Increasing Humic Acid Concentration Research Articles

Performance evaluation of the effect of humic acid on Anammox granular sludge: Apparent morphology, nitrogen removal and microbial community

Humic acid (HA) is a typical refractory organic matter, so it is of great significance to investigate its effect on the performance of Anammox granular sludge. When the dosage of HA ≤ 50 mg/L, HA promotes the total nitrogen removal rate (NRR) to 1.45 kg/(m3·day). When HA was between 50 and 100 mg/L, the NRR of Anammox was stable. At this time, the adsorption of HA causes the sludge to gradually turn from red to brown, but the activities of heme and enzymes showed that its capacity was not affected. When HA levels reached 250 mg/L, the NRR dropped to 0.11 kg/(m3·day). Moderate HA levels promoted the release of extracellular polymeric substance (EPS), but excessive HA levels lead to a decrease in EPS concentrations. HA inhibited Anammox activity, which indirectly hindered the transmission of substrate and accumulated substrate toxicity. Although HA promoted the increase of heterotrophic microbial abundance in Anammox system, the microbial diversity decreased gradually. With the increase of HA concentration, the abundance of Candidatus_Brocadia, the main functional microorganism of Anammox system, decreased gradually, while the abundance of Candidatus_Kuenenia increased gradually.

The influence of humic acid on metal(loid)s leaching in landfill leachate for enhancing landfill mining

The aim of this study was to investigate the effect of different concentrations of humic acid on the recovery rate of metal(loid)s in landfill leachate. The study focused on the release of 12 selected metal(loid)s, including critical raw materials (CRM) in landfills that were less than five years old and those that were more than ten years old. The experimental setup involved using different concentrations of humic acid (w/v) (0 %, 0.1 %, and 0.5 %) at pH 4 and 6. The results of the study showed that humic acid was effective in releasing Al, Cr, Co, Ni, Cu, Zn, As, Cd, and Pb. On the other hand, an increase in humic acid concentration led to a decrease in the release of Li, Mn, and Hg. The immobilization of Li, Mn, and Hg was due to the coordination and adsorption of humic acid. The presence of humic acid accelerated the release of metal(loid)s by carboxylic acidity compared to the recovery rate of metal(loid)s in landfill leachate without humic acid. However, a higher concentration of humic acid did not always result in a stronger recovery rate. The recovery rate of metal(loid)s was related to the solubility and concentration of humic acid. These findings can inform the development of more efficient and environmentally-friendly methods of recovering metal(loid)s using humic acid as a leaching agent.

Reduced adsorption of norfloxacin on UV aging microplastics in anoxic environment

Over recent years, much attention has been paid to aging problem of microplastics and adsorption behavior of antibiotic on microplastics. In this study, four microplastics, including polystyrene (PS), polypropylene (PP), polyamide (PA) and polyethylene (PE), were photoaged by UV light in anoxic environment. The surface characteristics of microplastics and adsorption behavior of norfloxacin (NOR) on microplastics were investigated. Results indicated that the specific surface area and crystallinity increased, and hydrophobicity weakened of microplastics after UV aging. The content of C element decreased and the content of O barely changed in the aged microplastics. In addition, the adsorption of NOR on microplastics yielded a better fitness for the pseudo-second-order kinetics, Langmuir and Freundlich models. The adsorption capacities of NOR on PS, PA, PP, and PE at 288K were 16.01, 15.12, 14.03, and 13.26mg·g-1, respectively, while the adsorption capacities of NOR on aged microplastics were reduced to 14.20, 14.19, 11.50, and 10.36mg·g-1, respectively, due to decrease of hydrophobicity and increase of crystallinity of microplastics after UV aging. The adsorption of NOR on microplastics decreased with the increase of temperature, implying the adsorption process was exothermic. Adsorption mechanism analysis showed that Van der Waals force was the main influential mechanism of the adsorption of NOR on PP and PE, and hydrogen bond was the main factor affecting the adsorption of NOR on PA, while the π-π interaction was the main mechanism impacting the adsorption of NOR on PS. Aging time and salinity significantly affect the adsorption of NOR on microplastics. With the increase of humic acid concentration and pH, the adsorption of NOR on microplastics first reduced and then rose. This study provides a basis for further clarifying the mechanism of UV aging microplastics and a reference for the study of combined pollution behavior of microplastics and antibiotics.

