Ternary Surface Complexes Research Articles

Synergistic release of Cd(II) and As(V) from ternary sorption systems containing ferrihydrite nanoparticles: The role of binary and ternary surface complexes

Cadmium (Cd) and arsenic (As) generally exhibit mutually beneficial co-sorption behavior on iron oxyhydroxides through multiple mechanisms, including surface precipitation, ternary surface complexes, and electrostatic interactions. However, the numerous factors that control the immobilization of Cd and As in turn complicated the processes and mechanisms involved in their co-desorption from iron minerals, which hindered the full understanding of their geochemical behaviors. Here, the simultaneous release of Cd(II) and As(V) from newly precipitated ferrihydrite nanoparticles by either Ca or P was investigated through kinetics and isothermal desorption experiments. We showed that the Cd(II) and As(V) present two-phase desorption processes (rapid desorption and slow desorption) in both binary (Fe–Cd or Fe–As alone) and ternary systems (Fe–Cd–As co-presence). Compared to their binary counterparts, Cd(II) and As(V) in the ternary systems are more prone to detachment from ferrihydrite. Further desorption of Cd(II) and As(V) at different co-presence scenarios (different initial concentrations) demonstrated mutual promotion behaviour towards their counterparts; the co-presence of Cd(II) facilitates the desorption of As(V), while the co-presence of As(V) also promotes the desorption of Cd(II). XPS and FTIR results demonstrated that either Ca or P showed minor effects on the binding environment of Cd and As. Further results from the in-situ ATR-FTIR experiment and second derivative peak fitting analysis indicate that the enhanced detachment of Cd(II) and As(V) from the ternary system may be due to the synergistic desorption of the ternary surface complexes and other surface complex species. Our results provide new insights into the prediction of the environmental behaviour of the coexistence of Cd(II) and As(V) in iron-rich geological settings. The potential environmental risks of iron-based remediation methods should be considered due to the enhanced bioavailability of Cd(II) and As(V) in co-presence circumstances.

Trivalent lanthanide sorption onto illite in the presence of carbonate: A study combining thermodynamic sorption modeling, time-resolved laser fluorescence spectroscopy, and parallel factor analysis

Formation of aqueous actinide complexes with dissolved inorganic carbon (DIC) in groundwater decreases the sorption of actinides onto clay minerals in a geological disposal environment. Hence, the effect of DIC on actinide sorption must be evaluated quantitatively. In this study, batch sorption experiments using Eu(III) and Sm(III) as the chemical analogs of trivalent actinides were performed to assess their sorption behavior onto illite in the presence of DIC. In the studied pH region between 8.4 and 11, the sorption was significantly lower in the presence of DIC, with the largest effect being observed at pH 9–10. The effect increased with increasing DIC from 10 to 50 mM. A thermodynamic sorption model (TSM) was used to evaluate the results of batch sorption experiments. The experimental results can only be predicted when ternary-surface complexes with carbonate ions are considered. Moreover, information on the chemical forms of sorbed Eu on illite was obtained via time-resolved laser fluorescence spectroscopy (TRLFS) measurements. Parallel factor analysis of TRLFS data indicated the presence of two chemical species of Eu. These chemical species exhibited pH dependence, which is consistent with that of the surface species predicted using the TSM. In the presence of DIC, the dominant chemical species showed a long fluorescence lifetime and was inferred to coordinate with carbonate ions, supporting the validity of the TSM.

Immobilization of hexavalent uranium U(VI) by hydroxyapatite under oxic conditions

