Humic Acid Colloids Research Articles

Transforming contaminant ligands at water–solid interfaces via trivalent metal coordination

In environmental matrices, the migration and distribution of contaminants at water–solid interfaces play a crucial role in their capture or dissemination. Scientists working in environmental remediation and wastewater treatment are increasingly aware of metal–contaminant coordination; however, interfacial behaviors remain underexplored. Here, we show that trivalent metal ions (e.g. Al3+ and Fe3+) mediate the migration of pollutant ligands (e.g. tetracycline (TC) and ofloxacin) to the organic solid interface. In the absence of Al3+, humic acid (HA) colloids (50 mg/L) capture 26.1 % of the TC in water (initial concentration: 10 mg/L) via weak intermolecular interactions (binding energy: −5.71 kcal/mol). Adding Al3+ (2.5 mg/L) significantly enhances the binding of TC to an impressive 94.2 % via Al3+ mediated coordination (binding energy: −84.89 kcal/mol). The significant increase in binding energy results in superior interfacial immobilization. However, excess free Al3+ competes for TC binding via direct binary coordination, as confirmed based on the unique fluorescence of Al3+–TC complexes. Density functional theory calculations reveal the intricate process of HA–Al3+ binding via carboxyl and phenolic hydroxyl sites. The HA–Al3+ flocs then leverage the remaining coordination capacity of Al3+ to chelate with TC. As well as providing insights into the pivotal role of metal ion on the self-purification of natural water bodies, our findings on the interfacial behavior of metal–contaminant coordination will propel coagulation technology to the capture of microscale pollutants.

Investigating the influence of bentonite colloids on strontium sorption in granite under various hydrogeochemical conditions

The disposal of high-level radioactive waste in deep geological repositories is a critical environmental issue. The presence of bentonite colloids generated in the engineering barrier can significantly impact the transport of radionuclides, but their effect on radionuclide sorption in granite remains poorly understood. This study aimed to investigate the sorption characteristics of strontium (Sr) on granite as well as on the coexistence system of granite and colloids under various hydrogeochemical conditions, through batch experiments. Fourier transform infrared spectroscopy was employed to analyze the sorption forms of Sr on granite before and after sorption. Several hydrogeochemical factors were examined, including contact time, pH, ionic strength, coexisting ions, and bentonite and humic acid colloid concentration. Among these factors, the concentration of bentonite colloids exhibited a significant effect on Sr sorption. Within a specific range of colloid concentration, the sorption of Sr on the solid system increased linearly with the bentonite colloid concentration. pH and ionic strength were also found to play crucial roles in the sorption process. At low pH, Sr sorption primarily occurred through the outer sphere's surface complexation and Na+/H+ ion exchange. However, at high pH, inner sphere surface complexation dominated the process. As the ionic strength increased, electrostatic repulsion gradually increased, resulting in fewer binding sites for particle aggregation and Sr sorption on bentonite colloids. The results also indicate that with increasing pH, the predominant forms of Sr in the solution transitioned from SrHCO3+ and SrCl+ to SrCO3 and SrCl+. This was mainly due to the ion exchange of Ca2+/Mg2+ in plagioclase and biotite, forming SrCO3 precipitation. These findings provide valuable insights into the transport behavior of radionuclides in the subsurface environment of the repository and highlight the importance of considering bentonite colloids and other hydrogeochemical factors when assessing the environmental impact of high-level radioactive waste disposal.

Adsorption and migration of ammonia nitrogen in sediments in the presence of SiO2 and HA colloids