Competitive Removal of Antimony and Humic Acid by Ferric Chloride: Optimization of Coagulation Process Using Response Surface Methodology

The co-contamination of aquatic systems with antimony (Sb) and humic acid (HA) is a global concern due to their potential risks to human health and environment. In this research, three-level-three-variable Box–Behnken design (BBD) was investigated for simultaneous Sb(III) and total organic carbon (TOC) removal responses from groundwater by use of ferric chloride (FC) as a coagulant. This study focuses on three operating variables, including initial Sb(III) concentration (100, 550, and 1000 μg/L), HA concentration (2, 6, and 10 mgC/L) and FC dosages (20, 60, and 100 mg/L). The proposed quadratic model presented good correlation with experimental values having R2 and adjusted R2 values of response variables (Sb(III): 0.9981 and 0.9956) and (TOC: 0.9935 and 0.9851), respectively. The most pronounced influence of FC dosage was observed in the removal responses of TOC and Sb(III). Interestingly, the model revealed that a high level of FC dosage had the same TOC removal potential regardless of increasing HA concentration. The statistical model for both Sb(III) and TOC responses was used to optimize the experimental conditions. Moreover, the experimental results were successfully validated with predicted values having high accuracy. The sludge volume produced for studied variables followed the decreasing order as FC dosage > HA concentration > Sb(III) concentration. Under optimum conditions, 0.45 mL/L sludge volume was produced in the treatment of ground water in Nawabshah. In general, the results of the current study may provide some insights into the predictability of simultaneous removal performance of Sb species and organic substances from groundwater.

Role of humic acid in the transformation of hexavalent chromium in a sulfidated ferrihydrite system

Ferrihydrite (Fh) often coexists with organic matter (i.e., humic acid (HA)) in the environment; however, its impacts on the transformation of hexavalent chromium (Cr(VI)) is poorly understood during the sulfidation of Fh. Upon exposed to 2 mM sulfide for 12 h, the total amount of Fe(II) (Fe(II)tot, 0.78 mM) in treatments with HA (300 mg HA/L) was higher than that (0.67 mM) in treatments without HA, since HA could enhance the solubility of Fe(II). After then, the Cr(VI) was reduced by sulfidated Fh. Aqueous Cr(VI) concentration (Cr(VI)aq) declined from 0.67 to 0.43 mM with the increase of HA concentration from 0 to 400 mg/L, which was partly ascribed to the inhibition of surface passivation by HA. Moreover, the increase in Fe(II) during the sulfidation of Fh also exerted a strong impact on the transformation of Cr(VI) subsequently. In addition of HA, batch experiments suggested that EDTA could also promote the formation of Fe(II) (Fe(II)tot, 0.80 mM), rendering a lower Cr(VI)aq (0.59 mM) in EDTA-300 treatments. This study further demonstrated that HA played an important role in the transformation of Cr(VI), hence providing a theoretical basis for in-situ remediation of Cr(VI) in the future.

Sulfide perturbation of NOM-bearing oxic environments induces dark •OH production

The photo-irradiation of natural organic matter (NOM) is a well-documented pathway of •OH production in surface waters. Using humic acid (HA) as a model compound, we show that the perturbation of NOM-bearing oxic environments by naturally occurring reduced species, sulfide, contributes to •OH production under dark conditions. Sulfide concentration had a more prominent influence on •OH production than HA. When HA was controlled at 16 mM C under oxic and neutral pH conditions, the increase of sulfide concentration from 0 to 5 mM increased the cumulative •OH concentration from <2 to 72.2 μM within 120 min. When sulfide was maintained at 2.5 mM, the increase of HA concentration from 0 to 16 mM C increased the cumulative •OH concentration from <0.6 to 44.8 μM. •OH accumulation showed good consistency with sulfide removal and O2 consumption. O2 consumption increased with increasing HA concentration at a fixed sulfide concentration, and also increased with increasing sulfide concentration at a fixed HA concentration. UV–Vis and FTIR characterizations suggested that the reduced HA moieties resulting from sulfide interaction with HA acted as the redox-active moieties responsible for •OH production. Additionally, 3DEEM fluorescence suggested the reaction of sulfide with oxic HA as a partially reversible reduction-oxidation process with production of labile HA fluorophoric intermediates. Our findings implicate that the perturbation of NOM-bearing oxic environments by sulfide, i.e., from the discharge of anoxic groundwater, springs, seafloor black smokers, and hydrothermal vents, is another important source of •OH in natural environments in addition to photo-irradiation.

