Efficient removal of Cd2+ and Pb2+ from aqueous solution with amino- and thiol-functionalized activated carbon: Isotherm and kinetics modeling

Hydroxyapatite modified sludge-based biochar for the adsorption of Cu2+ and Cd2+: Adsorption behavior and mechanisms

This study prepared sewage sludge, a municipal solid waste, into a biochar modified by hydroxyapatite (HAP) as a new and efficient absorbent (HAP-SSBC) for removal of Cu²⁺ and Cd²⁺ from aqueous solution. Adsorption experiment revealed that HAP-SSBC exhibited significantly higher adsorption performance than raw sludge-based biochar (SSBC). At 298.15 K, the maximum adsorption capacity of Cu²⁺ and Cd²⁺ via Langmuir model were 89.98 and 114.68 mg/g, respectively. Adsorption kinetic experiment revealed that chemisorption was the main reaction. Analysis of X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectrum (XPS) further confirmed that the main mechanisms were ion exchange with Ca²⁺, complexion by -OH and -COOH, and forming Cu-π or Cd-π binding with aromatic CC on HAP-SSBC surface. Overall, combing HAP and SSBC to be a new adsorbent is beneficial to the resource utilization of sludge and shows a good prospect for heavy metal removal in aqueous solution.

Effect of high hydrostatic pressure-assisted pectinase modification on the Pb2+ adsorption capacity of pectin isolated from sweet potato residue

Novel pectin derived from sweet potato residue was modified by high hydrostatic pressure (HHP)-assisted pectinase and then used for Pb²⁺ removal from aqueous solutions. The removal characteristics and mechanisms were also investigated. Results showed that modified sweet potato pectin exhibited greater adsorption performances for Pb²⁺ than that of natural ones, and showed excellent eco-friendly properties and good potential for adsorption of some other heavy metals (such as Cu²⁺). The adsorption curves were much more conformed to Langmuir model, and the highest capacity for Pb²⁺ adsorption was 263.15 mg/g with 1.00% pectin at pH 7. Chemical adsorption process of pectin for Pb²⁺ absorption involved O-containing functional groups (O-H, COO-), cation exchange, and along with electrostatic interactions. Overall, the results in this study indicated that sweet potato pectin modified with HHP-assisted pectinase had the potential to become an environmentally friendly coagulant-flocculant agent for the heavy metal adsorption, especially for Pb²⁺.

Theoretical and experimental studies on uranium(vi) adsorption using phosphine oxide-coated magnetic nanoadsorbent

In this study, novel Cyanex-923-coated magnetite nanoparticles (Fe₃O₄@Cyanex-923) were prepared, comprehensively characterized, and employed for uranium(vi) ion adsorption from aqueous solutions. FTIR and TGA data confirmed that Fe₃O₄ has successfully gained Cyanex-923 surface functionality. Particle size and morphological studies via DLS, HR-TEM, and SEM showed uniform-dispersed quasi-spherical nanoparticles with a mean diameter of ca. 44 nm. Magnetism measurement data revealed the superparamagnetic properties of the Fe₃O₄@Cyanex-923 nanoadsorbent. The effect of different experimental settings on the adsorption efficiency was studied to determine the best operational conditions. The experimental results were analyzed using Langmuir, Freundlich, and Temkin isotherms; where the adsorption data obeyed the Langmuir model showing a theoretical adsorption capacity of 429.185 mg g⁻¹ at 298 K. Kinetics data analysis revealed a fast adsorption process that could reach equilibrium within 60 min and is well-fitted to the pseudo-2nd-order model. Temperature affected the adsorption process and the thermodynamic data indicated that uranium(vi) adsorption was spontaneous and exothermic. Fe₃O₄@Cyanex-923 nanoparticles displayed a good regeneration behavior over three sequential adsorption–desorption cycles. The Fe₃O₄@Cyanex-923 nanoadsorbent showed a high uranium adsorption capacity, fast equilibration time, economic nature, good reusability, and easy separation; making it a promising candidate for uranium(vi) removal from nuclear waste streams.

A Fe₃O₄@Cyanex-923 nanoadsorbent was prepared and applied as an efficient candidate for uranium(vi) removal from aqueous solutions.

Adsorption Properties for La(III), Ce(III), and Y(III) with Poly(6-acryloylamino-hexyl hydroxamic acid) Resin

Using polyacrylic resin followed by the substitution reaction with 6-aminohexyl hydroxamic acid, poly(6-acryloylamino-hexyl hydroxamic acid) resin (PAMHA) was successfully synthesized. PAMHA, a spherical resin with the particle size of 0.4 mm, is a novel polyamide hydroxamic acid chelating resin containing acylamino and hydroxamic acid functional groups. A series of influences (pH, contact time, temperature, and the initial concentrations of rare earth ions) were investigated to determine the adsorption properties. The adsorption capacity for La(III), Ce(III), and Y(III) ions were 1.030, 0.962, and 1.450 mmol·g-1, respectively. Thermodynamic and kinetic studies were also carried out to show that the uptake of rare earth ions onto PAMHA fitted well the pseudo-second-order model and Langmuir isotherm, and the adsorption process was spontaneous endothermic. In addition, desorption of rare earth ions was achieved by using 2 mol·L-1 HNO3 and desorption efficiencies for La(III), Ce(III), and Y(III) ions were 98.4, 99.1, and 98.8%, respectively. Properties of PAMHA resin were characterized by scanning electron microscope (SEM), Fourier transform infrared spectrometry (FTIR), and X-ray photoelectron spectrometer (XPS). The results showed that there was coordination between the rare earth ions with PAMHA and rare metal ions were chemically adsorbed on the surface of the PAMHA.

