Adsorption Of Dissolved Organic Matter Research Articles

Multiple effects of iron oxides on the adsorption and oxidation of dissolved organic matter by manganese oxides

Adsorptive fractionation and oxidation of dissolved organic matter (DOM) on manganese (Mn) oxide surfaces alter the molecular composition and stability of DOM, but the impact of iron (Fe) oxides on the coupled adsorption-oxidation processes of DOM by Mn oxides is largely unknown. In this study, the underlying mechanisms of molecular transformation of DOM on birnessite (Bir) in the presence of ferrihydrite (Fh), with varying Fh/Bir mass ratios, were investigated at both molecular levels and microscopic scales with a suite of characterization techniques, including Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). We found that higher Fh/Bir mass ratio impeded the adsorption of DOM by birnessite and the reductive dissolution of birnessite to release dissolved Mn, and the impediment on the reductive dissolution of birnessite was proportional to Fh/Bir mass ratio. Overall, during the interactions of DOM with ferrihydrite and birnessite, phenolic compounds were preferentially adsorbed by Fe and Mn minerals, and compounds with higher oxygen contents and polymeric substances were formed. FT-ICR-MS analysis further suggested that higher Fh/Bir mass ratio inhibited the occurrence of aromatic ring-opening and carboxylation of substituted groups on aromatic rings, but promoted polymerization of phenolic compounds. Cs-STEM analysis revealed that DOM distributions on ferrihydrite, birnessite, and their mixtures were regulated by their microscopic structures and reactivity. Compared with aromatic carbon (C), carboxylic and phenolic C were more likely to associate with birnessite. Our results highlighted the significance of organo-mineral associations with the mixed minerals in regulating the distribution and reactivity of organic C. This study has provided molecular evidences for molecular transformation of DOM mediated by both Mn and Fe oxides, which contributed to advancing our understanding on coupled reactions of organic C at the mineral–water interfaces in the environment.

Molecular-level insight into the origin-dependent adsorption fractionation of dissolved organic matter on ferrihydrite in aquatic environment: Implications for carbon sink in eutrophic lakes

Molecular fractionation of dissolved organic matter (DOM) induced by adsorption on minerals plays a significant role in carbon biogeochemical cycling in water environment. However, the molecular-level adsorption of algae-derived DOM (ADOM) on minerals in eutrophic lakes remains poorly understood. Herein, we investigated the ferrihydrite-induced adsorptive fractionation of ADOM and humic DOM for comparison by using spectral analysis, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and solid characterization methods. We observed higher adsorption rate but lower adsorption capacity for ADOM than humic DOM. Spectroscopy and FT-ICR-MS indicated compounds with large molecular weight (>350 Da), high unsaturated and oxygen-rich were preferentially adsorbed for both ADOM and humic DOM. Specifically, the adsorption affinity order was aromatics > lignin-like > aliphatic compounds. Furthermore, tannin-like and condensed aromatics were predominantly adsorbed in humic DOM and protein-like formulas in ADOM. The solid characterization of ferrihydrite before and after adsorption further confirmed the dominant adsorption of aromatic and carboxylic-containing species in humic DOM and aliphatic materials in ADOM. Additionally, the contribution variation of oxygen atom to double bond in the molecular formula before and after adsorption identified absorbed oxygen-containing substances were predominantly aromatic carboxylic acids for humic DOM, but few aliphatic hydrocarbons substituted with ethers and alcohols for ADOM. These findings provided new insights into mineral-induced carbon sequestration, that is, increased release of ADOM in eutrophic lakes might influence the amount of organic carbon settled by ferrihydrite, and alter its quality from aromatic to aliphatic compounds transfer from water column to sediment through ferrihydrite adsorption.