Engineering mesoporous algal-based biochars for efficient remediation of norfloxacin pollution in marine environment

• Marine biomass was chosen as raw material for the synthesis of biochar. • An appropriate H 3 PO 4 ratio was more favorable for GAB x to remove NOR from seawater. • The excellent adsorption capability reaches as high as 166.48 mg/g. • The interaction mechanism between GABx and norfloxacin was clarified in seawater. Gelidium amansii -derived biochars (GAB) were synthesized through one-step H 3 PO 4 chemical activation at impregnation ratios of 1.5–5 and applied for norfloxacin (NOR) removal in the marine environment. The effects of contact time and initial concentration, especially the salinity and humic acid on NOR adsorption were emphasized. The results showed that the increased H 3 PO 4 impregnation ratio prompted the development of mesoporous and the elimination of oxygen-containing functional groups. Thus, GAB with an appropriate impregnation ratio of 3 (GAB3) exhibited the largest NOR adsorption capability (166.48 mg/g) compare to the others (105.71–159.49 mg/g). NOR adsorption capacity slightly decreased with the increase of salinity, while the adsorption was almost unaffected with the increase of humic acid concentration. The adsorption mechanisms of NOR on GAB in seawater were dominantly through pore-filling, electrostatic attraction, π-π electron donor-acceptor interaction, cation bridging, and negative charge-assisted H-bond. This study provides strategies for efficient remediation of antibiotic-contaminated seawater using algal-based biochars.

Transport characteristics of polystyrene microplastics in saturated porous media with biochar/Fe3O4-biochar under various chemical conditions

Magnetically modified biochar, with a rougher surface and more positive surface charge, may interact with microplastics (MPs) after being applied to soil, potentially altering the fate and transport of MPs in porous media. In this study, the transport and retention behavior of polystyrene microplastics (PSMPs) in a sandy porous media mixed with biochar/Fe3O4 modified biochar (Fe3O4-biochar) was investigated under various chemical conditions (humic acid (HA), ionic strength (IS) and cationic types (Na+/Ca2+)). The results showed that the addition of biochar and Fe3O4-biochar can hinder the transport of PSMPs in porous media without HA, and that Fe3O4-biochar was more effective in inhibiting the transport of PSMPs through electrostatic adsorption and complexation, with an optimum retention efficiency of 92.36 %. HA significantly attenuated the retention of PSMPs in both porous media through electrostatic repulsion, steric resistance and competitive adsorption under 1 mM Na+ solutions, and the mobility of PSMPs in Fe3O4-biochar/sand was enhanced more significantly than in biochar/sand with the increase of HA concentration. IS significantly inhibited the transport of PSMPs in both porous media in the absence of HA, but there was an antagonistic effect of HA and IS on the transport of PSMPs in the presence of HA, with the facilitative effect of HA being stronger than the inhibitory effect of IS. Ca2+ was consistently more effective in inhibiting the transport of PSMPs than Na+ under all test conditions, and HA promoted the transport of PSMPs in all Na+ solutions, while it inhibited the transport of PSMPs in high IS (10 mM) with Ca2+ solutions. In addition, HA, Fe3O4-biochar and PSMPs tend to form larger aggregates under the complex interactions of Ca2+, leading to increased retention of PSMPs in porous media. The two-site kinetic retention models suggested that the retention of PSMPs in porous media with biochar was predominantly reversible attachment effect, while retention in porous media with Fe3O4-biochar was predominantly an irreversible straining effect.

Transport and retention of microplastics in saturated porous media with peanut shell biochar (PSB) and MgO-PSB amendment: Co-effects of cations and humic acid

Biochar particles are extensively used in soil remediation and interact with microplastics (MPs), especially metal oxide-modified biochar may have stronger interactions with MPs. The mechanism of interactions between humic acid (HA) and different valence cations is different and the co-effect on the transport of MPs is not clear. In this study, the co-effects of HA and cations (Na+, Ca2+) on the transport and retention of MPs in saturated porous media with peanut shell biochar (PSB) and MgO-modified PSB (MgO-PSB) were systematically investigated. Breakthrough curves (BTCs) of MPs were fitted by the two-site kinetic retention model for analysis. In the absence of HA, the addition of PSB and MgO-PSB significantly hindered the transport of MPs in saturated porous media, and the retention of MPs increased from 34.2% to 59.1% and 75.5%, respectively. In Na+ solutions, the HA concentration played a dominant role in controlling MPs transport, compared to the minor role of Na+. The transport capacity of MPs always increased gradually with the increase of HA concentration. Whereas, in Ca2+ solutions, Ca2+ concentrations had a stronger effect than HA. The transport ability of MPs was instead greater than that in Na+ solutions as the HA concentration increased at low ionic strength (1 mM). However, the transport capacity of MPs was significantly reduced with increasing HA concentrations at higher ionic strength (10, 100 mM). The two-site kinetic retention model indicated that chemical attachment and physical straining are the main mechanisms of MPs retention in the saturated porous media.