The immobilization of hexavalent uranium U(VI) by hydroxyapatite (HAP) was investigated under oxic conditions as a function of pH and surface loading ([U(VI)]0/[HAP]0) to evaluate the applicability of HAP as migration barriers or waste forms in nuclear waste repositories. This study focused on the retention mechanisms, which were sought by integrating the solid-phase characterization (e.g., X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and U LIII-edge X-ray absorption spectroscopy) with the solution-phase analysis of dissolved Ca, P, and U. At acidic pH, due to the increasing solubility of HAP, U(VI) retention is mainly controlled by the formation of a uranyl phosphate following HAP dissolution. Importantly, this phosphate is in the form of a solid solution with Na meta-autunite being the principal component, which considerably expands the stable domain of uranyl phosphates compared to the pure phases (e.g., chernikovite, Na meta-autunite, and meta-autunite). At neutral to basic pH, however, the uranyl phosphate formation does not prevail till [U(VI)]0/[HAP]0 = 25 mg/g, below which the formation of U(VI)-phosphate ternary surface complexes mainly accounts for U(VI) retention. At basic pH, the surface substitution of PO43− for CO32− and the dominance of uranyl carbonate species make surface complexation less favorable under this pH condition. In this study, the initially present U(VI) is near completely immobilized (> 99.99 %) by HAP, with the greatest retention noted at circumneutral pH, thus demonstrating the effectiveness of HAP for the containment of U(VI) under ambient geochemical conditions.

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces.

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Synergistic mechanism of iron manganese supported biochar for arsenic remediation and enzyme activity in contaminated soil

This study prepared and characterized bamboo-derived biochar loaded with different ratios of iron and manganese; evaluated its remediation performance in arsenic-contaminated soil by studying the changes in various environmental factors, arsenic speciation, and arsenic leaching amount in the soil after adding different materials; proposed the optimal ratio and mechanism of iron–manganese removal of arsenic; and explained the multivariate relationship between enzyme activity and soil environmental factors based on biological information. Treatment with Fe–Mn-modified biochar increased the organic matter, cation exchange capacity, and N, P, K, and other nutrient contents. During the remediation process, O-containing functional groups such as Mn–O/As and Fe–O/As were formed on the surface of the biochar, promoting the transformation of As from the mobile fraction to the residual fraction and reducing the phytotoxicity of As, and the remediation ability for As was superior to that of Fe-modified biochar. Mn is indispensable in the FeMn-BC synergistic remediation of As, as it can increase the adsorption sites and the number of functional groups for trace metals on the surface of biochar. In addition to electrostatic attraction, the synergistic mechanism of ferromanganese-modified biochar for arsenic mainly involves redox and complexation. Mn oxidizes As(Ⅲ) to more inert As(V). In this reaction process, Mn(Ⅳ) is reduced to Mn(Ⅲ) and Mn(II), promoting the formation of Fe(Ⅲ) and the conversion of As into Fe–As complexes, while As is fixed due to the formation of ternary surface complexes. Moreover, the effect of adding Fe–Mn-modified biochar on soil enzyme activity was correlated with changes in soil environmental factors; catalase was correlated with soil pH; neutral phosphatase was correlated with soil organic matter; urease was correlated with ammonia nitrogen, and sucrase activity was not significant. This study highlights the potential value of FM1:3-BC as a remediation agent in arsenic-contaminated neutral soils.

Rigidity and Flexibility: Unraveling the Role of Fulvic Acid in Uranyl Sorption on Graphene Oxide Using Molecular Dynamics Simulations.

Using molecular dynamics simulations, this work targets a molecular understanding on the rigidity and flexibility of fulvic acid (FA) in uranyl sorption on graphene oxide (GO). The simulations demonstrated that both rigid Wang's FA (WFA) and flexible Suwannee River FA (SRFA) can provide multiple sites to cooperate with GO for uranyl sorption and act as "bridges" to connect uranyl and GO to form GO-FA-U (type B) ternary surface complexes. The presence of flexible SRFA was more beneficial to uranyl sorption on GO. The interactions of WFA and SRFA with uranyl were primarily driven by electrostatics, and the electrostatic interaction of SRFA-uranyl was significantly stronger owing to the formation of more complexes. The flexible SRFA could markedly enhance the bonding strength of uranyl with GO by folding itself to provide more sites to coordinate with uranyl. The rigid WFAs tended to be adsorbed on the GO surface in parallel due to π-π interactions, whereas the flexible SRFAs took more slant configurations resulting from intermolecular hydrogen bonds. This work provides new insights into the sorption dynamics, structure, and mechanism and addresses the effect of molecular rigidity and flexibility, with great significance for FA-based remediation strategies of uranium-contaminated sites.