Activation of iron oxide minerals in an aquifer by humic acid to promote adsorption of organic molecules

In aquifers, the sequestration and transformation of organic carbon are closely associated with soil iron oxides and can facilitate the release of iron ions from iron oxide minerals. There is a strong interaction between dissolved organic matter (DOM) and iron oxide minerals in aquifers, but the extent to which iron is activated by DOM exposure to active iron minerals in natural aquifers, the microscopic distribution of minerals on the surface, and the mechanisms involved in DOM molecular transformation are currently unclear. This study investigated the nonbiological reduction transformation and coupled adsorption of iron oxide minerals in aquifers containing DOM from both macro- and micro perspectives. The results of macroscopic dynamics experiments indicate that DOM can mediate soluble iron release during the reduction of iron oxide minerals, that pH strongly affects DOM removal, and that DOM is more efficiently degraded at low rather than high pH values, suggesting that a low pH is conducive to DOM adsorption and oxidation. Spherical aberration-corrected scanning transmission electron microscopy (SACTS) indicates that the reacted mineral surfaces are covered with large amounts of carbon and that dynamic agglomeration of iron, carbon, and oxygen occurs. At the nanoscale, three forms of DOM are found in the mineral surface agglomerates (on the surfaces, inside the surface agglomerates, and in the polymer pores). The microscopic organic carbon and iron mineral reaction patterns can form through oxidation reactions and selective adsorption effects. Fourier transform ion cyclotron resonance mass spectra indicate that both synergistic and antagonistic reactions occur between DOM and the minerals, that the release of iron is accompanied by DOM decomposition and humification, that large oxygen- and carbon-containing molecules are broken down into smaller oxygen- and carbon-containing compounds and that more molecules are produced through oxidation under acidic rather than alkaline conditions. These molecules provide adsorption sites for sediment, meaning that more iron can be released. Microscopic evidence for the release of iron was acquired. These results improve the understanding of the geochemical processes affecting iron in groundwater, the nonbiological transformation mechanisms that occur at the interfaces between natural iron minerals and organic matter, groundwater pollution control, and the environmental behavior of pollutants.

Unravelling molecular fractionation of dissolved organic matter on ferrihydrite-phosphate complexes

Association of dissolved organic matter (DOM) with iron (Fe) oxyhydroxides leading to significant molecular fractionation, plays a significant role in biogeochemical cycling of carbon in soils. However, the molecular effects of coexisting phosphate, a widely distributed oxyanion in agricultural soils that has a strong affinity with Fe oxyhydroxides on such a process remain poorly understood. Here, using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) with a superiority in deciphering DOM characteristics at the molecular level, this study reveals the DOM molecular fractionation characteristics on ferrihydrite-phosphate (Fh-P) complexes. Results show that the adsorption of DOM on Fh-P complexes can be significantly inhibited by ∼50% compared to that on ferrihydrite. Optical spectroscopy and FT-ICR-MS further indicate that phosphate can greatly change the molecular fractionation of DOM on Fh-P complexes. Phosphate preferentially inhibits the highly aromatic, hydrophobic, and high molecular weight compounds (> 400 Da) of DOM on Fh-P complexes, such as condensed hydrocarbon and tannin; more CHONS compounds preferentially adsorb on the Fh-P complexes than thoseon ferrihydrite. Phosphate inhibits the adsorption of DOM with the characteristics of high oxygen atoms number, aromaticity index (AI), and double bond equivalents (DBE) on ferrihydrite. Our study demonstrates that the inhibitory effects of phosphate on the adsorption of the DOM can significantly influence the fractionation and adsorption of DOM by ferrihydrite. The findings of this work deepen our understanding of biogeochemical cycling of DOM in soil and aquifer environments.

New insights into the adsorptive characteristics of trihalomethane precursors from surface water using magnetic powdered activated carbon

