Ocean carbon removal represents a promising pathway for mitigating residual anthropogenic carbon dioxide (CO2), yet existing methods are constrained by high energy demands and potential ecological risks. Here, inspired by the natural calcification process of corals, we present a bio-inspired capacitive decarbonization (CDC) reactor that sequesters dissolved inorganic carbon (DIC) from seawater as CaCO3 using only seawater-derived Ca2+ and renewable electricity. The CDC system integrates a Ca2+-selective electrode with a weak electric field to regulate ion transport and disrupt the hydration shell of Ca2+, enhancing its reaction with CO32−. To address the limited concentration of CO32− relative to Ca2+ in seawater, we introduce an asymmetric electrosorption strategy to preferentially enrich CO32− at the electrode interface, achieving a DIC conversion rate of up to 34% with an ultralow intrinsic electrochemical energy input of 2.5 kJ mol−1 CO2 for the CDC reactor. The reactor exhibits stable continuous operation for over 100 h without fouling, enabled by spatially decoupled CaCO3 precipitation. To mitigate the reduction in seawater alkalinity, we introduce a mineral-assisted re-alkalinization step that effectively restores pH and supports continued CO2 absorption. A global integrated analysis model shows the CDC technology could remove up to 11–438 million tonnes of CO2 by 2050–2100. This work demonstrates a scalable and low-energy solution for durable ocean carbon removal.

The dissolution of waste glasses by groundwater presents a key mechanism for immobilised or encapsulated contaminant release over geological timescales. Accurately predicting glass dissolution rates remains a challenge to waste management, where a complete understanding of glass dissolution mechanisms is required to model the release and fate of contaminants. Here, this work investigated the suitability of boron isotope fingerprinting techniques for studying glass dissolution mechanisms, focussing on solid-state diffusion processes during boron release. Two glasses (magnesium-free 10B-ISG and magnesium-bearing 6Li-Mg-EM) were altered in deionised water at 90 °C for 0.25 to 112 d. Solution renewal experiments were used to further study altered surface layer properties. At ≤ 28 d, solution boron isotope (11B/10B) ratios for 6Li-Mg-EM were consistent with the apparent congruent release of boron alongside sorption/coprecipitation processes with secondary minerals, but decreasing solution 11B/10B ratios at > 28 d suggested diffusion occurred across the altered layer at a dissolution front spatially separated from that of lithium. Contrastingly, solution 11B/10B ratios for 10B-ISG at ≤ 28 d were fitted well using a diffusion model assuming a time-dependent apparent diffusion coefficient, but those at > 28 d were better explained by either sorption/coprecipitation processes with secondary minerals or a spatially-dependent apparent diffusion coefficient. The altered layer formed for 10B-ISG after 28 d was not protective following solution renewal, and renewed solution 11B/10B ratios were instead consistent with an apparent congruent release mechanism. This study presents boron isotopes as in situ tracers for studying glass dissolution mechanisms, assisting in predicting contaminant releases during waste glass-aqueous solution interactions.

Impurity-containing iron hydroxides, abundant in many natural and engineered soil and aqueous environments, control the fate and transport of multiple aqueous contaminants. Fe(III) hydroxide was reported to simultaneously detoxicate As(III) and Cr(VI). However, the mechanisms and reaction intermediates are not clear, and the effects of impurities in ferrihydrite were far from being well understood. Here, Cr(III)-incorporated Fe(III) hydroxides were precipitated from acidic solutions (pH ∼ 3.0) with varied Fe(III)/Cr(III) molar ratios (10 : 0 to 8 : 2) for simultaneous removal of As(III) and Cr(VI). Multiple characterization techniques were combined to investigate the effects of Cr-incorporation on the size, band gap, adsorption, and catalytic efficiency of Fe hydroxides. With the amounts of Cr-incorporation increasing, the particle size of Fe hydroxides rapidly decreased (from 16.7 to 6.0 nm), and the removal of total As/Cr increased, as the Cr-incorporated Fe hydroxides with smaller size had larger surface area, promoting As/Cr removal by adsorption. Based on As/Cr speciation analysis of both aqueous and solid phases, the molar ratios of the oxidized As(III) (88%) to reduced Cr(VI) (∼56%) were calculated to be ∼1.5, indicating that the coupled redox conversion was the dominant removal mechanism over As(III)/Cr(VI) adsorption and As(III) oxidation. Intermediate characterization and molecular simulation found that Cr-incorporation promoted the early formation of H2O2 and Cr(V) intermediates, and enhanced the adsorption of reaction intermediates on Cr-incorporated Fe hydroxides, thus promoting their catalytic efficiency for coupled As(III)/Cr(VI) redox reactions.