Removal of lead from two polluted soils by magnetic wheat straw biochars

Due to high sorption capacity for heavy metals, magnetic biochar (MBC) has the potential to adsorb heavy metals in soils, which are then removed together with MBC from soils by a magnetic field. In this study, two magnetic biochars (MBC300 and MBC700) were derived from the magnetization of wheat straw biochars pyrolyzed at 300 and 700 °C. Strong binding of Pb with iron oxide particles deposited on biochar was observed. After the MBCs (7.5%, w/w) were applied to two naturally Pb-polluted soils (named as He-soil and Hu-soil) for 720 h, the removal efficiency of Pb from the soil by MBC300 (26.8-40.1%) was similar (p > 0.05) to that by MBC700 (25.1-42.1%). This is because MBC300 has lower sorption capacity for Pb but higher recovery percentage from soils as a result of lower saturation magnetization. The removal efficiencies of Pb by the two MBCs were 13-17% higher for He-soil than for Hu-soil, which was due to higher proportion of mobile forms of Pb in He-soil (82.3%) than in Hu-soil (51.5%). Spectroscopic analysis indicated that Pb in soils tended to bind onto the surface of MBC in more stable forms. Moreover, removing Pb from soils by MBC could decrease Pb concentration in ryegrass by about 30%. Therefore, it might be a potential method to remedy Pb-polluted soils.

One-dimensional graphene for efficient aqueous heavy metal adsorption: Rapid removal of arsenic and mercury ions by graphene oxide nanoribbons (GONRs)

There is a knowledge gap for the application of one-dimensional graphene in the adsorption process. Our hypothesis was based on the fact that graphene oxide nanoribbons (GONRs) as one-dimensional graphene with more desired edges and specific surface area than other carbonaceous nanomaterials have more oxygen containing functional groups (active sites) on their edges and basal planes and therefore are more capable in adsorption of pollutants. In this regard, we synthesized GONRs by unzipping of multi-walled carbon nanotubes (MWCNTs) and investigated the adsorption behavior of GONRs by ultrasonic-assisted adsorptive removal of As(V) and Hg(II) ions from aqueous solution. The obtained results showed that As(V) ions are more favorably adsorbed onto the GONRs than Hg(II) ions and with increasing initial As(V) and Hg(II) ions concentration to 300 ppm, the equilibrium adsorption uptake of the synthesized GONRs increases to 155.61 and 33.02 mg/g for As(V) and Hg(II) ions, respectively through a rapid separation process in just 12 min. Also, three kinetic models and Freundlich and Langmuir adsorption isotherms were applied to evaluate the obtained experimental results. Our findings highlight the potential application of GONRs as one-dimensional graphene adsorbent with more desired edges than MWCNTs and graphene oxide (GO) and high adsorption capacity for selective removal of heavy metals.

Magnetic supramolecular polymer: Ultrahigh and highly selective Pb(II) capture from aqueous solution and battery wastewater

For the practical capture of heavy metal ions from wastewater, fabricating environmental friendly adsorbents with high stability and super adsorption capacity are pursuing issue. In this work, we develop magnetic supramolecular polymer composites (M-SMP) by using a simple two-step hydrothermal method. Systematical characterizations of morphological, chemical and magnetic properties were conducted to confirm the formation of M-SMP composites. The resulting M-SMP composites were applied to remove Pb(II) from aqueous solution and from real battery wastewater, and easy separation was achieved using a permanent magnet. By investigating the effects of various parameters, we optimized their operating condition for Pb(II) adsorption by the M-SMP. The uptake of Pb(II) onto M-SMP fitted well the pseudo-second-order and Langmuir isotherm models, and favourable thermodynamics showed a spontaneous endothermic process. The SMP endowed M-SMP with ultrahigh adsorption capacity for Pb(II) (946.9 mg g^-1 at pH = 4.0, T = 298 K), remarkable selectivity, satisfactory stability and desirable recyclability. In Pb-contaminated lead-acid battery industrial wastewater, the concentration of Pb(II) declined from 18.070 mg L^-1 to 0.091 mg L^-1, which meets the current emission standard for the battery industry. These merits, combined with simple synthesis and convenient separation, make M-SMP an outstanding scavenger for the elimination of industrial Pb(II) wastewater.

Exploring a New Method to Study the Effects of Surface Functional Groups on Adsorption of CO2 and CH4 on Activated Carbons

The commercial coconut shell-activated carbon was modified to change the number of oxygen-containing functional groups. N₂ adsorption/desorption isotherms, Fourier transform infrared (FT-IR), and Boehm titration were adopted to describe the physical and chemical properties of the samples. The adsorption isotherms of CO₂ and CH₄ on both the unmodified and modified samples were measured. To better understand the effects of surface oxygen-containing functional groups on adsorption of CO₂ and CH₄, the overall adsorption could be considered as the result of adsorption within the pores and adsorption onto the oxygen-containing functional groups. Thus, a new way to understand different adsorption mechanisms by calculation was proposed. On the basis of the results, there is a significant correlation between the saturation adsorption capacity of CO₂ and the number of oxygen-containing functional groups, especially carboxyl and hydroxyl. According to the values of enthalpy (-12.2 to -20 kJ/mol), it can be known that the adsorption caused by oxygen-containing functional groups is exothermic and belongs to physisorption. A semiempirical relationship between the variation of the surface oxygen-functional groups and the variation of the adsorbed amount was established. The method proposed in this paper provides a new way to study the effects of surface functional groups on the adsorption of CO₂ and CH₄ and can be even promoted in studying the adsorption mechanism of other adsorbates.