Powdered activated carbon (PAC) can effectively eliminate dissolved organic matter (DOM) as a precursor of trihalomethane (THM), but its application has raised concerns regarding its separation and the overelevated formation of brominate THM species in treated water after chlorination. Although magnetic PAC (Mag-PAC) has been proposed as an easily separable adsorbent for water treatment, its removal by Mag-PAC remains unknown. In this study, a novel Mag-PAC adsorbent for removing DOM and controlling THM formation from surface water was fabricated, evaluated using batch experiments, and compared with PAC. Our results found that Mag-PAC blended the advantages of PAC and magnetic particles, having an efficient removal performance toward DOM with an adsorption capacity of 2.84–3.69 mg-C/g and good magnetic separability of 10.15 emu/g. The iron oxide coating on the carbon matrices was predominantly distributed in the presence of crystalline goethite and magnetite structures. Chemisorption was the dominant mechanism of DOM adsorption by Mag-PAC. The sorption rate of DOM was influenced by the impregnation of iron oxide, but such impregnation did not significantly affect the DOM adsorption capacity. Aromatic DOM, humic-acid and fulvic-acid like compounds were efficiently adsorbed by the Mag-PAC. Compared with PAC, Mag-PAC exhibited a higher reduction in lifetime cancer risks from THMs through the ingestion pathway by decreasing the formation potential of trichloromethane and bromodichloromethane, which generated high unit risks among various THM species. These findings highlight the potential of Mag-PAC as an effective sorbent for DOM removal to control the formation of THM in surface water.

Adsorption and thermal stability of dissolved organic matter on Ca- and Mg-exchanged montmorillonite: Implications for persistence in soils and sediments

Dissolved organic matter (DOM) represents a vital and highly dynamic component of organic matter in the environment. Despite its significance, there is still limited knowledge regarding how different base cations and chemical conditions influence the preservation of complex natural DOM through adsorption. This study aimed to investigate the adsorption of natural DOM onto montmorillonite (Mt) by systematically exploring the effects of exchanging cations (Ca2+ and Mg2+), pH levels (4 and 7), and DOM molecular weight (< 10, 10–100, and > 100 kDa) on the mechanisms and thermal stability of adsorbed DOM. To this end, we conducted a series of adsorption experiments, complemented by spectroscopic and thermal analyses. Our results revealed that lower pH and higher DOM molecular weight enhanced adsorption, with affinity for carbohydrate-like compounds. Notably, changes in d001 spacing and peak broadening indicated intercalation of DOM into the interlayer space of Mt, which was positively correlated with the amount of DOM adsorbed. Conversely, increased adsorption was found to negatively correlate with thermal stability. This was attributed to the prevalence of weaker DOM-DOM interactions occurring at higher DOM weight and lower pH, owing to reduced repulsion among protonated organic molecules, in contrast to stronger electrostatic interactions near the Mt surface that appeared to dominate at lower DOM weight and higher pH conditions. Intriguingly, no clear preference for DOM adsorption on Ca- or Mg-exchanged Mt was observed, suggesting similar efficiencies of these cations in stabilizing DOM through cation-bridging. Our findings provide significant insights into the mechanisms involved in mineral-organic associations and have implications for understanding the persistence of DOM in soils and sediments.

Unveiling the molecular responses of dissolved organic matter in landfill leachate to activated carbon adsorption

Powdered activated carbon (PAC) is an effective method for removing dissolved organic matter (DOM) in membrane bioreactor effluent of landfill leachate (MBR effluent). Among commercially available PACs, wood-based PAC (WoAC) and coal-based PAC (CoAC) are two most common ones. However, the interactions between two PACs and DOM in MBR effluent at the molecular level as well as the effect of PACs properties on the adsorption of DOM compounds are not fully understood. To address the gap, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy and other spectroscopic techniques were employed to study the change of DOM in MBR effluent when adsorbed by WoAC and CoAC. The results showed that CoAC had a better removal performance of DOM (89.2% for UV254 and 78.4% for DOC) than WoAC (80.9% for UV254 and 70.8% for DOC), which was likely due to the Fe component in CoAC playing an important role in the DOM adsorption process. Both WoAC and CoAC exhibited preferential adsorption for compounds with high unsaturation or poor oxygen. Moreover, the Fe component in CoAC may bind with some functional groups (e.g., carboxyl groups) in unsaturated compounds, which may enhance the adsorption of unsaturated compounds. These findings deepen our understanding of PAC-DOM interactions at the molecular level and provide guidance for designing cost-effective PACs for landfill leachate treatment.