Persulfate-based advanced oxidation processes (PS-AOPs) catalyzed by carbon-based catalysts are promising for removing organic pollutants via radical/non-radical pathways. However, the activation efficiency of peroxymonosulfate (PMS) or peroxydisulfate (PDS) usage and the reaction mechanism remain insufficiently understood. In this study, the effects of PMS/PDS dosage on the degradation of bisphenol A (BPA, 10 mg/L) were evaluated using N-doped biochar (N-BC, 0.2 g/L) assisted PS-AOPs. The reaction pathways were comprehensively investigated through a combination of characterization techniques and molecular simulations. With low PS dosages (0.05 and 0.1 mM), the degradation rate constants () were higher in N-BC/PDS (0.04 and 0.07 min−1) compared to N-BC/PMS (0.02 and 0.04 min−1), likely due to higher PDS utilization, which enhanced the contribution of the non-radical pathway. Interestingly, with higher PS dosages (0.5 and 1.5 mM), the  values were 0.16 min−1 and 0.18 min−1 in N-BC/PMS, respectively, significantly exceeding those determined in N-BC/PDS (0.11 and 0.11 min−1). This result stemmed from the greater adsorption capacity of N-BC for PMS compared to PDS, leading to increased formation of 1O2. The contribution of non-radical pathways for both PMS and PDS increased with higher PS dosage. The results highlighted that BPA degradation improved significantly with the increase in PMS dosage; meanwhile, BPA degradation was insensitive to PDS dosage. The optimal PMS dosage for BPA degradation was found to be 1.5 mM and 0.1 mM for PDS. This study offered valuable insights for optimizing PS-AOPs in environmental remediation, helping to guide the selection of appropriate oxidants and dosages for maximizing pollutant removal.

Calcium phosphate is widely used for the remediation of lead-contaminated sites, where calcium/lead phosphate coprecipitates (Ca/Pb CoPs) form. This research investigated such coprecipitation with model low-molecular-weight organics (LMWOs), produced in the rhizosphere with representative functional groups, which were found to regulate both the composition (Ca/Pb and C/Pb ratios) and stability (aggregation and transformation) of Ca/Pb CoPs. The strong complexation ability of –SH in l-cysteine with aqueous Ca2+/Pb2+ ions inhibited coprecipitation to a great extent. Meanwhile, coprecipitates with lysine containing both –NH2 and –COOH had a higher Ca/Pb ratio than those with citrate containing only –COOH, probably due to the elevated local supersaturation caused by both –NH2 and –COOH in lysine that attracted phosphate ions and cations, promoting Ca doping in CoPs. Also, the strong binding of both –NH2 and –COOH with coprecipitates resulted in a higher C/Pb ratio for the CoPs with lysine than citrate. The oriented aggregation of nano-CoPs formed needle-shaped hydroxylpyromorphite crystals without organics. Unexpectedly, lysine/citrate disrupted and l-cysteine promoted such oriented aggregation, resulting in inhibited and promoted crystallinity, respectively. This study provided new mechanistic insights on LMWO effects on Ca/Pb CoP formation and their stability and can help understand Pb speciation and availability in the rhizosphere.

Iron (oxyhydr)oxide nanoparticles (IONPs) are formed in many aquatic and soil systems through nucleation and growth in solution (homogeneous precipitation), and at soil-water interfaces (heterogeneous precipitation). This review summarizes the roles of metal ions, organics, and microbes in the nucleation and growth of IONPs in natural settings. Metal ions can adsorb onto mineral surfaces that act as substrates to modify heterogeneous precipitation processes at soil (mineral/organic)–water interfaces. Further, metal ions could also affect homogeneous precipitation through lattice substitution or surface adsorption onto IONPs. Similarly, organic matter can interfere with heterogeneous IONP formation through adsorbing onto mineral surfaces, and can affect homogeneous IONP formation by complexing with iron ions and adsorbing onto IONP surfaces. Indeed, the physicochemical diversity of mineral surfaces and organic matter properties, especially regarding organic functional groups which have varied complexation and (de)protonation capabilities, can profoundly affect these processes. Microbial influences arise through the production of extracellular polymeric substances (EPS) and the redox modulation of the surrounding environment, which alter electron transfer dynamics and surface reactivity to affect the formation of IONPs. This review provides an integrated view of the roles of metals, organics and microbes in IONP formation, which can not only help in the understanding of the iron cycle, but also the biogeochemical fate of contaminants.