Removal of Aquatic Cadmium Ions Using Thiourea Modified Poplar Biochar

Removal of aquatic cadmium ions using biochar is a low-cost method, but the results are usually not satisfactory. Modified biochar, which can be a low-cost and efficient material, is urgently required for Cd-polluted water and soil remediation. Herein, poplar bark (SB) and poplar sawdust (MB) were used as raw materials to prepare modified biochar, which is rich in N- and S- containing groups, i.e., TSBC-600 and TMBC-600, using a co-pyrolysis method with thiourea. The adsorption characteristics of Cd2+ in simulated wastewater were explored. The results indicated that the modification optimized the surface structure of biochar, Cd2+ adsorption process by both TSBC-600 and TMBC-600 was mainly influenced by the initial pH, biochar dosage, and contact time, sthe TSBC-600 showed a higher adsorption capacity compared to TMBC-600 under different conditions. The Langmuir adsorption isotherm model and pseudo-second-order kinetic model were more consistent with the adsorption behavior of TSBC-600 and TMBC-600 to Cd2+, the maximum adsorption capacity of TSBC-600 and TMBC-600 calculated by the Langmuir adsorption isotherm model was 19.998 mg/g and 9.631 mg/g, respectively. The modification method for introducing N and S into biochar by the co-pyrolysis of biomass and thiourea enhanced the removal rate of aquatic cadmium ions by biochar.

Facile synthesis of graphene-based hyper-cross-linked porous carbon composite with superior adsorption capability for chlorophenols

In this work, we proposed a green and cost-effective method to prepare a graphene-based hyper-cross-linked porous carbon composite (GN/HCPC) by one-pot carbonization of hyper-cross-linked polymer (HCP) and glucose. The composite combined the advantages of graphene (GN) and hyper-cross-linked porous carbon (HCPC), leading to high specific surface area (396.93 m²/g) and large total pore volume (0.413 cm³/g). The resulting GN/HCPC composite was applied as an adsorbent to remove 2,4-dichlorophenol (2,4-DCP) from aqueous solutions. The influence of different solution conditions including pH, ionic strength, contact time, system temperature and concentration of humic acid was determined. The maximum adsorption capacity of GN/HCPC composite (calculated by the Langmuir model) could reach 348.43 mg/g, which represented increases of 43.6% and 13.6% over those of the as-prepared pure GN and HCPC, respectively. The Langmuir model and pseudo-second-order kinetic model were found to fit well with the adsorption process. Thermodynamic experiments suggested that the adsorption proceeded spontaneously and endothermically. In addition, the GN/HCPC composite showed high adsorption performance toward other organic contaminants including tetracycline, bisphenol A and phenol. Measurement of the adsorption capability of GN/HCPC in secondary effluent revealed a slight decrease over that in pure water solution. This study demonstrated that the GN/HCPC composite can be utilized as a practical and efficient adsorbent for the removal of organic contaminants in wastewater.

Adsorption of aqueous Hg2+ and inhibition of Hg0 re-emission from actual seawater flue gas desulfurization wastewater by using sulfurized activated carbon and NaClO

The potential impacts of seawater flue gas desulfurization (SFGD) process used in coal-fired power plants have been greatly concerned because the wastewater containing Hg is directly discharged into the ocean environment without proper treatment. Furthermore, the re-emission of Hg as Hg⁰ to the atmosphere from SFGD wastewater caused by the reduction of aqueous Hg²⁺ has also been observed. This study investigated the dependence of Hg²⁺ adsorption behavior for sulfurized activated carbon (SAC) in actual SFGD wastewater on various influencing factors, including initial Hg²⁺ concentration, solution pH, contact time, temperature, and the addition of oxidant (sodium hypochlorite, NaClO). SAC exhibited greater Hg²⁺ adsorption than raw activated carbon at an initial Hg²⁺ concentration of more than 4,723 ng L^-1. The Hg²⁺ removal efficiency of SAC was slightly larger at pH 7.0 and 8.0 than that at pH within 2.0-6.0. Hg²⁺ adsorption on SAC was well correlated with the linear adsorption model. Kinetic analysis results indicate that pseudo-second-order adsorption may serve as the rate-limiting reaction of Hg²⁺ adsorption on SAC. Thermodynamic analyses confirmed the endothermic and spontaneous adsorption behavior of Hg²⁺ on SAC in the seawater environment. Notably, the addition of NaClO significantly reduced the Hg²⁺ removal efficiency when SAC was used as the adsorbent. Nevertheless, NaClO addition also inhibited the reduction reaction of Hg²⁺ to Hg⁰ by forming strong HgCl complexes, which decreased the risk of Hg⁰ reemitted into the atmosphere via a SFGD system.