The release of internal pollutants in shallow lakes is a widely concerning environmental issue. Colloids widely existing in eutrophic shallow lakes will affect the release and migration of pollutants. In this context, the different effects of two colloids [humic acid (HA) and silica (SiO2)], either individual or in coexistence, on the adsorption and migration of ammonia nitrogen (NH4+-N) in sediments under different ionic strengths (IS) were analyzed, through a series of batch and column experiments. It was found that, under a relatively low IS condition (1 mM Na+), SiO2 colloids greatly enhanced the mobility of NH4+-N in sediments. With IS increased (20 mM Na+), the presence of colloids exhibited an impeding effect on the mobility of NH4+-N due to colloidal flocculation and precipitation. While HA colloids largely suppressed the mobility of NH4+-N in sediments under either low or high IS conditions. HA colloids are more likely to form aggregates and have more adsorption sites than SiO2 colloids. In the NH4+-N–SiO2–HA system, HA colloids also showed a retardant effect on the transport of both SiO2 colloids and NH4+-N, and the retardant effect increased with the increase of HA colloid concentration. Exchange coordination and hydrogen bond force, as well as the decreased stability of the colloid system, played an important role in such a retardant effect, which consequently resulted in more NH4+-N adsorption. The transport of colloids in the NH4+-N–colloids co-transport system can be successfully described by Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. In addition, with an increase of IS, NH4+-N transport was enhanced because of competitive adsorption of the cations and NH4+-N on colloids and the porous medium. These results implied that abundant HA colloids and low ion concentration can reduce the risk of NH4+-N release and migration in sediments and consequently its exchange flux across the sediment-water interface.

Natural colloids facilitated transport of steroidal estrogens in saturated porous media: Mechanism and processes

Steroid estrogens (SEs) as typical endocrine disrupting compounds (EDCs) are widely detected in terrestrial environment, whilst the transport of SEs in groundwater remains unwell understood. Specifically, the effects of ubiquitous natural colloids on the SEs transport are unclear in subsurface environment, especially in aquifer systems. Here, the influence of inorganic colloids (i.e. silica and illite) and organic colloids, i.e. Humic acid (HA), on the transport of estrone (E1) and estradiol (E2) in saturated porous media was studied utilizing laboratory scale column experiments. Characterization on the colloids and porous aquifer material was conducted to provide a basis for interpretation of the experimental findings. Results showed that the transport of SEs was clearly affected by the natural colloids migrating through the saturated porous media. About 38.5% of E1 and 24.6% of E2 were retained in the column when colloids were absent in the system. When transporting with silica colloids, illite colloids, and HA colloids, the transport of E1 was enhanced by 15.64%, 11.17% and 25.60%, respectively; whilst the transport of E2 was improved by 19.56%, 23.06% and 36.40%, respectively. The SEs transport enhancement by colloids depended upon not only the mobility of the colloids but also their geochemical characteristics. The organic colloids showed 1.5–2.5 times greater ability on promoting the transport of SEs than the inorganic ones tested in this study. The proposed mechanisms of nature colloids facilitated transport of SEs including competing for adsorption sites on the sand surfaces by the colloids resulting mobilization of adsorbed SEs from solid matrix, and transport of colloids as carriers for SEs.

Cotransport of uranyl carbonate loaded on amorphous colloidal silica and strip-shaped humic acid in saturated porous media: Behavior and mechanism

Uranyl carbonate (UC(VI)) is a stable form of uranyl (U(VI)) that widely coexists with amorphous colloidal silica (ACSi) and humic acid (HA) in carbonate-rich U-contaminated areas. In this context, the cotransport behavior and mechanism of UC(VI) with ACSi (100 mg L−1) and HA colloids in saturated porous media were systematically investigated. It was found that the ACSi and strip-shaped HA have a strong adsorption capacity for UC(VI), and their adsorption distribution coefficient (Kd) is 4–5 orders of magnitude higher than that of quartz sand (QS). In the ternary system, UC(VI) was mainly existing in the colloid-associated form at low UC(VI) concentration (4.2 × 10−6 M). Compared with the individual transport of UC(VI), the presence of ACSi and strip-shaped HA in the binary system promotes the transport of low-concentration UC(VI) (4.2 × 10−6 M) but shows a hindering effect when UC(VI) = 2.1 × 10−5 M. When ionic strength (IS) increased from 0 to 100 mM, the individual transport of UC(VI) and ACSi was weakened owing to the masking effect and the compression of the electrical double layer, respectively; this weakening effect is more pronounced in the binary (UC(VI)–ACSi) system. Notably, the transport of UC(VI) and ACSi in the ternary system is independent of the changes in IS due to the surface charge homogeneity strengthening the electrostatic repulsion between HA and QS. The Derjaguin–Landau–Verwey–Overbeek theory and retention profiles reveal the co-deposition mechanism of ACSi and UC(VI) in the column under different hydrochemical conditions. The nonequilibrium two-site model and the mathematical colloidal model successfully described the breakthrough data of UC(VI) and ACSi, respectively. These results are helpful for evaluating the pollution caused by UC(VI) migration in an environment rich in HA and formulating corresponding effective control strategies.