Effects of nanometer alumina and humic acid on the retention and transport of hexavalent chromium in porous media

Column experiments were conducted to investigate the effects of ion type, ion strength, humic acid (HA), and nanometer alumina (NA) particles on the transport of hexavalent chromium (HC) in saturated porous media. A one-dimensional model is developed to simulate the migration of HC affected by NA particles. The results show that nano-alumina particles would enhance the mobility of HC in saturated porous media. However, the influence of NA on the migration of HC in porous media is complex. When the concentration of NA reaches 30 mg/L, HC has minimum retention parameter and best mobility. The transport of HC also is affected by ion strength and ion type. Higher ionic strength would decrease the retention of HC and enhance its mobility. Compared with sodium ion, calcium ion has larger effects on the transport of HC. Moreover, HA can improve the mobility of HC in saturated porous media, but the corresponding promoting effect decreases with the increase of HA concentration. As nanometer contaminants and HC come into the subsurface environment, findings from this study elucidate the key factors and processes controlling the transport of HC in porous media, which can promote the prediction and assessment of HC in the groundwater system.

Effect of humic acid on the single-stage nitrogen removal using anammox and partial nitritation (SNAP) process: performance and bacterial communities

The present research investigated the effect of humic acid on nitrogen removal performance and microbial community structure in a single-stage nitrogen removal using anammox and partial nitrification (SNAP) reactor. The results showed that 576 mg·L−1 humic acid enhanced nitrogen removal efficiency maximally to 96% and nitrogen removal rate to 1.61 kgN·m−3·d−1. COD concentration (provided by humic acid) of 120–233 mg·L−1, the nitrogen removal rate (NRR) was 1.27 kgN·m−3·d−1. COD concentration (provided by humic acid) of 259–658 mg·L−1 enhanced the NRR above 1.5 kgN·m−3·d−1, and COD concentration increased to 691–1637 mg·L−1, the average NRR decreased to 1.45 kgN·m−3·d−1. The results indicated COD concentration of 691–1637 mg·L−1 has the strongest positive effect on the anammox reaction. The analysis of high throughput sequencing presented COD concentration of 500 mg·L−1 promoted the growth of Candidatus_brocadia, and 1500 mg·L−1 COD concentration improved Candidatus kuenenia growth. Functional genes from KEGG and COG database showed that the expressions of nitrate reductase and nitrite reductase were highly promoted with the increase of humic acid concentration, which proved the nitrification enhanced by humic acid. All findings in this study were inferred that humic acid enhanced nitrogen removal in SNAP reactor. This result will improve the practical and effective use of the SNAP process to treat landfill leachate in engineering application.

Removal of arsenic(III) via nanofiltration: contribution of organic matter interactions

The removal of arsenic(III) (As(III)) with nanofiltration (NF) was investigated with emphasis on the role of salinity, pH and organic matter on retention mechanisms. While no measurable impact of salinity on As(III) retention with NF membranes (NF270 and NF90) was observed, a significant increase in As(III) retention occurred from pH 9 to pH 12. This was explained by As(III) deprotonation at pH > 9 that enhanced Donnan (charge) exclusion. Of the five different organic matter types investigated at 10 mgC/L, only humic acid (HA) increased As(III) retention by up to 10%. Increasing HA concentration to 100 mgC/L enhanced As(III) retention by 40%, which was attributed to As(III)-HA complexation. Complexation was confirmed by field-flow fractionation inductively coupled plasma mass spectrometry (FFF-ICP-MS) measurements, which showed that the bound As(III) increased with HA concentration. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that NF90, which exhibited lower permeability reduction than NF270, has accumulated a lower amount of As(III) in the presence of HA, where As(III)-HA complex was formed in the feed solution. This finding implies that As(III) retention with NF technology can be enhanced by complexation, instead of using other methods such as oxidation or pH adjustement.