Enhancement of Pb (II) adsorption by boron doped ordered mesoporous carbon: Isotherm and kinetics modeling

Boron doped ordered mesoporous carbon (BMC) was prepared to improve the adsorption of Pb(II). The effects of several parameters such as contact time, pH, and ionic strength on the adsorption by both pristine ordered mesoporous carbon (OMC) and BMC were investigated. Thermodynamic, sorption isotherm and adsorption kinetics models were used to study the adsorption mechanisms by each of the adsorbents. Based on intraparticle diffusion model, the adsorption process by the two adsorbents mainly involved the quick liquid-film diffusion stage and slow pore diffusion portion, and fitting experimental data with Temkin model indicates that the adsorption process by both of the adsorbents involve physisorption and chemisorption. Based on Langmuir model, the estimated maximum adsorption capacity for BMC was about 1.3 times higher than the pristine OMC. Moreover, BMC retained good adsorption performance in tap and lake water, and could be regenerated effectively and recycled using EDTA. The results suggested that BMC, with enhanced adsorption performance compared with OMC, could be considered as very effective and promising materials for Pb (II) removal from wastewater.

Heavy metal ions removed from imitating acid mine drainages with a thermoacidophilic archaea: Acidianus manzaensis YN25

Metals in acid mine drainages (AMD) have posed a great threat to environment, and in situ economic environment-friendly remediation technologies need to be developed. Moreover, the effects of acidophiles on biosorption and migrating behaviors of metals in AMD have not been previously reported. In this study, the extremely thermoacidophilic Archaea, Acidianus manzaensis YN25 (A. manzaensis YN25) was used as a bio-adsorbent to adsorb metals (Cu²⁺, Ni²⁺, Cd²⁺ and Zn²⁺) from acidic solutions which were taken to imitate AMD. The values of their maximum biosorption capacities at both high (1 mM) and low (0.1 mM) metal concentrations followed the order: Cu²⁺ > Ni²⁺ > Cd²⁺ > Zn²⁺. With the elevations of temperature and pH value, the adsorption amounts of metals increased. The results also indicated that A. manzaensis YN25 had the highest adsorption affinity to Cu²⁺ in coexisting system of quaternary metals. Acid-base titration data revealed that carboxyl and phosphoryl groups provided adsorption sites for metals via deprotonation. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) further corroborated that amino played an important role in the biosorption process. The fitted Langmuir model illustrated monolayer adsorption occurring on cell surface. The possible adsorption mechanism of A. manzaensis YN25 mainly involved in electrostatic attraction and complexes formation. This study gives a profound insight into the biosorption behavior of heavy metals in acidic solution by thermoacidophilic Archaea and provides a probable novel strategy for in situ remediation of heavy metals pollution in AMD.

Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution

In order to achieve the purpose of simultaneous removal of coexisting heavy metal ions, in this work, functionalized magnetic mesoprous nanomaterials (Fe₃O₄-HBPA-ASA) with high density and multiple adsorption sites were designed and prepared. The obtained Fe₃O₄-HBPA-ASA was characterized by SEM, FTIR, VSM, TGA and zeta potential. Cu(II), Pb(II) and Cd(II) were chosen as the model heavy metal ions, the adsorption experiments showed that Fe₃O₄-HBPA-ASA showed hightheoretical adsorption capacitiesin individual system, and the maximum adsorption capacity was 136.66 mg/g, 88.36 mg/g and 165.46 mg/g, respectively. In the binary and ternary systems, the competitive adsorption leads to a decrease in the adsorption capacity of Cu(II), Pb(II) and Cd(II). However, in the ternary system with a concentration lower than 15 mg/L, the simultaneous removal rate was still higher than 90%. The adsorption isotherms and kineticswere well fitted by Langmuir and pseudo-second-order models, respectively. The XPS and density functional theory (DFT) analysis have confirmed that the adsorption of metal ions was related to various types of functional groups on the surface of Fe₃O₄-HBPA-ASA, while the adsorption mechanisms of Cu(II), Cd(II) and Pb(II) were different.

Highly efficient and irreversible removal of cadmium through the formation of a solid solution

Sulfur-containing materials are very attractive for the efficient decontamination of some heavy metals. However, the effective and irreversible removal of Cd²⁺, coupled with a high uptake efficiency, remains a great challenge due to the relatively low bond dissociation energy of CdS. Herein, we propose a new strategy to overcome this challenge, by the incorporation of Cd²⁺ into a stable Zn_xCd_1-xS solid solution, rather than into CdS. This can be realised through the adsorption of Cd²⁺ by ZnS nanoparticles, which have exhibited a Cd²⁺ uptake capacity of approximate 400 mg g^-1. Through this adsorption mechanism, the Cd²⁺ concentration in a contaminated solution could effectively be reduced from 50 ppb to <3 ppb, a WHO limit acceptable for drinking water. In addition, ZnS continued to exhibit this noteworthy uptake capacity even in the presence of Cu²⁺, Pb²⁺, and Hg²⁺. ZnS displayed high chemical stability. Particles aged in air for 3 months still retained a> 80% uptake capacity for Cd²⁺, compared with only 9% uptake capacity for similarly-aged FeS particles. This work reveals a new mechanism for Cd²⁺ removal with ZnS and establishes a valuable starting point for further studies into the formation of solid solutions for hazardous heavy metal removal applications.