Co-adsorption mechanisms of Cd(II) and As(III) by an Fe-Mn binary oxide biochar in aqueous solution

Biochar can remove oppositely charged ionic species of Cd(II) and As(III)/As(V) from water but the co-adsorption mechanisms of the metal(loid)s are poorly understood. This study aims to unravel the co-absorption mechanisms of Cd(II) and As(III) by a Pennisetum sp. straw biochar (BC) modified with Fe-Mn oxide (FMBC). The maximum adsorption capacity of Cd(II) and As(III) by FMBC was 141.1 and 31.8 mg/g, respectively, which was 2.71 and 3.89 times higher than that of BC. The adsorption of Cd(II) by FMBC demonstrated a homogeneous monolayer chemical process, while the As(III) adsorption presented a heterogeneous multilayer process. In the dual-adsorbate system, Cd(II) increased the adsorption of As(III) on FMBC by 5.1–123.9%, while As(III) reduced the adsorption of Cd(II) by 0.7–24.8%, indicating respective synergism and competition effect. Systematic co-adsorption experiments and adsorbent characterization results suggested that the synergistic mechanism occurred via electrostatic attraction and ternary surface complexation, whereas the competitive effect emerged due to complexing reaction with oxygen-containing functional groups of FMBC. The FMBC could be a potential adsorbent to adsorb Cd(II) and As(III) simultaneously.

The occurrence characteristics and underlying removal mechanism of combined copper and sulfamethoxazole pollution in constructed wetlands

Constructed wetlands (CWs) can effectively remove heavy metals and antibiotics, providing a promising solution for rural wastewater treatment. However, mechanisms of the interaction and removal of coexisting contaminants in CWs remain unclear. The removal efficiency of coexisting copper (Cu) and sulfamethoxazole (SMZ), main removal pathways, and interactions between Cu and SMZ in CWs were investigated using both in small-scale CWs and laboratory batch-adsorption experiments. The removal efficiencies of SMZ and Cu in CWs were approximately 99 %, and the presence of Cu accelerated SMZ removal within 10 h. Cu removal was mainly dependent on ceramsite adsorption, and its contribution ratio decreased by 39 %–41 % with increasing concentrations. The coexistence of high SMZ concentrations enhanced the Cu bioaccumulation. Over 92 % of SMZ removal in CWs occurred via microorganisms; SMZ was more likely to accumulate in plant roots than in aboveground parts. Adding low levels of Cu increased bacterial diversity in CWs, and addition of 20 mg/L Cu under SMZ stress conditions significantly increased the relative abundance of anaerobic denitrifiers and SMZ-degrading Microscillaceae. Copper and SMZ interactions with ceramsite presented competitive and synergistic effects depending on SMZ concentration. Copper and SMZ adsorptions were stable under neutral and weakly alkaline conditions, respectively. Cation addition promoted SMZ adsorption via hydrophobic interactions but did not promote Cu adsorption. Sulfamethoxazole and Cu(II) competed for –NH sites, and the adsorbed Cu bridged additional anionic SMZ to form surface ternary complexes on ceramsite particles. SMZ − NH2+ bonding facilitated additional Cu–N coordination bonding via electrostatic interactions. These findings provide insights for optimising the purification efficiency of CWs and mitigating the environmental risks of compound pollution.

The immobilization of Sr(ll) and Co(ll) via magnetic easy-separation organophosphonate-hydroxyapatite hybrid nanoparticles

The magnetic Hexamethylene diamine tetramethylene phosphonic acid (HDTMP)-hydroxyapatite nanoparticles (HD-MHAP) were firstly prepared and applied for the immobilization of radioactive Sr2+ (Strontium ion) and Co2+ (Cobalt ion). The molar ratio of organic phosphine in HDTMP to total phosphorus was 10% (10HD-MHAP) and can also be easily separated from aqueous solution. At the equilibrium concentration of Sr2+ and Co2+ (200 mg/L), the adsorption capacity of MHAP and 10HD-MHAP on Co2+ was 28.4 mg/g and 172.3 mg/g, respectively and for Sr2+, the adsorption capacity was 54.9 mg/g and 320.7 mg/g. Additionally, the desorption percentage of Sr2+ (%) and Co2+ (%) was very low, which was 0.66 and 1.61, respectively. This is significant because the irreversible adsorption of HDTMP/Fe3O4/HAP composite for Sr2+ and Co2+ ensures the captured radionuclides will not be released back into the aquatic systems. Moreover, the affinity between HDTMP and HAP and the coordination of HDTMP and Sr2+ or Co2+ was probed by Multiwfn wavefunction analysis. The results indicated that the formation of a ternary surface complex of MHAP-HDTMP-M (Sr2+ or Co2+) was the key to the excellent immobilization performance of HD-MHAP composite for Sr2+ or Co2+.