Transport characteristics of fragmental polyethylene glycol terephthalate (PET) microplastics in porous media under various chemical conditions

Transport characteristics of fragmental polyethylene glycol terephthalate (PET) microplastics in porous media were elucidated via column experiments under a series combination of electrolytes, pH, and humic acid (HA) conditions. Fragmental PET microplastics showed low mobility in porous media with a small mass recovery rate (<50.1%) even under unfavorable retention conditions. The electrolyte, pH, and HA showed combined impact on PET microplastic transport. PET microplastics mobility was enhanced with decreasing electrolyte concentration, increasing pH, and increasing HA concentration. Basic properties (e.g. destiny and shape) of PET microplastics showed stronger effect on their transport behaviors in porous media rather than the experimental chemical conditions. In general, both environmental factors and basic properties played important roles in controlling the retention and transport of PET microplastics in porous media. A numerical model considering the second order kinetic deposition sites was applied to depict the retention and transport of PET microplastics in porous media. Model simulations well matched the experimental breakthrough curves. Given the fragmental PET microplastics have more realistic and irregular shapes, results from this study can improve present knowledge of the environmental fate and risk of microplastics in underground soil and water systems.

Effects of solution chemistry and humic acid on the transport of polystyrene microplastics in manganese oxides coated sand

Microplastics as the emerging persistent pollutants have attracted more attention in the terrestrial environments. In this study, the transport behavior of polystyrene microplastics (PSMPs) in manganese oxides coated sand (MOCS) was examined under different pH, ionic strength (IS), cationic type and humic acid (HA) conditions. Compared with the transport behavior of PSMPs in the bare sand, the mobility of PSMPs in MOCS was significantly lower and less affected by pH, IS and cation type, which can be attributed to the existence of attractive electrostatic force and rougher collector surfaces of MOCS. Specifically, the transport of PSMPs was inhibited when cotransport with Cd2+. Furthermore, the HA significantly increased the transport of PSMPs in the MOCS, and the mobility increased with the increase of HA concentration ranged from 0 to 10 mg L−1. The results can contribute to the further understanding of the migration mechanism and fate of microplastics in the soil system.

Evolution of dissolved organic matter during artificial groundwater recharge with effluent from underutilized WWTP and the resulting facilitated transport effect

Currently, the interaction between contaminants and dissolved organic matter (DOM) during artificial groundwater recharge (AGR) with effluent from underutilized wastewater treatment plant (WWTP) is unclear. The present study investigated DOM evolution in this AGR scenario. The DOM composition in the inflow was identified to be distinct to that of the outflow due to the release of soil humic acid (HA). The soluble soil HA was then extracted and used in co-transport experiments with tetracycline (TC). The separated HA transport through the soil column exhibited high mobility with mass recovery >92.5% in the effluent. Following the mixing of injected TC and HA, the TC breakthrough in the column increased with HA concentration. TC was heavily adsorbed by the soil without the presence of HA, yet the retention ratios decreased from 63.60% to 53.30% for the HA range of 0–20 mg L−1. An advection-dispersion-retention (ADR) numerical model was developed to effectively quantify the HA-TC co-transport, with results demonstrating the reduction in the TC attachment rate with increasing HA concentrations. Furthermore, batch adsorption experiments, kinetics and isotherms models, and FTIR spectra analysis were implemented to determine the underlying mechanism. The co-transport behavior was observed to be a function of the relative soil sorption affinity between HA and TC. The weaker sorption of the HA-coated TC compared to the separated TC consequently suggests that HA is likely to compete for available soil adsorption sites. Thus, the release of soil humus during AGR can potentially facilitate the transport of the introduced contaminants.

Effects of humic acid on enhanced removal of lead ions by polystyrene-supported nano-Fe (0) nanocomposite

Polymer-supported nanozero-valent iron composites (D001-nZVI) were fabricated for the removal of lead ions from aqueous solutions by embedding nZVI into the porous polystyrene anion exchanger D001. Humic acid (HA) was selected as a model species because of its ubiquitous existence to gain insight into the influencing factors in the actual application process. The iron contents of the composites were approximately 11.2%, and the smallest ZVI particle size was ~ 5 nm. The experimental results showed that the effect of HA on the reduction of lead ions by D001-nZVI was a concentration-dependent process. At low HA concentrations, the surface-competitive adsorption of HA and Pb2+ dominated; therefore, the removal efficiency of Pb2+ by D001-nZVI decreased from 97.5 to 90.2% with an increasing HA concentration. When the HA concentration increased to 30 mg/L or more, the lead ions removal remained constant with the following possible cooperation mechanism: the competitive adsorption of HA and Pb2+ on the nZVI surface and the well-dispersed particles were caused by electrostatic interactions between the HA coating and the nZVI surface. In addition, the adsorption complexation between HA and Pb2+ also had a positive effect on the removal of Pb2+ at higher concentrations of HA.