Rapid and efficient recovery of silver with nanoscale zerovalent iron supported on high performance activated carbon derived from straw biomass

High performance activated carbon (HPAC) supported nanoscale zerovalent iron (nZVI) was prepared and used for recovery of silver. This composite material was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The removal amount of Ag⁺ increased with pH values and temperature. The removal process achieved equilibrium within 40 min and the maximum removal capacity was 986.5 mg/g at 298 K. The composite material showed fast adsorption rate and high adsorption capacity because the presence of high surface area activated carbon could effectively inhibit aggregation of nanoscale zerovalent iron, thus enhancing its reactivity. The Ag⁺ removal followed pseudo-second-order kinetic model and Langmuir isotherm model. XPS and XRD characterizations were performed to elucidate removal mechanism. It could be concluded that both coordination adsorption and reductive precipitation contributed to removal of Ag⁺ on the nZVI/HPAC.

Linear and nonlinear behavior of crosslinked chitosan/N-doped graphene quantum dot nanocomposite films in cadmium cation uptake

In this research, crosslinked nanocomposite (NC) films involving chitosan (CS) and various percentages of nitrogen-doped graphene quantum dot (NGQD) were prepared via ultrasonic acoustic accompanied by adding glutaraldehyde as a crosslinking agent (henceforth nominated as CCS/NGQD NC). The objective of this study is the design of a safe adsorbent of CCS/NGQD NC under easy and low-cost conditions to investigate the mechanisms of Cd(II) ion sorption and find an appropriate model for the kinetics of removal. By comparing adsorption ability of CCS/NGQD NC films 2, 5 and 8 wt% under the same conditions, the CCS NC film with 5 wt% of NGQD was selected as the best mass ratio to investigate the adsorption process. To understand the nature of the sorption behavior, the experimental data were used to calculate pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models, and various isotherm models in linear and nonlinear regression. In addition, some error functions were applied to detect, either linear or nonlinear model is suitable to examine the experimental data and prevent any huge mistakes. The linear Freundlich equation well describes the uptake of Cd(II) ion by CCS film and CCS/NGQD NC film 5 wt%. Based on linear Langmuir, the maximum adsorption capacities of CCS film and CCS/NGQD NC film 5 wt% were 34.46 and 35.00 mg·g^-1, respectively. Kinetic analysis indicates that the mechanism of removal is described by nonlinear pseudo-second order model for CCS film and linear pseudo-second order model for the CCS/NGQD NC film 5 wt%. Also, thermodynamic parameters were analyzed in different temperatures. The obtained thermodynamic values prove that Cd(II) ion adsorption on both adsorbents is feasible, spontaneous and endothermic.

Adsorption of Pb2+ from Aqueous Solutions Using Novel Functionalized Corncobs via Atom Transfer Radical Polymerization

The present study developed novel functionalized corncobs introducing brushes with dense and active carboxyl groups (-COOH), named MC-g-PAA, for the highly efficient adsorption of Pb2+ from aqueous solutions. MC-g-PAA were synthesized via atom transfer radical polymerization (ATRP) and characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The amount of Pb2+ adsorbed on MC-g-PAA by hydrolysis with t-BuOK was 2.28 times greater than that with NaOH, attributed to the larger steric effect of t-BuOK, which reduced the hydrolysis of the bromo-ester groups. The influence of different parameters including the solid/liquid ratio, working solution pH, sorption temperature, and initial concentration and sorption time on the adsorption of Pb2+ were investigated in detail in batch experiments. Thermodynamic studies have shown that the adsorption process was spontaneous, endothermic, and accompanied by an increase in randomness. A better fit for the isotherm data was obtained using the Langmuir model than for the other four models and the maximum amount ( q max ) of Pb2+ adsorbed on MC-g-PAA was 342.47 mg/g, which is 21.11 times greater when compared with that of pristine corncobs (16.22 mg/g). The adsorption of Pb2+ on MC-g-PAA was very fast and followed the pseudo-second-order kinetic equation with a correlation coefficient of 0.99999. This monolayer adsorption process was dominated by chemical adsorption, and may proceed according to complexation and electrostatic interactions between Pb2+ and the carboxylate groups. This study indicated that MC-g-PAA could be successfully used as an adsorbent for the removal of Pb2+ from aqueous solutions due to its excellent efficiency.

Effect of thiourea-modified biochar on adsorption and fractionation of cadmium and lead in contaminated acidic soil

Biochar was obtained through pyrolysis of carrot pulp (CP) and then further modified with thiourea (CH₄N₂S). We investigated the effect of carrot pulp biochar (CPB) and modified CPB (MCPB) for adsorption and chemical fractionation of cadmium (Cd) and lead (Pb) in contaminated acidic soil. Application of modified biochar significantly (p < 0.05) increased the pH, soil organic carbon (SOC), and cation exchange capacity (CEC) of the soil, especially at the 8% application rate. The adsorption equilibrium data showed that the adsorption behavior of Cd and Pb could be described more reasonably by the pseudo-second-order kinetic model and the Langmuir isotherm model more accurately fitted the experimental data than Freundlich and Temkin isotherm models. The maximum adsorption capacity of soil treated with MCPB at the 8% application rate for Cd and Pb were 4122.7 and 5219.6 mg kg^-1, respectively. Sequential chemical extractions revealed that incorporation soil with MCPB induced the transformation of the acid-soluble fraction of Cd to oxidizable and residual fractions, and the acid-soluble fraction of Pb to reducible, oxidizable, and residual fractions. The results demonstrated that the application of MCPB could effectively immobilize Cd and Pb, thereby reducing their mobility in contaminated acidic soil.