Iron (oxyhydr)oxide nanoparticles (IONPs), which are ubiquitous in many natural aquatic and soil systems, can strongly interact with nutrient and contaminant species in the environment through their large specific surface areas and redox reactivity, thus controlling the transport and fate of these elements. Following their formation, IONPs often undergo aggregation and phase transformation processes that collectively determine their long-term environmental stability. The aggregation of IONPs reduces colloidal stability and can lead to deposition and immobilization, whereas stable dispersed colloids can remain mobile and transport associated elements over long distances. The phase transformations of metastable, poorly crystalline IONPs (e.g., ferrihydrite) into more crystalline iron (oxhydr)oxides (e.g., goethite, hematite, and magnetite) profoundly alter particle properties and influence the retention or release of sorbed or structurally incorporated species. This review focuses on IONP aggregation and phase transformation as key processes controlling long-term IONP stability and critically examines how they are influenced by three common environmental factors: metal ions, organic matter (OM), and microbial activity. Metal ions can adsorb to IONP surfaces to modify surface charges or be structurally incorporated to affect IONP crystallography, thereby modulating inter-particle forces and transformation rates. OM can adsorb to IONP surfaces, and, depending on its concentration and molecular characteristics, it can either stabilize particles via electrostatic and/or steric repulsion, or promote aggregation through charge neutralization and bridging effects. Further, organic ligands can also often inhibit IONP transformation or alter transformation pathways by binding to reactive surface sites. Microbial activity influences IONP stability through extracellular polymeric substances (EPS) that coat or bridge particles, and through redox processes that generate or consume Fe(II), thereby either dispersing IONPs or accelerating their transformation into more stable mineral phases. This review summarizes present research on the effects of IONP interactions with metals, organics, and microbes on IONP aggregation and transformation. Such an understanding is crucial for predicting IONP stability and transport in the environment and the long-term cycling of associated organic and inorganic contaminants and nutrients.

Under acidic pH conditions, the mobility of Cr is controlled by (Fe, Cr)(OH)3 coprecipitation in solution (homogeneous) and on soils (heterogeneous), and natural organic matter (NOM) adsorption onto soils could affect heterogeneous (Fe, Cr)(OH)3 precipitation on soils and, thus, Cr transport. Here, Suwannee River NOM (SRNOM) adsorption onto Al2O3 was investigated under varied carbon concentrations, and properties of SRNOM-coated surfaces were characterized using spectroscopic and interfacial techniques. Heterogeneous (Fe, Cr)(OH)3 precipitation on SRNOM-coated surfaces was studied at acidic pH via metal analysis and phase/size characterization. With lower NOM concentrations, preferential adsorption of aromatic moieties occurred, rendering more hydrophobic surfaces, which promoted nucleation and resulted in precipitates with higher Cr/Fe ratios. With higher NOM concentrations, NOM-coated surfaces became more negatively charged, attributed to enrichment of acidic (i.e., carboxylate) structures. Therefore, the amount of heterogeneous precipitates increased as enriched carboxylates and negative charge promoted heterogeneous nucleation and deposition. The controlling mechanisms were further validated with model OMs: For humic acid and fulvic acid, similar phenomena were observed with SRNOM. For polyacrylic acid with high acidity and no aromaticity, its adsorption onto Al2O3 made the surface highly negatively charged and hydrophilic, resulting in promoted heterogeneous precipitation with low Cr incorporation. Preferential adsorption of OMs with higher molecular weights (MWs) onto Al2O3 also occurred, but the MW did not affect either the amount or composition of heterogeneous (Fe, Cr)(OH)3 precipitates. The new knowledge learnt here could help in understanding Cr immobilization under acidic environments with diversified NOMs.

Mineral crystallization is central to myriad natural processes from the formation of snowflakes to stalagmites, but the molecularscale mechanisms are often far more complex than models reflect. Feedbacks between the hydro-, bio-, and geo-spheres drive complex crystallization processes that challenge our ability to observe and quantify them, motivating an expansion of crystallization theories. In this article, we discuss how the driving forces and timescales of nucleation are influenced by factors ranging from simple geometric confinement to distinct interfacial solution structures involving solvent organization, electrical double layers, and surface charging effects. Taken together, these ubiquitous natural phenomena can preserve metastable intermediates, drive precipitation of undersaturated phases, and modulate crystallization in time and space.