Eco-Corona Formation on Plastics: Adsorption of Dissolved Organic Matter to Pristine and Photochemically Weathered Polymer Surfaces.

Plastic fate in freshwater systems is dependent on particle size, morphology, and physicochemical surface properties (e.g., charge, surface roughness, and hydrophobicity). Environmental aging processes, such as photochemical weathering and eco-corona formation due to dissolved organic matter (DOM) adsorption on plastic surfaces, can alter their physicochemical properties, affecting fate and transport. While plastic aging has been studied from a materials science perspective, its specific implications in environmental contexts remain less understood. Although photochemical weathering and eco-corona formation occur simultaneously in the environment, in this work, we systematically assessed the effects of photochemical weathering on the physicochemical properties of polymers (polyethylene, polypropylene, polyethylene terephthalate, and polystyrene) and how this influences the adsorption of DOMs (Suwannee River humic acid, fulvic acid, and natural organic matter) relative to pristine polymers. Pristine polymers initially had different and distinct physicochemical surface properties, but upon aging, they became more similar in terms of surface properties. Photochemical weathering resulted in a decrease in polymer film thickness, an increase in surface roughness, and hydrophilicity. DOM adlayers on the polymer surfaces resulted in more comparable wettability, effectively masking the initial polymer properties. Collectively, this study explores the physiochemical changes polymers undergo in laboratory studies mimicking environmental conditions. Understanding these changes is the initial step to rationalizing and predicting processes and interactions such as heteroaggregation that dictate the fate of plastics in the environment.

Understanding the Nanoscale Affinity between Dissolved Organic Matter and Noncrystalline Mineral with the Implication for Water Treatment.

In recent decades, the concentration of dissolved organic matter (DOM) in aquatic ecosystems has gradually increased, leading to water pollution problems. Understanding the interfacial chemical processes of DOM on natural minerals is important to the exploration of high-efficiency absorbents. However, studying DOM chemical processes and adsorption mechanisms are still challenging due to the complex DOM structure and environmental system. Hence, we characterized the microstructure changes after the formation of amorphous calcium phosphate (ACP) at the interface of montmorillonite (Mt) minerals in a simulated environment system. Combined with atomic force microscopy and density functional theory (DFT) simulation, the mechanism of interfacial interaction between Mt-ACP and DOM was characterized at the molecular level. Moreover, we further evaluated the adsorption behavior of Mt-ACP as a potential adsorbent for organic matter. The comprehensive investigation of humic acid adsorption, intermolecular force, and DFT simulation is conducive to our understanding of the interfacial interaction mechanism between organic matter and noncrystalline minerals in aquatic environments and provides new perspectives on the application of clay-based mineral materials in pollutant removal under exposure from DOM.

Effects of incorporating Mn into goethite on adsorption of dissolved organic matter and potentially toxic elements in soil: Isotherms, kinetics, and mechanisms

Goethite is ubiquitous in the environment and plays key role in preserving dissolved organic matter (DOM) and deactivating potentially toxic elements (PTEs) by adsorbing DOM and PTEs. Various non-Fe metals are usually incorporated into natural goethite, substituting Fe in the goethite structure, which dramatically influence the physico-chemical properties and adsorption behavior of the goethite. In the present study, adsorption of DOM and Pb(II) on Mn-substituted goethite samples was investigated. The results displayed that the specific surface area (SSA) of mineral samples increased by 67.6% as the incorporation of Mn for Fe, from 25.71 m2 g−1 for pure goethite to 43.09 m2 g−1for Mn-goethite. Besides, the Mn substitution caused more hydroxyl groups and relatively fewer positive charges on mineral surface, and Mn in the Mn-goethite samples was predominantly present as Mn(III). The amount of DOM adsorbed to per unit mass of goethite was increased as Mn content increased, which was attributed to Mn incorporation increasing the SSA of mineral samples. However, the SSA-normalized absorption capacity for goethite to DOM was decreased by Mn because Mn substitution decreased the number of positive charges of mineral samples, which weakened the electrostatic attraction between DOM and the minerals. The amount of Pb(II) adsorbed to per unit mass of goethite was increased by Mn substitution, and the amount of Pb(II) adsorbed to per unit SSA of goethite increased as the amount of Mn substitution increased, indicating that the increased capacity for adsorbing Pb was not only caused by the SSA increasing but also by there were more surface hydroxyl groups on the Mn-goethite than pure goethite and Pb(II) preferentially adsorbed to Mn sites on the Mn-goethite. The present study results showed that Mn-goethite could be used to sequester DOM and remediate soil contaminated with PTEs because Mn-goethite has a high adsorption capacity and is environmentally benign.