Removal of Cd(II) by modified maifanite coated with Mg-layered double hydroxides in constructed rapid infiltration systems

To improve the adsorption performance of Cd(II) by maifanite in constructed rapid infiltration systems (CRIS), Mg-layered double hydroxides (MgAl-LDHs, MgFe-LDHs) are prepared by a co-precipitation method and in-situ coated on the surface of original maifanite. Characterization of the successful LDHs-coating modification is realized by the following: scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Brunauer Emmett Teller (BET). In the purification experiments, the average removal rates of Cd(II) were 97.66% for maifanite/MgAl-LDHs and 97.54% for maifanite/MgFe-LDHs, approximately 11% greater than for the original maifanite. Isothermal adsorption experiments and adsorption kinetic experiments were conducted to explore the Cd(II) adsorption mechanism. The modified maifanite demonstrated a higher Langmuir adsorption capacity and stronger surface bond energies compared to the original maifanite. The adsorption type of Cd(II) by maifanite/Mg-LDHs and original maifanite was monolayer adsorption based mainly on chemical adsorption. Furthermore, the extracellular polymeric substances and dehydrogenase activities of the microorganisms were measured and analyzed to study the effect of microorganisms on the removal of Cd(II) in the test columns. High-throughput sequencing technology was also applied to analyze the composition and diversity of bacterial communities. Based on a simple estimation, the synthesis cost of maifanite/MgAl-LDHs was only ¥ 0.33/Kg. In brief, maifanite/Mg-LDHs is an efficient and economical substrate for a CRIS for Cd(II) removal.

Adsorption and removal of chromium (VI) contained in aqueous solutions using a chitosan-based hydrogel

The aim of this work was to study the adsorption and removal of chromium (VI) ions contained in aqueous solutions using a chitosan-based hydrogel synthesized via chemical crosslinking of radical chitosan, polyacrylic acid, and N,N'-methylenebisacrylamide. Fourier-transform infrared spectroscopy confirmed the hydrogel synthesis and presence of reactive functional groups for the adsorption of chromium (VI) ions. The chromium (VI) adsorption mechanism was evaluated using non-linear Langmuir, Freundlich, Redlich-Peterson, and Sips isotherms, with the best fit found by the non-linear Redlich-Peterson isotherm. The maximum chromium (VI) adsorption capacities of the chitosan-based hydrogel were 73.14 and 93.03 mg metal per g dried hydrogel, according to the non-linear Langmuir and Sips isotherm models, respectively. The best kinetic fit was found with the pseudo-nth order kinetic model. The chromium (VI) removal percentage at pH 4.5 and 100 mg L^-1 initial metal concentration was 94.72%. The results obtained in this contribution can be useful for future works involving scale-up of a water and wastewater treatment method from a pilot plant to full-scale plant.

Sulfamic acid modified hydrochar derived from sawdust for removal of benzotriazole and Cu(II) from aqueous solution: Adsorption behavior and mechanism

A novel hydrochar adsorbent derived from sawdust (SAHC) was prepared for highly efficient simultaneous removal of benzotriazole (BTA) and Cu(II) from aqueous solution. The prepared adsorbent was characterized by several methods such as SEM, FTIR, and XPS. Batch adsorption experiments showed that the maximum adsorption capacity of SAHC for BTA and Cu(II) was 159.91 and 298.86 mg/g, respectively. Additionally, the study of competitive adsorption showed that the adsorption of Cu(II) was barely affected by the existence of BTA while the BTA adsorption was significantly improved with the coexistence of Cu(II). The study of adsorption mechanism found that Cu(II) could chelate with BTA to form complex, and the complexing-bridging interaction improved BTA adsorption. SAHC exhibited high adsorption ability after six adsorption cycles, which indicated excellent stability and regeneration performance of SAHC. All the results suggested that SAHC could be a promising adsorbent for simultaneous removal of BTA and Cu(II) from wastewater.

Adsorption, recovery, and regeneration of Cd by magnetic phosphate nanoparticles

Adsorption plays an important role in removing cadmium (Cd²⁺) from water, and magnetic adsorbents are increasingly being used due to their ease of separation and recovery. Magnetic Fe₃O₄-coated hydroxyapatite (HAP) nanoparticles (nHAP-Fe₃O₄) were developed by co-precipitation and then used for the removal of Cd²⁺ from water. The properties of these nanoparticles were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and magnetization curves. Experiments were conducted to investigate the effects of adsorption and mechanisms. Results illustrated that kinetic data were well fitted by a pseudo-second-order model. The adsorption capacity of nHAP-Fe₃O₄ was 62.14 mg/g. The mechanisms for the adsorption of Cd²⁺ on nHAP-Fe₃O₄ included rapid surface adsorption, intraparticle diffusion, and internal particle bonding, with the ion exchange with Ca²⁺ and chemical complexation being the most dominant. The regeneration efficiency and recovery rate of nHAP-Fe₃O₄ eluted by EDTA-Na₂ after the fifth cycle were 63.04% and 40.2%, respectively. Results revealed that the feasibility of nHAP-Fe₃O₄ as an adsorbent of Cd²⁺ and its environmental friendliness make it an ideal focus for future research.