The heterogeneous coprecipitation of nanocrystals with metals on substrates plays a significant role in both natural and engineered systems. Due to the small dimensions and thereby the large specific surface area, nanocrystal coprecipitation with metals, which is ubiquitous in natural settings, exerts drastic effects on the biogeochemical cycling of metals on the earth’s crust. Meanwhile, the controlled synthesis of nanocrystals with metal doping to achieve tunable size/composition enables their broad applications as adsorbents and catalysts in many engineered settings. Despite their importance, complex interactions among aqueous ions/polymers, nanocrystals, substrates, and metals are far from being well-understood, leaving the controlling mechanisms for nanocrystal formation with metals on substrates uncovered. In this Account, we discuss our systematic investigation over the past 10 years of the heterogeneous formation of representative nanocrystals with metals on typical substrates. We chose Fe(OH)3 and BaSO4 as representative nanocrystals. Mechanisms for varied metal coprecipitation were also investigated for both types of nanocrystals (i.e., Fe, Al, Cr, Cu, and Pb)(OH)3 and (Ba, Sr)(SO4, SeO4, and SeO3)). Bare SiO2 and Al2O3, as well as those coated with varied organics, were selected as geologically or synthetically representative substrates. Through the integration of state-of-the-art nanoscale interfacial characterization techniques with theoretical calculations, the complex interactions during nanocrystal formation at interfaces were probed and the controlling mechanisms were identified. For BaSO4 and Fe(OH)3 formation on substrates, the local supersaturation levels near substrates were controlled by Ba2+ adsorption and the electrostatic attraction of Fe(OH)3 monomer/polymer to substrates, respectively. Meanwhile, substrate hydrophobicity controlled the interfacial energy for the nucleation of both nanocrystals on (in)organic substrates. Metal ions’ (i.e., Cr/Al/Cu/Pb) hydrolysis constants and substrates’ dielectric constants controlled metal ion adsorption onto substrates, which altered the surface charges of substrates, thus controlling heterogeneous Fe(OH)3 nanocrystal formation on substrates by electrostatic interactions. The sizes and compositions of heterogeneous (Fe, Cr)(OH)3 and (Ba, Sr)(SO4, SeO4, SeO3) formed on substrates were found to be distinct from those of homogeneous precipitates formed in solution. The substrate (de)protonation could alter the local solution’s pH and the substrates’ surface charge; substrates could also adsorb cations, affecting local Fe/Cr/Ba/Sr ion concentrations at solid–water interfaces, thus controlling the amount/size/composition of nanocrystals by tuning their nucleation/growth/deposition on substrates. From slightly supersaturated solution, homogeneous coprecipitates of microsized (Ba, Sr)(SO4, SeO4, SeO3) formed through growth, with little Sr/Se(VI) incorporation due to higher solubilities of SrSO4 and BaSeO4 over BaSO4. While cation enrichment near substrates made the local solution highly supersaturated, nanosized coprecipitates formed on substrates through nucleation, with more Sr/Se(VI) incorporation due to lower interfacial energies of SrSO4 and BaSeO4 over BaSO4. The new insights gained advanced our understanding of the biogeochemical cycling of varied elements at solid–water interfaces and of the controlled synthesis of functional nanocrystals.

Molybdenum (Mo) is an essential nutrient for almost all organisms. However, at high concentrations, it can be toxic to animals and plants. This study investigated the interactions of Mo(VI) with iron oxyhydroxides during ferrihydrite bioreduction in the presence of Fe(III)-reducing Geobacter sulfurreducens. Here, we showed that Mo concentration controlled ferrihydrite phase transformation, leading to Mo release. With the biotic reduction of ferrihydrite and Fe(II) production, Mo(VI) reduction and Mo(IV)O2 formation were observed for the first time, which further immobilized Mo after surface adsorption of Mo(VI). At low Mo levels (Mo/Fe molar ratios of 1–2%), sufficient Fe(II) adsorption onto ferrihydrite resulted in its transformation into magnetite nanoparticles (>80%, ∼25 nm), which catalyzed the reduction of Mo(VI) to form Mo(IV)O2 and immobilized Mo. Contrastingly, at high Mo concentrations (Mo/Fe molar ratios of 5–10%), Mo(VI)O42– adsorption onto ferrihydrite limited Fe(II) adsorption; subsequently, less magnetite (<8–12%) formed while more goethite (∼30–50%, width and length >15 and 100 nm, respectively) and siderite (∼20–30%, width and length >100 and 200 nm, respectively) with larger particle sizes formed instead, causing Mo(VI) release due to lower Mo adsorption. This study provides a comprehensive understanding of the interaction mechanisms among Geobacter sulfurreducens, Mo(VI), and iron oxyhydroxides, enabling predictions and controls of long-term Mo mobility and Fe mineral transformation under a variety of biogeochemical scenarios.