Hydration states of europium(III) adsorbed on silicas with nano-sized pores

The confinement in small spaces influences the complicated reaction involving water molecules under electrostatic potentials at the solid–liquid interface. It is unclear, in particular, how the hydration state of metal ions adsorbed on the surface changes as a function of the pore size. This study analyzed the adsorption states of europium aqua ions (Eu3+) on mesoporous silicas with different pore distributions in comparison with nonporous silica using time-resolved laser-induced fluorescence spectroscopy. The parallel factor analysis was used to differentiate the contribution of different chemical species of Eu3+ to the fluorescence signal and determine the influence of the pore size on each chemical species. The results show that Eu3+ mainly form outer-sphere complexes with silica below pH 6, where Eu3+ adsorption is low. Within nano-sized pores, distortion of the hydration structure and a decrease in the hydration number were suggested in this pH range. As the concentration of the silicate ions derived from the dissolution of silica increases with increasing pH, Eu3+ form the silica/Eu3+/silicate ternary surface complexes. Within nano-sized pores, the concentration of silicate ions decreases due to the overlap of the electric double layer, which inhibits the formation of the ternary surface complex. Furthermore, at high pH, Eu3+ multinuclear complexes formed only on the mesoporous silica surface. This adsorption behavior specific to nano-sized pores could not be concluded by macroscopic adsorption experiments alone because the amount of Eu3+ adsorbed per unit surface area did not differ between the mesoporous and nonporous silicas. Consideration of the silicate complexes should be indispensable in future studies on the adsorption of lanthanide ions using mesoporous silica.

Nano-ferrihydrite colloidal particles mediated interfacial interactions of arsenate and cadmium: Implications for their fate under iron-rich geological settings

Arsenic (As) and cadmium (Cd) often coexist in paddy soils. Nano-ferrihydrite colloidal particles (NFPs) are ubiquitous at redox active interfaces of the paddy system and are well-known to play a critical role in controlling the solubility and bio-availability of As and Cd. However, the mutual interaction between As and Cd on NFPs remains elusive. Herein, batch experiments and in-situ spectroscopic techniques were used to investigate the effects of the interaction pattern (sequential reaction) of Cd(II) and As(V) on their respective adsorption on the surfaces of NFPs. Two scenarios were designed: Cd(II) pre-saturated NFPs and As(V) pre-saturated NFPs. Adsorption of Cd(II) was increased by 1.67, 4.08, and 5.21 times in As(V)-saturated NFPs, but only by 1.05, 1.11, and 1.15 times for As(V) in Cd(II)-saturated NFPs. Further, we determined the pH-dependent mutually beneficial cooperation pathways as mediated by the surface of NFPs. At lower pH (5), As(V) tended to promote Cd(II) adsorption, whereas Cd(II) tended to enhance As(V) adsorption at higher pH (> 5.5). X-ray photoelectron spectroscopy (XPS) indicated that both pre-saturated Cd(II) and As(V) altered the local coordination environment of their counterpart ions. Furthermore, results from in-situ attenuated total reflection-Fourier-transform infrared spectroscopy (ATR-FTIR) and second derivative peak shape fitting revealed two types of ternary surface complexes, namely Cd(II)-bridged and As(V)-bridged complexes, which were responsible for the distinct Cd(II) and As(V) co-adsorption behavior on the surface of NFPs under different conditions. These findings help us understand how co-presence Cd and As behave in an iron-rich geological setting and will aid in the development of related restoration technologies.