Biochar-mediated sequestration of Pb and Cd leads to enhanced productivity in Mentha arvensis

Immobilization of cadmium (Cd) and lead (Pb) along with the alleviation of their phytotoxicity in Mentha arvensis by biochar was examined in this investigation. A greenhouse experiment was executed to evaluate the effect of biochar (BC) amended Cd and Pb spiked soil on their immobilization and uptake, plant growth, photosynthetic attributes (total chlorophyll, photosynthetic rate, transpiration rate, and stomatal activity) and oxidative enzymes (guaiacol peroxidase: POD; catalase: CAT and superoxide dismutase: SOD). In the present study, the photosynthetic attributes showed that BC significantly improved the total chlorophyll, photosynthetic, transpiration rates, and stomatal activity in the plants. The incorporation of BC in soil increase the Pb and Cd tolerance in M. arvensis vis-à-vis improved the biomass yield and nutrient intake. In addition, biochar has also reduced the POD, CAT, and SOD in the plant as well as improved the soil pH and enzymatic activities. Overall, BC immobilized the Cd and Pb in soil by providing the binding site to the metals and reduced the phytotoxicity in M. arvensis. However, large-scale field trials of BC are required for safe cultivation of M. arvensis which is known for its phytopharmaceuticals importance.

Enhanced adsorption of cationic Pb(II) and anionic Cr(VI) ions in aqueous solution by amino-modified nano-sized illite-smectite clay

A raw illite-smectite mixed-layered clay (RI/S) was ground for preparing nano-sized I/S clay (NI/S) and subsequently amino-functionalized via grafting of 3-aminopropyltrithoxysilane (APTES) (NH₂-RI/S and NH₂-NI/S, respectively). The samples were characterized by particle size analysis, specific surface area measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and ²⁹Si nuclear magnetic resonance (²⁹Si NMR). Compared to RI/S, NI/S has a narrow particle size distribution and appears in a platelet-like morphology due to the disintegration/exfoliation of RI/S after grinding. Based on the ²⁹Si NMR spectra, the appearances of tri-silicate units indicate the chemically grafting of APTES molecules on NH₂-RI/S and NH₂-NI/S, respectively. NH₂-NI/S can adsorb greater amounts of Pb(II) cations and Cr(VI) anions rather than NH₂-RI/S since NH₂-NI/S grafts more amounts of amine groups (-NH₂). The isotherm data for adsorption of Pb(II) cations and Cr(VI) anions can be described by the Langmuir model at different temperatures (i.e., 10 °C, 30 °C, and 50 °C), respectively. The maximum adsorption amounts of Pb(II) cations and Cr(VI) anions onto NH₂-NI/S calculated by the Langmuir isotherm model are 131.23 mg/g and 36.91 mg/g at 50 °C, respectively. The adsorptions of Pb(II) cations and Cr(VI) anions onto NH₂-NI/S involve in the surface complexation of NI/S and amine groups.

The effect of interaction between Bacillus subtilis DBM and soil minerals on Cu(II) and Pb(II) adsorption

The effects of interaction between Bacillus subtilis DBM and soil minerals on Cu(II) and Pb(II) adsorption were investigated. After combination with DBM, the Cu(II) and Pb(II) adsorption capacities of kaolinite and goethite improved compared with the application of the minerals independently. The modeling results of potentiometric titration data proved that the site concentrations of kaolinite and goethite increased by 80% and 30%, respectively after combination with DBM. However, the involvement of functional groups in the DBM/mineral combinations resulted in lower concentrations of observed sites than the theoretical values and led to the enhancement of desorption rates by NH₄NO₃ and EDTA-Na₂. The DBM-mineral complexes might also help to prevent heavy metals from entering DBM cells to improve the survivability of DBM in heavy metal-contaminated environments. During the combination process, the extracellular proteins of DBM provided more binding sites for the minerals to absorb Cu(II) and Pb(II). In particular, an especially stable complexation site was formed between goethite and phosphodiester bonds from EPS to enhance the Pb(II) adsorption capacity. So, we can conclude that the DBM-mineral complexes could improve the Cu(II) and Pb(II) adsorption capacities of minerals and protect DBM in heavy metal-contaminated environments.

Preparation of various thiol-functionalized carbon-based materials for enhanced removal of mercury from aqueous solution

In this work, biochar (BC), activated carbon (AC), and graphene oxide (GO) were thiol-functionalized using 3-mercaptopropyltrimethoxysilane (3-MPTS) (named as BCS, ACS, and GOS, respectively). BCS, ACS, and GOS were synthesized mainly via the interaction between hydrolyzed 3-MPTS and surface oxygen-containing functional groups (e.g., -OH, O-C=O, and C=O) and π-π interaction. The materials before and after modification were characterized and tested for mercury removal, including sorption kinetics and isotherms, the effects of adsorbent dosage, initial pH, and ionic strength. Pseudo-second-order sorption kinetic model (R² = 0.992~1.000) and Langmuir sorption isotherm model (R² = 0.964~0.998) fitted well with the sorption data of mercury. GOS had the most -SH groups with the largest adsorption capacity for Hg²⁺ and CH₃Hg⁺ (449.6 and 127.5 mg/g), followed by ACS (235.7 and 86.7 mg/g) and BCS (175.6 and 30.3 mg/g), which were much larger than GO (96.7 and 4.9 mg/g), AC (81.1 and 24.6 mg/g), and BC (95.6 and 9.4 mg/g). GOS and ACS showed stable mercury adsorption properties at a wide pH range (2~9) and ionic strength (0.01~0.1 mol/L). Mercury maybe removed by ligand exchange, surface complexation, and electrostatic attraction.