The aggregation behavior of ferrihydrite nanoparticles (FNPs) can control the fate of associated aqueous contaminants, trace elements, and organic compounds. However, FNP aggregation is difficult to predict in the presence of organic matter (OM), given the heterogeneity in the OM properties. Five model OMs based on (poly)acrylic acid (PAA or AA) and polyethylene glycol with or without terminal carboxyl groups (PEG or PEGbis, respectively) were chosen to probe the influence of key OM properties─specifically, carboxyl richness and molecular weight (MW)─and the dominant mechanisms by which they influence OM adsorption onto FNPs and the resulting aggregation. For OMs with similar MWs, those with a higher carboxyl richness adsorbed more extensively onto FNPs: PAA2k > PEGbis > PEG. Meanwhile, for OMs with the same carboxyl richness, higher MW OMs adsorbed more: PAA25k > PAA2k > AA. Furthermore, the subsequent aggregation of FNPs was largely controlled by the adsorbed mass. OMs with negligible adsorption (i.e., PEG and AA) did not change the aggregation behavior of FNPs. For OMs with low carboxyl richness (PEGbis), accelerated aggregation occurred through a bridging effect with low adsorbed mass. For OMs with high carboxyl richness (PAA2k and PAA25k), aggregation was accelerated at moderate adsorbed OM masses by patch-charge attraction and was inhibited with high adsorbed OM mass due to steric repulsion. This study provided new insights into understanding and predicting the transport and fate of FNPs and natural organic matter (NOM) in natural environments with various NOM compositions.

ron/chromium hydroxide coprecipitation controls the fate and transport of toxic chromium (Cr) in many natural and engineered systems. Organic coatings on soil and engineered surfaces are ubiquitous; however, mechanistic controls of these organic coatings over Fe/Cr hydroxide coprecipitation are poorly understood. Here, Fe/Cr hydroxide coprecipitation was conducted on model organic coatings of humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA). The organics bonded with SiO2 through ligand exchange with carboxyl (–COOH), and the adsorbed amounts and pKa values of –COOH controlled surface charges of coatings. The adsorbed organic films also had different complexation capacities with Fe/Cr ions and Fe/Cr hydroxide particles, resulting in significant differences in both the amount (on HA > SA(–COOH) ≫ BSA(–NH2)) and composition (Cr/Fe molar ratio: on BSA(–NH2) ≫ HA > SA(–COOH)) of heterogeneous precipitates. Negatively charged –COOH attracted more Fe ions and oligomers of hydrolyzed Fe/Cr species and subsequently promoted heterogeneous precipitation of Fe/Cr hydroxide nanoparticles. Organic coatings containing –NH2 were positively charged at acidic pH because of the high pKa value of the functional group, limiting cation adsorption and formation of coprecipitates. Meanwhile, the higher local pH near the –NH2 coatings promoted the formation of Cr(OH)3. This study advances fundamental understanding of heterogeneous Fe/Cr hydroxide coprecipitation on organics, which is essential for successful Cr remediation and removal in both natural and engineered settings, as well as the synthesis of Cr-doped iron (oxy)hydroxides for material applications.