Remediation mechanism of Cu, Zn, As, Cd, and Pb contaminated soil by biochar-supported nanoscale zero-valent iron and its impact on soil enzyme activity

Recently, heavy metal contaminated soil has been widespread concerned. Herein, rice straw-derived biochar-supported nanoscale zero-valent iron (nZVI@BC) composite was prepared and characterized, and its remediation performance in Cu, Zn, As, Cd, and Pb contaminated soil were evaluated. nZVI@BC demonstrated better immobilization effect on multiple heavy metals than biochar, and the residual component of Cu, Zn, As, Cd, and Pb increased by 10.27%, 7.18%, 7.44%, 9.26%, and 12.75%, respectively. After nZVI@BC treatment, the soil pH, the available iron, cation exchange capacity, total organic carbon, and dissolved organic carbon increased. Besides, soil enzyme (catalase, urease, and fluorescein diacetate hydrolase) activities significantly increased, especially, the catalase activity was doubled. Furthermore, the immobilization mechanism was explored, including surface adsorption, electrostatic attraction, ion exchange, coprecipitation, oxidation-reduction, and complexation. Additionally, the adsorption sites of Pb and Cu were more competitive, and As synergistically promoted the adsorption of Pb, forming a ternary surface complex PbFeAsO4OH. Therefore, it well demonstrates that nZVI@BC may be a potential technology for remediation of multiple heavy metals contaminated soil.

Adsorption of potentially harmful elements by metal-biochar prepared via Co-pyrolysis of coffee grounds and Nano Fe(III) oxides

Nano Fe(III) oxide (FO) was used as an amendment material in CO2-assisted pyrolysis of spent coffee grounds (SCG) and its impacts on the syngas (H2 & CO) generation and biochar adsorptive properties were investigated. Amendment of FO led to 153 and 682% increase of H2 and CO in pyrolytic process of SCG, respectively, which is deemed to arise from enhanced thermal cracking of hydrocarbons and oxygen transfer reaction mediated by FO. Incorporation of FO successfully created porous structure in the produced biochar. The adsorption tests revealed that the biochar exhibited bi-functional capability to remove both positively charged Cd(II) and Ni(II), and negatively charged Sb(V). The adsorption of Cd(II) and Ni(II) was hardly deteriorated in the multiple adsorption cycles, and the adsorption of Sb(V) was further enhanced through formation of surface ternary complexes. The overall results demonstrated nano Fe(III) oxide is a promising amendment material in CO2-assisted pyrolysis of lignocellulosic biomass for enhancing syngas generation and producing functional biochar.

Adsorption of glycine at the anatase TiO2/water interface: Effects of Ca2+ ions

Adsorption reactions of amino acids (AAs) on TiO2 nanoparticles (NPs) play an important role in the available nutrients in soils and sediments. The pH effects on glycine adsorption have been studied, but little is known about its coadsorption with Ca2+ at the molecular level. Combined attenuated total reflectance Fourier transform infrared (ATR-FTIR) flow-cell measurements and density functional theory (DFT) calculations were used to determine the surface complex and corresponding dynamic adsorption/desorption processes. The structures of glycine adsorbed onto TiO2 were closely associated with its dissolved species in the solution phase. The presence of Ca2+ exerted different influences on glycine adsorption within pH 4−11, thus affecting its migration rate in soils and sediments. The mononuclear bidentate complex at pH 4−7, involving the COO− moiety of zwitterionic glycine, remained unchanged in the absence and presence of Ca2+. At pH 11, the mononuclear bidentate complex with deprotonated NH2 can be removed from the TiO2 surface upon coadsorption with Ca2+. The bonding strength of glycine on TiO2 was much weaker than that of the Ca-bridged ternary surface complexation. Glycine adsorption was inhibited at pH 4 but was enhanced at pH 7 and 11.