Theoretical modeling of molecular fractionation of dissolved organic matter on ferrihydrite and its impact on proton and metal binding properties

Molecular fractionation of dissolved organic matter (DOM) at the mineral-liquid interfaces in soil changes its molecular composition, thus altering its reactivity, such as proton and metal binding properties. Therefore, a quantitative understanding of compositional change of DOM molecules after adsorptive fractionation by minerals is of great environmental significance for predicting the cycling of organic carbon (C) and metals in the ecosystem. In this study, we conducted adsorption experiments to investigate the adsorption behaviors of DOM molecules on ferrihydrite. The molecular compositions of the original and fractionated DOM samples were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). For all DOM molecules, three molecular groups with significantly different chemical properties were identified through Spearman correlation analysis between the relative intensities of DOM molecules and organic C concentrations in solutions after adsorptive fractionation. Three corresponding molecular models for the three molecular groups were constructed based on Vienna Soil-Organic-Matter Modeler and FT-ICR-MS results, which were used as base units to construct molecular models for the original or fractionated DOM samples (model(DOM)). The models well described the chemical properties of the original or fractionated DOM as compared with the experimental data. Furthermore, based on model(DOM), the proton and metal binding constants of DOM molecules were quantified by SPARC chemical reactivity calculations and linear free energy relationships. We found the density of binding sites of the fractionated DOM samples was negatively correlated with the adsorption percentage. Our modeling results suggested that adsorption of DOM on ferrihydrite gradually removed acidic functional groups from the solution, dominated by the adsorption of both carboxyl and phenol groups. This study proposed a new modeling approach to quantify the molecular fractionation processes of DOM on Fe oxides and their impact on proton and metal binding properties, which is expected to be applicable to DOM from different environments.

Retention of dissolved organic matter during podzolisation: Testing processes in laboratory experiments and at the submicron scale

Immobilisation of organic matter in illuvial subsoil horizons is a characteristic result of Podzol formation and development. We tested dissolved organic matter (DOM) immobilisation by mineral mixtures as models of initial illuvial horizons and conducted laboratory experiments by irrigating mineral mixtures with a DOM solution and analysing the eluate solutions. The mineral mixtures consisted of quartz with accessory H+-saturated illite, or Ca2+-saturated illite, or quartz with ferrihydrite. Furthermore, we studied the transport of colloidal proto-imogolite in the ferrihydrite-containing mixture and analysed the spatial distribution of elements by nanoscale secondary ion mass spectrometry (NanoSIMS). Retention of DOM in the presence of Ca2+-saturated illite did not exceed that in the presence of H+-saturated illite, as the promoting effect of the divalent cation was counterbalanced by the initially higher pH. The mineral mixture containing ferrihydrite retained most DOM. In all experiments, aromatic and oxidised DOM moieties were preferentially adsorbed. Colloidal proto-imogolite co-precipitated with DOM and reduced the pore space of the mineral mixture, which increased the residence time and DOM adsorption on ferrihydrite. NanoSIMS analyses showed proto-imogolite deposited on ferrihydrite, with ferrihydrite as the more important DOM adsorbent in ferrihydrite-proto-imogolite assemblages. Furthermore, NanoSIMS revealed less N in adsorbed DOM closer to the mineral surface. Thus, we differentiated between processes of DOM retention in simulated early podzolisation, detecting preferentially adsorbed species and compositional differences within adsorbed DOM layers.