Mineral scale refers to the hard inorganic solids nucleated on substrates or deposited from the aqueous phase. The formation and deposition of barium sulfate and strontium sulfate in various industries, such as water treatment and oilfield operations, can significantly impact facility operations, posing serious threats. Machine learning (ML) approaches have been adopted recently in scale threat predictions to address the limitations of conventional scaling prediction models. However, there are few reports on collecting sulfate mineral scaling data, employing ML methods for data analysis, and evaluating the modeling results to gain deeper insights of sulfate mineral scaling process and to improve the accuracy of sulfate scaling threat prediction. Despite comprehensive experimental studies, the literature does not provide adequate guidance for identifying the influence on the solubility of barium sulfate and strontium sulfate under different aqueous environments and actual operating conditions. To this end, this study collected 1600 experimental datasets of barium/strontium sulfate from the literature to construct and evaluate the reliability and versatility of a ML-based model for sulfate solubility calculations. Single neural networks, hybrid neural networks, and optimization algorithms were employed to build solubility prediction models for barium sulfate and strontium sulfate across a wide range of temperatures, pressures, and different ions. The model's applicability in predicting sulfate scaling threats in various actual operating environments demonstrated its broad usability, consistent with its actual performance. This study marks the first stride towards constructing a reliable model for identifying the scaling trends of barium sulfate and strontium sulfate across various operating conditions, underscoring the importance of developing robust and accurate prediction models to address challenges in various industrial systems.

he simultaneous precipitation of (Fe, Cr)(OH)3 nanoparticles in solution (homogeneous) and on soil surfaces (heterogeneous), which controls Cr transport in soil and aquatic systems, was quantified for the first time in the presence of model surfaces, i.e., bare and natural organic matter (NOM)-coated SiO2 and Al2O3. Various characterization techniques were combined to explore the surface-ion-precipitate interactions and the controlling mechanisms. (Fe, Cr)(OH)3 accumulation on negatively charged SiO2 was mainly governed by electrostatic interactions between hydrolyzed ion species or homogeneous (Fe, Cr)(OH)3 and surfaces. The elevated pH through protonation of Al2O3 surface hydroxyls resulted in higher Cr/Fe ratios in both homogeneous and heterogeneous coprecipitates. Due to ignorable NOM adsorption onto SiO2, the amounts of (Fe, Cr)(OH)3 precipitates on bare/NOM-SiO2 were similar; contrarily, attributed to favored NOM adsorption onto Al2O3 and consequently carboxyl association with metal ions or (Fe, Cr)(OH)3 nanoparticles, remarkably more heterogeneous precipitates harvested on NOM-Al2O3 than bare-Al2O3. With the same solution supersaturation, the total amounts of homogeneous and heterogeneous precipitates were similar irrespective of the substrate type. With lower pH, decreased electrostatic forces between substrates and precipitates shifted (Fe, Cr)(OH)3 distribution from heterogeneous to homogeneous phases. The quantitative knowledge of (Fe, Cr)(OH)3 distribution and the controlling mechanisms can assist in better Cr sequestration in natural and engineered settings.

The poor colloidal stability of magnetite nanoparticles (MNPs) limits their mobility and application, so various organic coatings (OCs) were applied to MNPs. Here, a comparative study on the colloidal stability of MNPs coated with acetic (HAc) and polyacrylic acids (PAA) was conducted under varied pH (5.0–9.0) in the presence of different concentrations of cations and anions, as well as humic acid (HA). Comparing the effects of various cations and anions, the stability of both HAc/PAA-MNPs followed the order: Na+ > Ca2+and PO43− > SO42− > Cl−, which could be explained by their adsorption behaviors onto HAc/PAA-MNPs and the resulting surface charge changes. Under all conditions even with more anion adsorption onto HAc-MNPs (0.14–22.56 mg/g) than onto PAA-MNPs (0.04–18.34 mg/g), PAA-MNPs were more negatively charged than HAc-MNPs, as PAA has a lower pHIEP (2.6 ± 0.1) than that of HAc (3.7 ± 0.1). Neither the HAc nor PAA coatings were displaced by phosphate even at considerably high phosphate concentration. Compared with HAc-MNPs, the stability of PAA-MNPs was greatly improved under all studied conditions, which could be due to both stronger electrostatic and additional steric repulsion forces among PAA-MNPs. Besides, under all conditions, Derjaguin-Landau-Verwey-Overbeek (DLVO) explained well the aggregation kinetic of HAc-MNPs; while extended DLVO (EDLVO) successfully predict that of PAA-MNPs, indicating steric forces among PAA-MNPs. The aggregation of HAc/PAA-MNPs was all inhibited in varied electrolyte solutions by HA (2 mg C/L) addition. This study suggested that carboxyl coatings with higher molecular weights and pKa values could stabilize MNPs better due to stronger electrostatic and additional steric repulsion. However, in the presence of HA, these two forces were mainly controlled by adsorbed HA instead of the organic pre-coatings on MNPs.