Soil colloids affect the aggregation and stability of biochar colloids

The stability of biochar colloids plays an important role in the transport and fate of contaminants and nutrients in soil. This study aimed to investigate the effects of main soil components, kaolin (Kao), goethite (Goe), and humic acid (HA) colloids on the aggregation kinetics of biochar colloids derived from dairy manure (DM), sewage sludge (SS), and wheat straw (WS). The WS biochar colloid had the highest critical coagulation concentration (CCC) (624 mM) than that of SS (200 mM) and DM (75 mM) due to its richest hydroxyl and carboxyl groups, showing the highest stability. Kao markedly improved the stability of DM and SS biochar colloids with 171% and 52.5% increase of CCC, respectively, by increasing the electrostatic repulsion of the system. However, the WS biochar colloid became more aggregated in the presence of Kao since the hydroxyl and carboxyl functional groups in WS biochar colloid could complex with Kao, generating electrostatic shielding. Goe could rapidly combine with biochar colloids via electrostatic attraction, resulting in the aggregation of SS and WS, while the aggregation rate of DM/Goe mixed colloids was inhibited. The HA increased the electrostatic repulsion of all biochar colloids through adsorbed on the surface of biochar colloids, resulting in the increased steric hindrance and stability of biochar colloids, with the CCC increased from 75 to 624 mM to 827–1012 mM. Our findings reveal that soil kaolin, goethite, and humic acid colloids have remarkable effects on the stability and aggregation of biochar colloid, which will advance understanding of the potential environmental fate and behaviors of biochar colloids.

Structural Study of Polystyrene-b-poly(acrylic acid) Micelles Complexed with Uranyl: A SAXS Core-Shell Model Analysis.

The interactions between natural colloidal organic matter and actinides in solutions are complex and not fully understood. In this work, a crew-cut polystyrene-b-poly(acry1ic acid) (PS-b-PAA) micelle is proposed as a model particle for humic acid (HA) colloid with the aim to better understand the sequestration, aggregation, and mobility of HA colloids in the presence of uranyl ions. The effects of uranyl ions on the structure of PS29k-b-PAA5k micelles in aqueous solution were mainly investigated by synchrotron small-angle X-ray scattering. A core-shell model, accounting for the thickness and contrast changes of the PAA corona induced by the adsorption of uranyl, was employed to analyze the scattering data. A combination of transmission electron microscopy, dynamic light scattering, and zetametry showed a strong affinity of uranyl ions to PAA segments in water at pH 4-5 that resulted in the shrinkage and improved contrast of the PAA corona, as well as colloidal destabilization at a high uranyl concentration.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions.

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH4+ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH4+ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH4+, the co-transport mechanism became significantly different. During transport by the NH4+-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH4+, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH4+. The co-transport of NH4+ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH4+ to be more likely to be transported at a higher ionic strength (IS = 0.05m, CaCl2) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH4+ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20mg/L then declines at 40mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Transport behaviors of humic acid, kaolin and kaolin\u2013humic acid compound colloids on naphthalene in floating islands with biofilmed fillers

As one of the persistent organic pollutants, the investigations of naphthalene transports with humic acid, kaolin and kaolin–humic acid compound colloids by passing through the floating islands with biofilmed filler matrix pores were checked out. Naphthalene was easily captured onto the biofilmed fillers while it was passing through the floating islands with the filling percentages of 0.0 to 60.0 by absorbing bigger contaminated colloids, reflecting that the naphthalene concentrations in influents were, respectively, decreased from 200 to 57 μg L−1 for the humic acid colloids, to 32 μg L−1 for the kaolin colloids and to 13 μg L−1 for the kaolin–humic acid compound colloids. With the filling percentages of 60.0 to 80.0, the naphthalene concentrations in effluents were maintained at 57 ± 2 μg L−1 for the humic acid colloids, at 32 ± 3 μg L−1 for the kaolin colloids and at 13 ± 3 μg L−1 for the kaolin–humic acid compound colloids, respectively. The naphthalene distribution amounts on the biofilmed fillers were, respectively, from 213 to 96 ng m−1 for the humic acid colloids, from 274 to 96 ng m−1 for the kaolin colloids and from 340 to 83 ng m−1 for the kaolin–humic acid compound colloids. Thus, the biofilmed fillers in the floating islands could decrease the naphthalene concentrations by the captures of the humic acid, kaolin and kaolin–humic acid compound colloids to make the contaminated colloid concentrations reduced and promote the water qualities.

