Cadmium (Cd) is one dangerous and widespread heavy metal that of great environmental concern. To cost-efficiently adsorb aqueous Cd under influence of various factors, this study succeeded in fabricating goethite-modified biochar (GBC) derived from distillers’ grains (DGs) for Cd sorption of different concentrations (10–100 mg L⁻¹) at pH of 3, 6 and 8 with and without microcystin-LR (MC-LR). Sorption kinetics and isotherms data revealed that Cd sorption capacity of GBC and unmodified BC increased as pH elevated from 3 to 6 but stabilized when pH further elevated to 8. Pseudo-second-order and Langmuir models more accurately fitted to sorption data for both BCs, implying monolayer chemisorption of Cd onto BCs. GBC exhibited more robust sorption for each Cd concentration than unmodified BC, with the maximum sorption capacity of around 28 mg g⁻¹ at neutral and weak alkaline pH. Notably, goethite-modification obviously increased bulk polarity, specific surface area, porosity and surface oxygenic group abundance of BC, thus strongly enhancing Cd sorption by creating more sorption sites mainly via pore-filling, electrostatic attraction, and also via complexation and cation exchange. Co-existing MC-LR of 100 μg L⁻¹ did not obviously affect Cd sorption by both BCs for most Cd levels at each pH, mostly because sorption mechanisms diverged between MC-LR and Cd to largely avoid their competition for sorption sties. Thus, goethite could modify DG-BC as promising and cost-efficient sorbent for Cd even with co-existing MC-LR, especially at neutral and weak alkaline pH that common in the nature. This study was greatly implicated in modifying and applying DG-BC for Cd immobilization in MC-LR laden waters with various pH circumstances.

Understanding the arsenic (As) enrichment mechanisms in the subsurface environment relies on a systematic investigation of As valence species and their partitioning with the Fe (oxyhydr)oxide phases in the subsoil profile. The present study explored the distribution, speciation, partitioning, and (im)mobilization of As associated with Fe in four subsoil cores (∼30 m depth) from Hong Kong using sequential chemical extraction and X-ray absorption near edge spectroscopy. The subsoil profiles exhibited relatively high concentrations of As at 26.1–982 mg/kg (median of 112 mg/kg), and the As was dominated by As(V) (85–96%) and primarily associated with the residual fraction (50.7–94.7%). A small amount of As (0.002–13.2 mg/kg) was easily mobilized from the four subsoil profiles, and a concentration of water-soluble As higher than 100 μg/L was observed for only some subsoil layers. The molar ratios of As:Fe in the oxalate-extractable Fe fraction ranged from 1.2 to 76.5 mmol/mol (median of 11.1 mmol/mol), revealing the participation of poorly crystalline Fe (oxyhydr)oxides in immobilizing most of the high geogenic As. The primary phases of ferric (oxyhydr)oxides were characterized as ferrihydrite (16–53%), lepidocrocite (0–32%), and goethite (0–62%), and these phases contributed to the sufficient ability of the subsoil to sequester 45.3–100% (median of 98.8%) of the exogenous As(V) (1.0 mg/L) in adsorption experiments. In contrast to As(V), exogenous As(III) showed a lower removal percentage (3.9–79.1%, median of 45.1%). The study revealed that the chemical speciation of As and Fe in the subsoil profiles is useful for predicting the immobilization of high geogenic As in the region, which is also helpful for the safe utilization of As-containing soil during land development worldwide.

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.

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g⁻¹ at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

Hexavalent chromium has aroused a series of environmental concerns due to its high mobility and toxicity. Iron and manganese oxides usually coexist in the environments and influence the speciation and geochemical cycling of chromium. However, the interaction mechanism of iron-manganese oxides with dissolved Cr(VI) remains largely unknown. In this work, the interaction processes of dissolved Cr(VI) and manganite in the presence of goethite coating were investigated, and the effects of pH (2.0–9.0) and iron oxide content were also studied. Manganite-goethite composites were formed with uniform micromorphologies in the system of manganite and Fe(II). In the reaction system of single manganite and Cr(VI), manganite could only adsorb but not reduce Cr(VI), with the adsorption amount decreasing at higher pHs. In the reaction system of manganite-goethite composites and Cr(VI), adsorbed Cr(VI) was reduced to Cr(III) by Fe(II) on composites surface. The generated Cr(III) was then retained as Cr(OH)₃ on the mineral surface. Goethite coating suppressed the re-oxidation of newly formed Cr(III) by manganite. The amounts of adsorbed Cr(VI) and generated Cr(III) increased with increasing iron oxide content, and increased first and then decreased with increasing pH. The Cr(III) formation and Cr(VI) adsorption amount reached the maximum at pH 5.0–6.0. The present work highlights the transformation and retention of Cr(VI) by iron-manganese oxides and provides potential implications for the use of such oxides in the remediation of Cr(VI) polluted waters and soils.