Phosphate addition is commonly applied to remediate lead contaminated sites via the formation of lead phosphate particles with low solubility. However, the effects of natural organic matter (NOM) with different properties, as well as the contributions of specific interactions (particle-particle, particle-NOM, and NOM-NOM) in enhanced stabilization or flocculation of the particles, are not currently well understood. This study investigates the influence of two aquatic NOM and two soil or coal humic acid (HA) extracts on the aggregation behavior of lead phosphate particles and explores the controlling mechanisms. All types of NOM induced disaggregation and steric stabilization of the particles in the presence of Na+ (100 mM) or low (1 mM) Ca2+ concentrations, as well as at low NOM concentrations (1 mgC/L). However, for the soil and coal HA, a threshold at NOM concentrations of 10 mgC/L and high (3 mM) Ca2+ concentrations was observed where bridging flocculation (rather than steric stabilization) occurred. In situ attenuated total reflectance – Fourier transform infrared characterization confirmed adsorption of the soil and coal humic acid extracts (10 mgC/L) onto the surface of the lead phosphate particles in 3 mM Ca2+, whereas dynamic and static light scattering demonstrated extensive HA flocculation that dominated the overall scattered light intensities. These results imply that the accelerated aggregation was induced by a combination of HA adsorption and bridging flocculation by Ca2+. Overall, this research demonstrates that the type of NOM is critical to predict the colloidal stability of lead phosphate particles. Aquatic NOM stabilized the particles under all conditions evaluated, but soil or coal HA with higher molecular weight and aromaticity showed highly variable stabilization or flocculation behavior depending on the HA and Ca2+ concentrations available to adsorb to the particles and participate in bridging. These results provide new mechanistic insights on particle stabilization or destabilization by NOM.

More than two billion people worldwide have suffered thyroid disorders from either iodine deficiency or excess. By creating the national map of groundwater iodine throughout China, we reveal the spatial responses of diverse health risks to iodine in continental groundwater. Greater non-carcinogenic risks relevant to lower iodine more likely occur in the areas of higher altitude, while those associated with high groundwater iodine are concentrated in the areas suffered from transgressions enhanced by land over-use and intensive anthropogenic overexploitation. The potential roles of groundwater iodine species are also explored: iodide might be associated with subclinical hypothyroidism particularly in higher iodine regions, whereas iodate impacts on thyroid risks in presence of universal salt iodization exhibit high uncertainties in lower iodine regions. This implies that accurate iodine supply depending on spatial heterogeneity and dietary iodine structure optimization are highly needed to mitigate thyroid risks in iodine-deficient and -excess areas globally.

Magnetite nanoparticles (MNPs) with varied organic coatings (OCs) which improved their stability have broad environmental applications. However, the adsorbed amounts and layer thickness of varied OCs onto MNPs during the synthesis were generally not or poorly characterized, and their interactions with natural organic matter (NOM) were still in progress. In this study, acetic (HAc), citric (CA), and polyacrylic acid (PAA) were selected as model OCs, the adsorption behaviors of OCs on MNPs were characterized under varied aqueous C/Fe ratios, and the aggregation behaviors of MNPs with varied OCs (OC-MNPs) at neutral pH (7.0 ± 0.2) with NaCl (5–800 mM) in the presence/absence of NOM were systematically investigated. Under low aqueous C/Fe ratio, the adsorbed amounts of model OCs as –COOH/Fe ratio followed the order: CA ≈ PAA >> HAc. With high aqueous C/Fe ratio, the maximum adsorbed masses of OC-MNPs were similar. The adsorbed layer thicknesses of OC-MNPs were thoroughly characterized using three different methods, all showing that the adsorbed layer of PAA was thicker than that of CA and HAc. Derjaguin–Landau–Verwey–Overbeek (DLVO) and extended DLVO (EDLVO) calculations showed that electrostatic and van der Waals forces were dominant for CA-MNPs and HAc-MNPs stabilization; while steric repulsion played major roles in stabilizing PAA-MNPs, probably due to a thicker PAA layer. In the presence of NOM, stability behaviors of all OC-MNPs were similar, ascribing to the much greater amounts of NOM adsorbed than the OCs, causing greater steric repulsion. This study provides new mechanistic insights which could help better understand the effects of varied OCs on MNPs' colloidal stability.