Characteristic of adsorption, desorption, and co-transport of vanadium on humic acid colloid.

Understanding the interactions between humic acid colloid (HAC) and vanadium (V) in soils is of great importance in forecasting the behaviors and fates of V in the soil and groundwater systems. This study investigated the characteristics and factors that affect V adsorption-desorption by the HAC; meanwhile, we also explored the co-transport of the HAC and V in a saturated porous media. Scanning Electronic Microscopy micrographs showed the variation of morphological features on the surface of the HAC before and after V adsorption. Fourier transform infrared spectroscopy spectra revealed that the presence of hydroxyl, carboxyl, carbonyl, carbon-carbon double bond, amino, and aromatic ring on the HAC participated in V adsorption. The adsorption isotherms were well described by the Langmuir model, and the adsorption kinetics of the HAC was better described by the pseudo-first-order kinetic models. The adsorption-desorption was strongly dependent on the initial V concentration, solution pH, and temperature. The maximum adsorption amount was 861.17mgg-1 by 200mgL-1 HAC at the initial V concentration of 500mgL-1, and the corresponding desorption amount was 15.13mgg-1. These results showed that the HAC had high fixation capacity of V in soil. In addition, the HAC sped up the mobility of V; however, it decreased mass of migration of V in the saturated quartz sand column. These results are expected to provide insight into the potential impact of HAC on geochemical behaviours of V in vulnerable ecosystems.

Effect of a humic acid colloid on the sorption behaviour of Sr onto soil in a candidate high-level radioactive waste geological disposal site

We explored the effect of the presence or absence of humic acid (HA) on the sorption behaviour of Sr onto soil. We examined three different experimental cases for Sr sorption: (1) sorption in the presence of only colloidal HA, (2) sorption in the presence of only soil and (3) sorption in the presence of both colloidal HA and soil (HS). A batch technique was used to study the influencing factors, including the amount of colloidal HA, solid content, pH, initial concentration of Sr and contact time. The experiments showed that the influencing factors significantly affected the sorption process. For example, in the case of soil and HS, the sorption percentage increased rapidly with increasing solid content at m/V < 20g/L, changing from 8.35% and 37.54% to 49.09% and 77.03%, respectively. Moreover, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize samples. The kinetics and isotherms of Sr were best described by the pseudo-second-order and Langmuir models, which indicated that the process was controlled by chemisorption and uniform monolayer sorption with constant energy on the outer surface. These findings provide valuable information for predicting strontium migration in radioactive waste disposal sites.

Metals and Metalloid Removal by Colloidal Humic Acid–Goethite: Column Experiments and Geochemical Modeling

Core Ideas Humic‐acid‐coated goethite effectively removes cationic metals and As(V) from water. Among five elements, sorption of As(V), Cu, and Zn is favored with higher selectivity. In cation–oxyanion binary systems, As(V) enhanced Zn sorption (synergism). Metals less strongly sorbed are desorbed in mixed contamination due to competition for sorption sites. Multi‐surface complexation model in PHREEQC described As(V), Cu, Cd, Ni, and Zn BTCs. Reactive barriers are proposed to scavenge groundwater contaminants such as metals and arsenate. Their remediation capacity is commonly investigated with sorption data disregarding the complexity of the interactions among elements when faced with mixed contamination. This study was set up to measure interactions during reactive transport of the anionic arsenate As(V) and cationic metals and test if that transport can be modeled with geochemical codes. Sand coated with colloidal humic acid–goethite was the adsorbent in line with field work where these colloidal particles have been injected in situ. Equilibrium batch adsorption experiments showed binding preferences in the order: As(V) ∼ Cu &gt;&gt; Zn &gt; Cd &gt; Ni. Column experiments with single and binary mixtures showed an important synergistic effect of As(V) on Zn sorption but the reverse interaction was not found; this interaction is not included in the PHREEQC geochemical code. The breakthrough curves (BTCs) of mixtures of As(V), Cd, Cu, Ni, and Zn showed that Cd and Ni concentrations in the effluent exceeded those of the influent, an overshooting effect attributed to competition among divalent cations. Simulations with the PHREEQC geochemical code using a surface complexation model for sorption reactions on goethite and humic acids coupled to the transport module successfully predicted the most essential trends of the BTCs, illustrating the potential of these codes to estimate the longevity of the reactive barriers used to remediate mixtures of contaminants. The novel aspect of this work is that a correct reactive transport model in PHREEQC was obtained by only adjusting the number of reactive surface sites of the adsorbent from fitting batch sorption distribution coefficient (Kd) values.