Multi-walled carbon nanotubes (MWCNTs) are increasing used in commercial applications and may be released into the environment with anionic surfactants, such as sodium dodecylbenzenesulfonate (SDBS), in sewer discharge. Little research has examined the transport, retention, and remobilization of MWCNTs in the presence or absence of SDBS in porous media with controlled chemical heterogeneity, and batch and column scale studies were therefore undertaken to address this gap in knowledge. The adsorption isotherms of SDBS on quartz sand (QS), goethite coated quartz sand (GQS), and MWCNTs were determined. Adsorption of SDBS (MWCNTs » GQS > QS) decreased zeta potentials for these materials, and produced a charge reversal for goethite. Transport of MWCNTs (5 mg L⁻¹) dramatically decreased with an increase in the fraction of GQS from 0 to 0.1 in the absence of SDBS. Conversely, co-injection of SDBS (10 and 50 mg L⁻¹) and MWCNTs radically increased the transport of MWCNTs when the GQS fraction was 0, 0.1, and 0.3, especially at a higher SDBS concentration, and altered the shape of retention profile. Mathematical modeling revealed that competitive blocking was not the dominant mechanism for the SDBS enhancement of MWCNT transport. Rather, SDBS sorption increased MWCNT transport by increasing electrostatic and/or steric interactions, or creating reversible interactions on rough surfaces. Sequential injection of pulses of MWCNTs and SDBS in sand (0.1 GQS fraction) indicated that SDBS could mobilize some of retained MWCNTs from the top to deeper sand layers, but only a small amount of released MWCNTs were recovered in the effluent. SDBS therefore had a much smaller influence on MWCNT transport in sequential injection than in co-injection, presumably because of a greater energy barrier to MWCNT release than retention. This research sheds novel insight on the roles of competitive blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention and release.

The behaviour of arsenic (As) from geogenic soil exposed to aerobic conditions is critical to predict the impact of As on the environment, which processes remain unresolved. The current study examined the depth profile of As in geologically derived subsoil cores from Hong Kong and investigated the mobilization, plant availability, and bioaccessibility of As in As-contaminated soil at different depths (0–45.8 m). Results indicated significant heterogeneity, with high levels of As in three layers of soil reaching up to 505 mg/kg at a depth of 5 m, 404 mg/kg at a depth of 15 m, and 1510 mg/kg at a depth of 27–32 m. Arsenic in porewater samples was <11.5 μg/L in the study site. X-ray absorption spectroscopy (XAS) indicated that main As species in soil was arsenate (As(V)), as adsorbed fraction to Fe oxides (41–69% on goethite and 0–8% on ferrihydrite) or the mineral form scorodite (30–57%). Sequential extraction procedure demonstrated that 0.5 ± 0.4% of As was exchangeable. Aerobic incubation experiments exhibited that a very small amount (0.14–0.48 mg/kg) of As was desorbed from the soil because of the stable As(V) complex structure on abundant Fe oxides (mainly goethite), where indigenous microbes partly (59 ± 18%) contributed to the release of As comparing with the sterilized control. Furthermore, no As toxicity in the soil was observed with the growth of ryegrass. The bioaccessibility of As was <27% in the surface soil using simplified bioaccessibility extraction test. Our systematic evaluation indicated that As in the geogenic soil profile from Hong Kong is relatively stable exposing to aerobic environment. Nevertheless, children and workers should avoid incidental contact with excavated soil, because high concentration of As was present in the digestive solution (<0.1–268 μg/L).