Removal of dissolved organic matter in road runoff by granules prepared using sludge from waterworks

Abstract This work assessed the elimination of dissolved organic matter (DOM) in road runoff by a granular sludge-clay (GSC) adsorbent. The rates of adsorption were found to be consistent with the pseudo-second-order kinetic model. The data at equilibrium resulted in a maximum adsorption capacity of 4.466 mg/g at 298 K, which was in good agreement with the Langmuir isotherm model. The adsorption of DOM relies on pH. The higher removal efficiency of DOM was observed at pH 4.0 and 7.0. To clarify the related adsorption mechanism, isolated DOM fractions and their removal potentials were identified. The results showed hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions which contained abundant fulvic-like substances were more preferentially removed by the GSC. The adsorption mechanism of DOM in road runoff by GSC involves both electrostatic attraction and ligand exchange reactions. GSC synthesized using the sludge from waterworks is a very promising filler to replace soils or gravels that can be applied in some green infrastructures for removing DOM from road runoff.

Perturbation and strengthening effects of DOM on the biochar adsorption pathway

Biochar is an effective adsorbent commonly used in pollutants adsorption. However, natural constituents, such as dissolved organic matter (DOM), could affect pollutants adsorption. In this study, we analyzed the mechanisms underlying phenol adsorption on pine biochar under perturbation by fertilizer-derived DOM. In addition, biochar property alterations were characterized and further analyzed. The results showed that phenol and DOM combined to a certain extent in the adsorption system. DOM affected the adsorption pathway, which increased the biochar adsorption efficiency for phenol. The addition of DOM2 promoted phenol adsorption efficiency (70.31%), with total DOM adsorption capacity of 61.45 mg g-1 onto biochar.

Coagulation removal of dissolved organic matter (DOM) in nanofiltration concentrate of biologically treated landfill leachate by ZrCl4: Performance, mechanism and coagulant recycling

Coagulation treatment is often applied for removing the residual refractory dissolved organic matter (DOM) in biologically treated landfill leachate nanofiltration concentrate (LLNC) before discharge or further desalination treatment. However, the DOM removal efficiency by traditional coagulant needs to be improved, and two problems including the coagulant loss and difficulty in disposal of coagulation sludge need to be resolved. Based on this practical demand, a new coagulant ZrCl4 was adopted for LLNC treatment for the first time. The results showed that, ZrCl4 was better than the traditional coagulants (FeCl3 and AlCl3) for DOM removal. Under the optimal condition of pH 6.0 and ZrCl4 dosage of 5.0mM, the DOC content, UV254 and chromaticity of the LLNC reduced by 73.32%, 83.17% and 93.59%, respectively. All of the coagulants tested in this study could efficiently remove the hydrophobic and high molecular organics. There was an obvious difference between them for removal of hydrophobic, and small or medium molecular organics, and ZrCl4 was more effective. This might be due to the stronger negative charge neutralization capacity and larger floc size of ZrCl4, which was beneficial for DOM combination and adsorption. The loss of zirconium was only 2.11%, which was much lower than that of iron and aluminum. Furthermore, being recycled for 3 times after coagulant regeneration, the recovered zirconium coagulant showed no obvious difference with the original ZrCl4 for DOM removal, indicating the disposal problem of the produced coagulation sludge can be resolved. This study could provide a promising method for LLNC treatment.

Influence of dissolved organic matter, kaolinite, and iron oxides on aggregation and transport of biochar colloids in aqueous and soil environments