Distinct effect of humic acid on ferrihydrite colloid-facilitated transport of arsenic in saturated media at different pH

Both humic acid and arsenic (As) have a strong affinity to ferrihydrite colloids (FHC). However, little is known about effects of humic acid colloid (HAC) and FHC interaction on transport and deposition of As in porous media. Co-transport of HAC-FHC and As at different pH was systematically investigated by monitoring their breakthrough curves (BTCs) in saturated sand columns. Colloid and solute transport models were used to reveal mechanisms of As transport with HAC-FHC in porous media. Results showed that HAC-FHC regulated As transport. Under neutral pH conditions, As transport was facilitated by FHC loading small chain-shaped HAC, while it was slightly retarded by FHC loading granular HAC. Under alkaline conditions, HAC was chain-shaped and coupled with FHC to facilitate As transport. However, mobile colloid-adsorbed As was less evident due to relatively low adsorption on FHC under alkaline pH in comparison with the transport under neutral pH. The presence of HAC occupied adsorption sites on FHC and thus decreased As adsorption. Arsenic adsorption on mobile FHC decreased with increasing HAC concentration. Therefore, HAC had an antagonistic effect on mixed colloid-facilitated As transport at neutral and alkaline conditions. Arsenic was readily co-deposited with FHC, especially at low pH. This study suggested that morphology and concentration of HAC, mobility of FHC, and solution pH would control As transport in porous media. The fate of As changed from co-transport with HAC-FHC to co-deposition when environmental pH decreased.

Fe–colloid cotransport through saturated porous media under different hydrochemical and hydrodynamic conditions

To investigate the effect of different colloids on Fe migration in saturated porous media under different hydrochemical and hydrodynamic conditions, experiments were performed using colloidal silicon (inorganic) and colloidal humic acid (HA, organic), which are representative of the colloids in groundwater. Transport of Fe with and without colloid was investigated by column experiments using various porous media, colloid concentrations, ionic strengths (ISs), cation valences, and flow rates. The results show that colloidal silicon promotes and colloidal HA inhibits Fe transport, which is mainly because of their different bonding ratio, bonding modes with Fe and opposite surface charges between Fe–colloidal silicon and Fe–colloidal HA. Almost 100% of HA binds to Fe through the deprotonated functional groups, whereas only 13.3% of colloidal silicon binds to Fe, which is by electrostatic forces. Cotransport is also dependent on the hydrochemical and hydrodynamic conditions. For the Fe–colloidal silicon system, increasing the colloid concentration and flow rate, and decreasing the IS enhances Fe transport. Compared with colloidal silicon concentration = 10 mg/L, flow rate = 0.25 mL/min, and IS = 0.05 with CaCl2, a higher colloidal silicon concentration (20 mg/L), a higher flow rate (0.50 mL/min), and a lower IS (<0.0005 M) increase Fe recovery by 1.69%, 94.49% and 38.92%, respectively. Fe migration is also different in different porous media. For the Fe–colloidal HA system, Fe recovery decreases by 81.46% as the colloidal HA concentration increases from 0 to 20 mg/L. The type of porous medium and flow rate conditions have the same effects on Fe–colloidal HA transport as for colloidal silicon, although the electrical conditions have the opposite effect. With increasing IS, Fe–colloidal HA transport is enhanced because of competitive adsorption of the cations and Fe to colloidal HA and the porous medium.