Taguchi design-based enhancement of heavy metals bioremoval by agroindustrial waste biomass from artichoke

The Taguchi method of designing experiments is based on a system of tabulated designs (arrays) that enables the maximum number of variables to be estimated in a neutral (orthogonal) balanced manner with a minimum number of experimental sets. Heavy metals remediation of aqueous streams is of special concern due to its highly toxic and persistent nature. Taguchi approach was used for enhanced bioadsorptive removal of Pb(II), Cu(II) and Cd(II) from aqueous solutions using agroindustrial waste biomass from globe artichoke as inexpensive sorbent. Sorbent biomass was characterized as to its chemical composition by infrared spectroscopy (FTIR), revealing the presence of hydroxyl, carboxyl, sulphonic and amine functional groups. Ranks of four factors (pH, temperature, sorbent dosage and initial metal concentration) at three levels each, in a L9 array were conducted, in batch sorption tests, for the individual metal ions of concern. The sorption capacity (q_e) values were transformed into an accurate signal-to-noise (S/N) ratio for a "higher is better" response. The best conditions for individual heavy metal sorption were determined reaching up to 86.2 mg·g^-1 for Pb, 35.8 mg·g^-1 for Cd and 24.4 mg·g^-1 for Cu. This paper also discusses the equilibria and kinetic aspects of the sorption process. Sorption isotherms were successfully described by the Sips model. In addition, the experimental data showed that the uptake kinetic profiles of the three metal ions closely fitted the pseudo-second order model. Conclusively, the agroindustrial waste biomass from globe artichoke represents a potentially viable sorbent for the bioremoval of Pb(II), Cu(II) and Cd(II) ions from aqueous systems.

Magnetic nanoferromanganese oxides modified biochar derived from pine sawdust for adsorption of tetracycline hydrochloride

In this study, a new type of composite material, namely modified biochar (MBC), was synthesized by loading the magnetic ferromanganese oxide nanoparticles on pine biochar. BET, SEM, and FTIR were employed to analyze the surface properties and pore structures of MBC. In addition, XRD was adopted to examine the crystal structure of MBC. Characterization results showed that the surface area and porosity of MBC have been greatly improved, and the functional groups have been introduced by ferromanganese oxides. Adsorption experiments of tetracycline hydrochloride (TC) including kinetics, isotherms, thermodynamics as well as the influence of pH, salt ion strength, and the environmental risk of MBC, were evaluated. The results revealed that the experimental data conformed to the pseudo-second-order kinetic model and the Freundlich isotherm model. In the adsorption process, MBC showed excellent adsorption ability (maximum capacity for TC 100.74 mg g^-1) to BC (33.76 mg g^-1). In isotherm experiments, the maximum adsorption capacity of TC by MBC reached 177.71 mg g^-1. Toxicity studies showed that the MBC had no harm to the environment. To conclude, MBC has great potential for applications in removing TC from water.

Novel combined method of biosorption and chemical precipitation for recovery of Pb2+ from wastewater

A novel combined biosorption-precipitation process has been designed and applied to recycle Pb²⁺ from low concentration lead containing wastewater. Pb²⁺ was firstly removed selectively from wastewater by pyromellitic dianhydride (PMDA) modified sugarcane bagasse (SB) fixed-bed column, and then, it was desorbed into the concentrated eluate and recycled by adding chemical precipitant. Adsorption performance of the column and optimum desorption and precipitation condition for Pb²⁺ were investigated in detail. Results showed that the as-prepared column could efficiently remove Pb²⁺ from aqueous solution and optimum condition for Pb²⁺ precipitation in eluate was at pH 3.0 and molar ratio of precipitant to Pb²⁺ of 5:1 by using Na₃PO₄ as precipitant. Recovery experiment illustrated that Pb²⁺ was selectively removed from wastewater containing ions of Pb²⁺, Zn²⁺, Cd²⁺, Ca²⁺, K⁺, and Na⁺ through competitive substitution adsorption on the modified SB, and mass ratio of the five metal ions in eluate was 96.8:0.7:0.7:0.7:0.5:0.5. Pb²⁺ in this concentrated and purified eluate solution was recycled efficiently by adding Na₃PO₄. The combined method had great potential in application of heavy metal recovery from wastewater.

Characteristics of Cadmium Sorption by Heat-Activated Red Mud in Aqueous Solution

Red mud as a waste material is produced in large quantities by the aluminum industry. Heat activation has been used to enhance sorption capacity of red mud for its beneficial reuse as an effective sorbent. In this study, heat-activated red mud (HARM) was investigated for its Cd(II) sorption capacity under various process conditions (Cd concentration, pH and contact time) using response surface methodology (RSM). Analysis with RSM identified pH as the most important process parameter. The positive correlation between higher pH and greater Cd(II) sorption was likely due to: (i) decreased proton competition with Cd(II) for sorption sites at higher pH; (ii) enhanced sorption via ion exchange by monovalent Cd species from hydrolysis at higher pH; and (iii) improved thermodynamics of sorption at higher pH as protons are being released as products. Further analysis indicated the sorption process was thermodynamically favorable with a negative change in Gibbs free energy. Additionally, the sorption process exhibited a positive change in enthalpy, indicative of endothermic nature of sorption; this is consistent with sorption increase at higher temperature. These findings provide needed insight into the mechanisms underlying Cd(II) sorption by HARM for more effective applications of heat-activated red mud as sorbents for Cd(II) removal.