The aggregation and transport of biochar colloids (BCs) in the soil and groundwater are critical for applying biochar in the field and assessing long-term environmental risk. This research aimed to study the influence of dissolved organic matter (DOM) with different molecular weights (including humic acid, HA; bovine serum albumin, BSA; deoxyribonucleic acid, DNA) and three minerals (including kaolinite, goethite, and hematite) on the aggregation and transport behaviors of BCs. The adsorption of DOM on the surface of BCs increased the stability, inhibited aggregation, and promoted the transport of BCs. As the molecular weight of DOM increased, the thicknesses of the adsorption layer of HA, BSA, and DNA on BCs surface were 2.2 nm, 5.3 nm, and 5.6 nm, respectively, resulting in increasing steric hindrance and improving the stability and mobility of BCs. Kaolinite also significantly enhanced the stability and mobility of BCs by increasing the electrostatic repulsion. Goethite and hematite quickly combined with BCs through electrostatic attraction, resulting in stronger aggregation and retention of BCs. Compared to hematite, goethite provided more adsorption sites for BCs due to its needle-like shape, so goethite caused a larger heteroaggregation rate. Overall, the presence of DOM with different molecular weights and the minerals with varying surface charges in the soil environment had a significant and distinct impact on the stability, aggregation, and transport of BCs, which advances the knowledge of colloidal biochar fate in the soil and groundwater.

Interactions of dissolved organic matter with short-range ordered aluminosilicates by adsorption and co-precipitation

Retention of dissolved organic matter (DOM) by short-range ordered aluminosilicates (SROAS) by adsorption and co-precipitation contributes to carbon accrual in soils and sediments. In this study, we investigated effects of SROAS composition on DOM adsorption, partitioning of carbon moieties by adsorption and co-precipitation, and the mineral structure of co-precipitates. We used four types of sorptive solutions, representing DOM collected in situ from topsoil and subsoil horizons of a Dystric Cambisol, and water-extracted DOM from beech and fir litter. We studied adsorption of soil DOM on three SROAS that structurally resemble proto-imogolites and Si-rich allophanes as a function of contact time (1–168 h) at initial pH 5. Co-precipitation of soil and litter-extracted DOM was quantified as a function of the molar Al:C ratio (0.3–1.4) and at two levels of Si concentration (molar Al:Si = 1 and 2). To resolve the impact of DOM on mineral structure, we first examined time-dependent structural evolution of SROAS within 1 to 72 h and subsequently investigated the effects of DOM interference in crystallization processes. Mineral structure of SROAS and co-precipitates was resolved by infrared, solid-state 27Al- and 29Si-NMR spectroscopy. Chemical composition of DOM prior to reaction with SROAS and in co-precipitates was analysed by solid-state 13C-NMR spectroscopy. Maximal C contents of adsorption complexes were 7.1 mg C g−1 for Al-rich and 20.4 mg C g−1 for Si-rich SROAS. We found selective adsorption of aromatic C and preferential exclusion of polysaccharide and alkyl C for both topsoil and subsoil DOM. Adsorption was larger for a Si-rich SROAS, since it exhibited a greater accessibility of aluminol groups. As a function of aromatic C content in initial DOM, 39.9 to 81% of C was retained by co-precipitation. Composition of co-precipitated organic matter was determined by C speciation in DOM supply, involving marked uptake of polysaccharides. In the absence of DOM, up to 50% of Si was present in imogolite-like configuration after 72 h, evidencing rapid development of short-range order. Complexation of Al by DOM during formation of SROAS caused partial exclusion of Si and slowed structural evolution, consequently enhancing abundance of ill-defied Si species in co-precipitates. Interactions of DOM with SROAS may cause selective accumulation of organic compounds and promote Si mobility in Andosols and Podzols.

High molecular weight fractions of dissolved organic matter (DOM) determined the adsorption and electron transfer capacity of DOM on iron minerals