Thermodynamic and kinetic modeling the interaction of goethite-ligand-metal ternary system

Low-molecular-weight organic acids may significantly influence the mobility of metal in environment, but the kinetics are not fully understood and have not been quantified. In this study, the thermodynamic and kinetic effects of citric acid (CA) on the adsorption of Cd(II) and Ni(II) on goethite were investigated using batch-adsorption and stirred-flow experiments. A charge distribution and multisite complexation model (CD-MUSIC) and a thermodynamically based multi-rate kinetic model were employed to describe the adsorption behaviors. Two ternary surface complexes, (≡FeO)2CitMe and (≡FeOH)2MeCit2−, were involved in the adsorption. In addition, CA differed in its effects on Cd(II) and Ni(II) adsorption, enhancing Cd(II) adsorption but inhibiting Ni(II) adsorption at high levels. Kinetically, in the presence of CA, the adsorption of Cd(II) was faster than that of Ni(II). Increasing CA concentration led to faster Cd(II) adsorption, but resulted in the dissolution of the adsorbed Ni(II), possibly due to the much higher complexation constants of Ni-CA than of Cd-CA in aqueous phase. This finding implied that, in the rhizosphere, high level of CA may lead to more dissolution of Ni(II) than Cd(II); while in acidic ferrosol, CA may alleviate Cd(II) mobility and toxicity. The proposed mechanistic model sheds light on ion partition in the soil environment and may improve predictions thereof.

Insight into the mechanism of phosphate and cadmium co-transport in natural soils

Phosphate is ubiquitous in the environment and can affect the transport of heavy metals in the subsurface systems. In this study, column experiments were conducted to systematically evaluate the effects of phosphate on the transport of Cd in natural soils (RS, BS) under different ionic strength (IS) conditions. The presence of phosphate significantly retarded the transport of Cd in the soils. The extent of retardation was closely associated with phosphate concentrations, IS and soil properties. Increasing phosphate adsorption induced more negative surface charges on soils, thereby contributing to greater retention of Cd through electrostatic attraction. In contrast, higher IS not only promoted mobility of Cd, but also reduced the retardation effect of phosphate on Cd transport in soils. Moreover, higher Fe/Al oxides contents in RS exhibited a more pronounced effect of phosphate on Cd retardation. Our results indicated that electrostatic interaction was the predominant mechanism controlling co-transport of Cd with phosphate, but no ternary surface complexes was observed in the Cd LIII-edge XANES spectra. Our findings highlight the critical role of phosphate in retarding Cd transport in natural soils, which should be considered in assessing environmental risks of heavy metals in the subsurface.

Insight into the stabilization mechanism and long-term effect on As, Cd, and Pb in soil using zeolite-supported nanoscale zero-valent iron

Nanoscale zero-valent iron-based materials exhibited excellently for simultaneous remediation of arsenic(As), cadmium(Cd), and lead(Pb) co-contaminated soil. However, the instinct mechanism for stabilizing the three heavy metal(loid)s remains unclear. This study used the simplified zeolite-supported nanoscale zero-valent iron (Z-NZVI) to reduce As, Cd, and Pb availability, the remediation process characteristics, and the long-term stabilization mechanism were systematically investigated. Compared with the control, 1.0% of Z-NZVI performed highly efficiently for simultaneous stabilization of As, Cd, and Pb, and their availability decreased by 91.6%, 85%, and 96.6%, respectively, mainly due to the transformation of labile forms into residual. Furthermore, the availability of As, Cd and Pb was significantly positive with soil pH and negative with soil Eh (P < 0.01), respectively, indicating the soil pH and Eh should be priority regulated during the stabilization process. HRTEM-EDS, FTIR, XRD, and XPS characterizations of the magnetically separated Z-NZVI from incubated soil confirmed that the formation of coprecipitation together and the surface complexes transformed labile As into stable forms, complexation dominated Cd remediation, whereas Pb stabilization was mainly attributed to reduction and the formation of B-type ternary surface complexes by various Fe-containing substances. The simulated acid rain leaching results showed that Z-NZVI effectively restrained As, Cd, and Pb leaching by 97.7%, 92.8%, and 93.4%, respectively, revealing the long-term stabilization effect on the three heavy metal(loid)s. This research demonstrated that Z-NZVI is a promising amendment for realizing the simultaneous stabilization of As, Cd, and Pb in co-contaminated soil.