Radionuclide uptake by colloidal and particulate humic acids obtained from 14 soils collected worldwide

Uptake of six particle-reactive and/or redox-sensitive radionuclides (210Pb, 234Th, 7Be, 59Fe, 237Np and 233Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions (pH~5.5). In HA-groundwater slurry, Pb, Be, Fe and Pa bound strongly to particulate HA (>0.45 µm), supporting their application as tracers of soil erosion. Th bound strongly to the colloidal HA (3 kDa-0.45 µm) and as such, would not be a good candidate as a tracer for monitoring soil erosion. HAs likely reduced the oxidized neptunyl form (Np(V)O4+) to Np(IV) based on its enhanced particle-reactivity and Np uptake by particulate HAs, partially retarding the movement of anthropogenic 237Np in field polluted environments. Particulate/colloidal carbonyl/O-aryl (likely through hydroquinone/quinone) functionalities in the HA correlated to Np and Pa uptake, but only particulate O-aryl functionalities was responsible for Fe uptake. The carboxylate- and carbonyl/O-aryl-containing organic functionalities in the HA correlated strongly with Th uptake. In contrast, no significant correlations between organic parameters and Pb or Be uptake implied their predominance of uniform surface adsorption onto particles. This study provides novel insight into the binding of six radionuclides with different organic functionalities of three size fractions, as well as its possible impact on their application in the soil-tracing research.

Effects of colloidal humic acid on the transport of sulfa antibiotics through a saturated porous medium under different hydrochemical conditions

Abstract Colloidal humic acid (HA) acts as a vector that can facilitate the transport of contaminants in groundwater. However, investigations of factors that enhance the transport of sulfa antibiotics when there are colloids present remain incomplete to date. In this study, column experiments were performed under different conditions (particle size, pH, ionic strength, cation valence, colloidal concentration) using 0.25 mg/L sulfamerazine (SM) with or without colloids. The results showed that antibiotics were more easily deposited on the surface of porous media with a diameter of 0.22 mm than 0.45 mm. As the pH increased from 6 to 8, adding colloidal HA increased the maximum breakthrough concentration from 0.94 to 1 for SM. Adding colloidal HA at different NaCl concentrations decreased the maximum C/C0 ratio from 0.97 to 0.92. However, adding colloidal HA changed the C/C0 ratio more when the divalent cation (Ca2+) was present. Overall, increasing the colloidal HA concentration clearly caused the effluent sulfamerazine concentration to increase.

Enhanced transport of ferrihydrite colloid by chain-shaped humic acid colloid in saturated porous media

Both humic acid and colloid particle size effectively regulate colloid transport. However, little is known about effect of particle size and configuration of humic acid colloid (HAcolloid) on enhanced-transport of ferrihydrite colloid (FHcolloid) in porous media. Co-transport of HAcolloid and FHcolloid at different pH was systematically investigated by monitoring breakthrough curves (BTCs) in saturated sand columns. The colloid transport model and the (X)DLVO theory were used to reveal the mechanism of HAcolloid-enhanced FHcolloid transport in the columns. Results showed that HAcolloid enhanced FHcolloid transport in neutral and alkaline conditions. In neutral conditions, small HAcolloid (F-HAcolloid) with chain-shaped structure enhanced FHcolloid transport more prominently than pristine granular HAcolloid. The chain-shaped F-HAcolloid caused osmotic repulsion and elastic-steric repulsion between colloids and sand, leading to enhanced transport. However, the granular HAcolloid readily occurred as deposition due to attachment and straining, which decreased the enhanced transport of FHcolloid. In alkaline conditions, both HAcolloid and F-HAcolloid were chain-shaped, with longer chains of HAcolloid than F-HAcolloid. Ferrihydrite colloid transport was enhanced by HAcolloid more significantly than F-HAcolloid due to stronger repulsion between mixed HAcolloid-FHcolloid and sand. It suggested that regulation of particle size and morphology of HAcolloid would enhance FHcolloid transport and further help in understanding FHcolloid-facilitated contaminants transport in porous media.

