinct correlation of the manufacturing, characteristics, and mechanisms involved in the remediation of Cr contamination by various designed biochars is not summarized. Herein, this review provides information about the production, modification, and characteristics of biochars along with their corresponding effects on Cr stabilization. Biochar could be modified via physical, hybrid, chemical, and biological methods. The remediating mechanisms of Cr contamination using biochars involve adsorption, reduction, electron shuttle, and photocatalysis. Moreover, the coexisting ions and organic pollutants change the pattern of the remediating process of biochar in actual Cr contaminated water and soil. Finally, the present limitations and future perspectives are proposed.

Novel biochar and hydrochar for the adsorption of 2-nitrophenol from aqueous solutions: An approach using the PVSDM model

Two new adsorbents, namely avocado-based hydrochar and LDH/bone-based biochar, were developed, characterized, and applied for adsorbing 2-nitrophenol. The pore volume and surface diffusion model (PVSDM) was numerically solved for different geometries and applied to interpret the adsorption decay curves. Both adsorbents presented interesting textural and physicochemical characteristics, which achieved maximum adsorption capacities of 761 mg/g for biochar and 562 mg/g for hydrochar. The adsorption equilibrium data were well fitted by Henry isotherm. Besides, thermodynamic investigation revealed endothermic adsorption with the occurrence of electrostatic interactions. PVSDM predicted the adsorption decay curves for different adsorbent geometries at different initial concentrations of 2-nitrophenol. The surface diffusion was the main intraparticle mass transport mechanism. Furthermore, the external mass transfer and surface diffusion coefficients increased with the increase of 2-nitrophenol concentration.

Sustainable wastewater treatment by biochar/layered double hydroxide composites: Progress, challenges, and outlook

Biochar/layered double hydroxide (LDH) composites have gained considerable attention in recent times as low-cost sustainable materials for applications in water treatment. This paper critically evaluates the latest development in applications of biochar/LDH composites in water treatment with an emphasis on adsorption and catalytic degradation of various pollutants. The adsorption of various noxious contaminants, i.e., heavy metals, dyes, anions, and pharmaceuticals onto biochar/LDH composites are described in detail by elaborating the adsorption mechanism and regeneration ability. The synergistic effect of LDH with biochar exhibited significant improvement in specific surface area, surface functional groups, structure heterogeneity, stability, and adsorption characteristics of the resulting biochar/LDH composites. The major hurdles and challenges associated with the synthesis and applications of biochar/LDH composites in water remediation are emphasized. Finally, a roadmap is suggested for future research to assure the effective applications of biochar/LDH composites in water purification.

Implications of layered double hydroxides assembled biochar composite in adsorptive removal of contaminants: Current status and future perspectives

In recent years, biochar composites have received considerable attention for environmental applications. This paper reviews the current state of research on Layered Double Hydroxides (LDHs) tailored biochar composites in terms of their synthesis methods, characteristics, and their use as adsorbents for the removal of various pollutants from water, highlighting and discussing the key advancement in this area. The adsorption potential of LDHs-biochar composites for different inorganic and organic contaminants, important factors affecting composites' properties and the adsorption process, and the mechanisms involved in adsorption are discussed in this review. Though the adsorption capacities are high for the composites studied, partition coefficient which suggest the performance of composites remain low for most adsorbents. Despite the recent progress in the synthesis of LDHs-biochar composites, further research is needed to improve the performance of composites for different classes of aquatic pollutants, and to test their applicability in pilot-scale with real wastewater under real environmental conditions.

Black liquor as biomass feedstock to prepare zero-valent iron embedded biochar with red mud for Cr(VI) removal: Mechanisms insights and engineering practicality

Black liquor (BL) is an agro-industrial residue with high number of lignocellulosic components which could be recognized as a biomass feedstock. In this work, BL coupled with red mud (RM), were applied to prepare cost-effective zero-valent iron (ZVI) embedded in biochar. The oligomers in BL acted as reductants for RM to generate ZVI, while the organic components could be converted into biochar during pyrolysis. The RM/BL demonstrated excellent performance in the removal of Cr(VI) (349.5 mg/g), as the mechanisms were reduction and adsorption. The fixed-bed column study was conducted and 1.7 L simulated wastewater could be treated by 1.0 g RM/BL. After reaction, 95.5% ± 0.8% and 82.5%±3.2% Cr-loaded adsorbents could be recovered by an external magnet for batch and fixed-bed experiments, respectively. All these results shed light on valorizing these two widespread agro-industrial byproducts, and bridged the knowledge gap between magnetic bio-adsorbent preparation and its industrial practicality on wastewater purification.