Environmental mobility of C60 fullerene can be significantly affected in the presence of naturally abundant α-FeOOH. However, α-FeOOH vary significantly in sizes, shapes and associated properties that can greatly influence the fate and transport of C60 fullerene in environmental media. Therefore, colloidal hetero-association between well crystallized low aspect (LAsp) α-FeOOH and nC60 fullerene may differ substantially to weakly crystallized high-aspect (HAsp) counterpart. In contrast to LAsp α-FeOOH, inherent crystal defects and surface charge generation in HAsp α-FeOOH facilitated strong Coulombic attraction and aggregation with fullerene in acidic pH. However, LAsp α-FeOOH demonstrated subtle entropic depletion mediated interaction with fullerene prevalent in hard rods. Humic acid (HA) encapsulation of HAsp α-FeOOH substantially blocked fullerene attachment. Minute enhancement in colloidal stability was detected for HA-coated HAsp α-FeOOH and fullerene mixture to HA-coated HAsp α-FeOOH alone. To investigate the interfacial assembly of α-FeOOH with fullerene “in situ” differential interference contrast (DIC) microscopic investigations were employed. This study showed significantly different interface behavior of the binary mixtures of fullerene and HAsp α-FeOOH NPs, and LAsp particles. On air-water interface, bare HAsp α-FeOOH displayed liquid crystalline packing. However, addition of fullerene to HAsp α-FeOOH suspension at pH5 produced closed-loop polygonal and circular ring structures. Head-to-tail alignment of magnetic dipoles as well as fullerene hydrophobicity facilitated such assembly formation. “Ex situ” AFM investigation revealed further the presence of magnetically derived ring structure which asserts that the formed “in situ” ensembles were not transient, hence, may abate fullerene transport through environmental interfaces. Barring hydrophobicity assisted attachment of fullerene to LAsp α-FeOOHs, the absence of any close-packed structures may unlikely abate fullerene transport as envisaged in case of HAsp α-FeOOH. Thus, aspect ratio variation and associated material properties of naturally abundant α-FeOOH may significantly impact fullerene transport through environmental media.

Critical factors leading to arsenic release and attenuation from the shallow subsurface were studied with multidisciplinary approach in the natural gold–arsenic geochemical anomaly at Mokrsko (Czech Republic). The results show that microbial reduction promotes arsenic release from Fe(III) (hydr)oxides and Fe(III) arsenates, thereby enhancing dissolved arsenic in the shallow groundwater at average concentration of 7.76 mg/L. In the organic-rich aggregates and wood particles, however, microbial sulfate reduction triggers the formation of realgar deposits, leading to accumulation of As in the distinct organic-rich patches of the shallow subsurface. We conclude that precipitation of realgar in the shallow subsurface of soil/sediment depends on specific and non-trivial combination of water and rock chemistry, microbial community composition and spatial organisation of the subsurface zone, where speciation in saturated environments varied on a centimeter scale from reduced (decomposed wood, H2S and realgar present) to oxidized (goethite and arsenate minerals are present).

Adsorption and transport of levofloxacin (LEV) and ofloxacin (OFL) enantiomers in a matrix containing goethite and natural organic matter (NOM) were investigated using batch and column experiments. In batch studies, competition and enantioselectivity were observed in the adsorption of LEV and OFL. Enantioselectivity upon adsorption was investigated by comparing changes in the enantiomer fraction (EF) (the ratio of LEV to the sum of LEV and OFL remaining in the solution) after and before adsorption. At pH < 7, there was hardly any selectivity in adsorption of OFL and LEV to goethite. At pH > 7, OFL showed a stronger adsorption than LEV to goethite, and this preference remained when NOM samples of Leonardite humic acid (LHA) and Elliott Soil fulvic acid (ESFA) were added. However, when Suwannee River NOM (SRNOM) was added, the preference was reversed, and LEV was adsorbed more strongly. In single systems, the presence of different types of NOM increased adsorption of LEV and OFL, especially LEV. In column studies, preloaded NOM decreased the transport of LEV and OFL through goethite-coated sand. The EF values in the effluent increased with retention time and reached the largest values (0.59–0.72) at around 1.5 pore volume (PV), and then decreased again, reaching a stable value at 5.0–30.0 PV. Both batch and column experiments showed that, fractionation of LEV and OFL occurred during adsorption and transport in the presence of NOM-goethite complexes, which would eventually affect their environmental fate