Erratum to: Uptake of nitrogen dioxide (NO2) on acidic aqueous humic acid (HA) solutions as a missing daytime nitrous acid (HONO) surface source

A comprehensive kinetic study of a potential daytime nitrous acid (HONO) source reaction, the photoenhanced reduction reaction of the nitrogen dioxide (NO2) on acidic humic acid (HA), was completed using a wetted-wall flow tube (WWFT) (Fickert et al.: J. Phys. Chem. A. 102, 10689, 1998) photoreactor integrated with a high sensitivity HONO analyser (Wall et al.: J. Atmos. Chem. 55, 31–54, 2006; Huang et al.: Atmos. Environ. 36, 2225–2235, 2002). The nature of this reaction, is of great interest since recently observed, unpredictably high HONO daytime concentrations demand its ordinarily proposed heterogeneous source to proceed 60 times more rapidly at noon than during the night (Kleffmann et al.: ChemPhysChem 8, 1137–1144, 2007). This study investigated the nature of the reduction reaction with simulated colloidal HA aqueous solutions characteristic of anaerobic environmental conditions, varying in acidity, concentration and composition. Typical urban NO2 levels were investigated. Increasing photoenhanced HONO production with weakening solution acidity was detected due to increased deprotonation of the carboxyl groups within the humic acid. It was deduced that the acidic HA substrate contains numerous feasible chromophoric sensitizer units capable of photochemically reducing NO2 to HONO, owing to its ‘biofilm’ (Donlan, 2002) function under UV exposure. The mechanism was found to be more effective for HA standards with higher levels of ‘bioactivity’ (refractivity). Using a complex mathematical model developed, incorporating both chemistry and diffusion, reaction probability datasets were produced from the experimental data, providing evidence that this is, indeed, an environmentally important daytime HONO surface source reaction. The parameters required to scale up the data of the photoreactor to that of a regional rural/urban scale were assessed.

Column experiments to investigate transport of colloidal humic acid through porous media during managed aquifer recharge

Colloids act as vectors for pollutants in groundwater, thereby creating a series of environmental problems. While managed aquifer recharge plays an important role in protecting groundwater resources and controlling land subsidence, it has a significant effect on the transport of colloids. In this study, particle size and zeta potential of colloidal humic acid (HA) have been measured to determine the effects of different hydrochemistry conditions. Column experiments were conducted to examine the effects on the transport of colloidal HA under varying conditions of pH (5, 7, 9), ionic strength (<0.0005, 0.02, 0.05 M), cation valence (Na+, Ca2+) and flow rate (0.1, 0.2, 0.4 ml/min) through collectors (glass beads) to model the properties and quality of artificial recharge water and changes in the hydrodynamic field. Breakthrough curves showed that the behavior of colloidal HA being transported varied depending on the conditions. Colloid transport was strongly influenced by hydrochemical and hydrodynamic conditions. With decreasing pH or increasing ionic strength, a decrease in the peak effluent concentration of colloidal HA and increase in deposition could be clearly seen. Comparison of different cation valence tests indicated that changes in transport and deposition were more pronounced with divalent Ca2+ than with monovalent Na+. Changes in hydrodynamic field (flow rate) also had an impact on transportation of colloidal HA. The results of this study highlight the need for further research in this area.

Sorption and toxicity reduction of pharmaceutically active compounds and endocrine disrupting chemicals in the presence of colloidal humic acid.

This study investigated the toxicity changes and sorption of pharmaceuticals and endocrine disrupters in the presence of humic acid (HA). For the sorption experiment, a dead end filtration (DEF) system was used to separate bound and free-form target compounds. An algae growth inhibition test and E-screen assay were conducted to estimate the toxic effect of pharmaceutically active compounds (PhACs) and endocrine disrupting chemicals (EDCs), respectively. The permeate concentration was confirmed using liquid chromatography-mass spectrometry. In the sorption test, we observed significant sorption of PhACs and EDCs on colloidal HA, except for sulfamethoxazole (SMX). The values of log KCOC derived from DEF determinations ranged from 4.40 to 5.03. The removal efficiency varied with the HA concentration and the target chemical properties. Tetracycline and 4-octylphenol showed the highest sorption or removal efficiency (≈50%), even at 5 mg C/L HA. The algal growth inhibition of PhACs and the estrogenic effects of EDCs were significantly decreased in proportion to HA concentrations, except for SMX. In addition, the chemical analysis results showed a positive relationship with the bioassay results. Consequently, the sorption of PhACs and EDCs onto colloidal HA should be emphasized in natural environments because it significantly reduces bioavailable concentrations and toxicity to aquatic organisms.