Simultaneous adsorption of As(III) and Cd(II) by ferrihydrite-modified biochar in aqueous solution and their mutual effects

A simply synthetic ferrihydrite-modified biochar (Fh@BC) was applied to simultaneously remove As(III) and Cd(II) from the aqueous solution, and then to explore the mutual effects between As(III) and Cd(II) and the corresponding mechanisms. The Langmuir maximum adsorption capacities of As(III) and Cd(II) in the single adsorbate solution were 18.38 and 18.18 mg g−1, respectively. It demonstrated that Fh@BC was a potential absorbent material for simultaneous removal of As(III) and Cd(II) in aqueous solution. According to the XRF, SEM–EDS, FTIR, XRD, and XPS analysis, the mechanisms of simultaneous removal of As(III) and Cd(II) by Fh@BC could be attributable to the cation exchange, complexation with R-OH and Fe-OH, and oxidation. Moreover, the mutual effect experiment indicated that Cd(II) and As(III) adsorption on Fh@BC in the binary solution exhibited competition, facilitation and synergy, depending on their ratios and added sequences. The mechanisms of facilitation and synergy between Cd(II) and As(III) might include the electrostatic interaction and the formation of both type A or type B ternary surface complexes on the Fh@BC.

Real wastewater micropollutant removal by wood waste biomass biochars: A mechanistic interpretation related to various biochar physico-chemical properties

Emerging organic micropollutants (OMPs) are ubiquitous in waterbodies and not fully eliminated by wastewater treatment plants (WWTP). A proposed WWTP upgrade includes OMPs sorption by biochar rather than activated carbon (AC). Activated biochar (AB) and non-activated biochar (NAB) sorption performance towards 4 target OMPs (benzotriazole, sulfamethoxazole, carbamazepine, diclofenac) were evaluated in real-wastewater. Sorption processes are discussed in light of sorption mechanisms that depend both on OMP and on biochar physico-chemical properties. AB and AC have similar average sorption efficiency (45%), while NABs are much less efficient (<19%). OMPs sorption significantly correlates to SSA, inversely to H/C (indicating hydrophobic interaction with graphene-like structures) and to CEC or ash content (indicating H-bond or ternary surface complexation sorption mechanisms). This highlights the need to mix AB sources with different physico-chemical characteristics to sorb a large OMP spectrum within WWTPs. Production and use of ABs alleviate the negative environmental impact of ACs.

Interaction of Zn with ferrihydrite and its cooperative binding in the presence of PO4

In nature, zinc (Zn) ions interact with natural organic matter and soil metal (hydr)oxides. The resulting solid-solution partitioning is of major importance for the geochemical cycling and environmental risks of toxicity and deficiency. Ferrihydrite (Fh) is an important proxy for the natural metal (hydr)oxide fraction in soils. At its surfaces, Zn2+ ions have multi-component interactions, for instance with phosphate (PO43−). In nature, the latter is particularly relevant in topsoils since PO4 is omnipresent and has a high affinity for Fh. Zinc and phosphate ions may bind cooperatively by electrostatic forces and/or by ternary complex formation. This was studied presently with batch adsorption experiments using freshly prepared and well-characterized Fh nanoparticles. The data have been interpreted using the charge distribution (CD) model combined with a surface structural model for Fh that includes site heterogeneity. The CD coefficients have been derived independently by optimization of the surface geometries with molecular orbital calculations applying density functional theory (MO/DFT/B3LYP/6-31+G**). These computations confirm that upon adsorption Zn can change spontaneously its coordination number from 6 to 4. In agreement with literature results from X-ray absorption spectroscopy (XAS), Zn is bound at low loading as a double-corner bidentate complex. At higher loading, the number of ions in the second shell of Zn decreases according to published XAS results. Our model with the formation of surface complexes with single corner-sharing can quantitatively predict this decrease. Zn polymerization only occurs at a very high molar Zn/Fe ratio (>0.1), which can be described using a neutral, hydrolyzed Zn-dimer species. The presence of PO4 enhances the Zn adsorption, especially in the pH range 5–6. At the Zn and PO4 levels studied, no ternary Zn-P surface complexes can be revealed. For comparison, we re-interpreted copper (Cu)-PO4-Fh adsorption literature data with similar metal and PO4 loadings. In line with the results for Zn, no ternary Cu-P surface complexes were found. We conclude that electrostatic interactions explain metal adsorption at background levels in multi-component systems, and that ternary complexes may form only at rather extreme metal or PO4 loading conditions.