Dissolved organic matter (DOM) in the natural environment has a significant effect on the biogeochemical cycles of iron (Fe), such as adsorption and reduction reaction. However, the roles of the molecular weight (MW) fractions of DOM extracted from plants in adsorption onto Fe minerals coupled with the Fe reduction process are incompletely characterized. In this study, the effects of DOM (extracted from Carex cinerascens) with different MW fractions, labeled as low molecular DOM (L-DOM), medium molecular DOM (M-DOM), and high molecular DOM (H-DOM), respectively, on the adsorption onto Fe minerals and the electrons transfer capacity (ETC) during the Fe reduction process is investigated. The oxygen-rich and highly aromatic H-DOM exhibited stronger adsorption affinity onto Fe minerals, such as ferrihydrite, a poorly crystalline mineral, than other DOM fractions. The X-ray photoelectron spectra and the Fourier transform infrared spectra revealed that the functional groups of the oxygen-rich DOM, such as CO bonds and OCO bonds, preferentially adsorbed onto the Fe minerals. In addition, the cyclic voltammograms (CV) and the chronoamperometry (CA) measurements showed that H-DOM significantly enhanced the Fe reduction to its maximum extent, indicating that the efficiency of DOM on Fe reduction was strongly related to the ETC. Therefore, our findings confirm that H-DOM plays a key arole in the adsorption onto Fe minerals and Fe redox reaction. The results of this study may help evaluate the performance of the different MW of DOM from plants on the biogeochemical cycles of Fe in the soil environment.

Sorption of Pb(II) onto biochar is enhanced through co-sorption of dissolved organic matter

Biochar plays an important role in controlling migration of pollutants in soils. However, little information is available on the interactions between soil-derived dissolved organic matter (DOM), biochar and soluble metal species. The aim of this work was to present the adsorption process of soil DOM by biochar (corn straw biochar produced at 700 °C) and to determine whether co-sorption of DOM would change the affinity for Pb(II). The adsorption rates of biochar and biochar + DOM for Pb(II) were best fitted with a pseudo-second order kinetic model, and the equilibrium adsorption isotherm data agreed well with both the Langmuir and Freundlich models. Adsorption of DOM to biochar reached equilibrium after 15 h with an uptake of 52% of the supplied DOM. We used fluorescence excitation-emission matrices (EEMs) with parallel factor (PARAFAC) analysis to demonstrate that protein-like, fulvic acid-like and humic acid-like substances were the primary constituents of the DOM, which were quenched over time in the presence of biochar. Synchronous fluorescence spectra indicated that the protein-like structures were the predominant fluorescence substances in DOM. Two-dimensional correlation spectroscopy (2D-COS) showed the binding of DOM to biochar resulted in the quenching of fluorescence in the order: protein-like substances > humic-like substances (280 > 355 nm). Data supports the notion that DOM can increase the adsorption capacity of biochar for metal-ions.

Changes in molecular dissolved organic matter and disinfection by-product formation during granular activated carbon filtration by unknown screening analysis with Orbitrap mass spectrometry

The minimization of disinfection by-product (DBP) formation by the removal of its precursors before water disinfection is a highly effective approach. Granular activated carbon (GAC) filtration is widely used for water treatment, but our understanding of molecular dissolved organic matter (DOM) remains insufficient. This research investigates the removal of DOM and the minimization of DBP formation by pilot-scale coal- and coconut-based granular activated carbon filtrations (coAC and ccAC, respectively) using unknown screening analysis with Orbitrap mass spectrometry. DOM adsorption rates by both GACs were fitted with pseudo-second order models with initial adsorption rates of 0.005 mg g−1 min−1 and 0.022 mg g−1 min−1 for ccAC and coAC, respectively. Based on observations, ccAC was more effective in the removal of dissolved organic carbon and prolonged adsorption longer than coAC, as the breakthrough of coAC was found on Day 10. ccAC removed compounds with carbon, hydrogen, and oxygen (CHO features) with a wide range of oxidation states, as indicated by the carbon oxidation state (Cos), and a wide range of unsaturation, as indicated by oxygen subtracted double bond equivalent per carbon ([DBE−O]/C), while coAC selectively removed only those CHO features with less oxidized characters. Less oxidized compounds (low Cos) were preferentially removed with less contact time, while more oxidized compounds needed more contact time to adsorb on the GACs. A biofilm was developed on Day 60, and many CHO features were found to have increased after GAC treatment on Day 60, indicating the formation of microbial products. Chlorination resulted in a decrease in many CHO and CHO with Cl atom (CHOCl) features and the formation of CHOCl DBPs more than CHO DBP features. ccAC was effective in the minimization of trihalomethane (THM) and CHOCl DBP feature formations on Day 10 and Day 60, while coAC was found to be much less effective.