Enhanced role of humic acid on the transport of iron oxide colloids in saturated porous media under various solution chemistry conditions

Humic acid (HA), a naturally occurring and widely distributed natural organic matter in the environment, is one of the most important factors controlling the transport and fate of mineral colloids. This study examined the influence of HA (0–10 mg/L) on the transport of goethite and hematite colloids in saturated quartz sand-packed columns under various solution chemistry conditions (including ionic strength and pH). We found that the transport-enhancement effect of HA on goethite colloids was greater than that on hematite colloids because the steric hindrance and electrostatic repulsion between HA-bound goethite colloids and sand stronger than those between HA-bound hematite colloids and sand. Besides, Ca2+ could be a bridging agent between HA and sand; and consequently, enhanced the steric hindrance between iron oxide colloids and sand, which was an important mechanism for the transport-enhancement effect of HA in the presence of Ca2+. Interestingly, with increasing HA concentration from 3 to 10 mg/L, the transport-enhancement effect of HA at higher ionic strength (i.e., 20 mM NaCl) was to a larger extent than that at lower ionic strength (i.e., 5 mM NaCl); meanwhile, the enhancement effect of HA at lower pH (i.e., 5.0) was to a larger extent than that at higher pH (i.e., 9.0). The observations were attributable to the surface charge and aggregation properties of HA-bound iron oxide colloids affected by solution chemistry coupled with HA concentration. Furthermore, DLVO theory extended by steric repulsion (XDLVO) could explain the observed trend of colloid transport under the test conditions.

Solar photocatalytic degradation of thidiazuron in Yangtze River water matrix by Ag/AgCl-AC at circumneutral condition.

It is well-known that the degradation of pollutants in real water environment is not only challenging but also has practical value. This paper focuses on the photocatalytic degradation of thidiazuron (TDZ), a popular defoliant, using Ag/AgCl-AC (Ag@AC 2:1); AC stands for activated carbon) in a matrix of Yangtze River water under sunlight irradiation. The prepared composite catalyst exhibits excellent performance in TDZ degradation under near neutral condition, the degradation rate reaches 94% in 200min under solar irradiation. The common inorganic anions (SO42-, Cl-, and HCO3-) and cations (Ca2+, Cu2+, and Mg2+) show inhibitory effect of different degrees on TDZ degradation. Humic substances such as humic acid and fulvic acid also have an effect on the photocatalytic degradation of TDZ. With the increase of humic acid concentration, there is enhancement of inhibitory effect. As for fulvic acid, its effect is complex due to competitive adsorption and photoinduction action. The degradation products as identified by UHPLC-MS are mainly CO2, SO2, and H2O, indicating that the degradation was thorough. The reusability test of four runs reveals that the performance of the photocatalytic system is stable. The results demonstrate that sunlight can be well utilized for the photocatalytic degradation of TDZ. The study offers a cheap and effective approach for the photocatalytic degradation of organic pollutants in circumneutral water bodies.

Characteristics and Influencing Factors of Monothioarsenate Adsorption on Goethite

The adsorption kinetic of monothioarsenate (MTA) on goethite was characterized in this study, and batch experiments were then designed to further explore the effects of arsenate, arsenite, humic acid (HA), nitrate, and phosphate on the adsorption of MTA on goethite, and to identify the adsorption mechanism. The results showed that:① When a single arsenic species was present in a solution, the adsorption equilibrium times of MTA, arsenate, and arsenite on goethite were 8, 2, and 4 h, respectively. The adsorption experimental data of these three arsenic species were well fitted to a pseudo-second-order kinetic model. The equilibrium adsorption capacities (qe) of MTA, arsenate, and arsenite on goethite were 2129.851, 3291.838, and 1788.767 mg·kg-1, respectively. When MTA coexisted with arsenate or arsenite in a solution, MTA adsorption on goethite continued to be well fitted to a pseudo-second-order kinetic model. The value of qe for MTA was significantly reduced to 1236.941 mg·kg-1 when MTA coexisted with arsenate, and to 1532.287 mg·kg-1 when MTA coexisted with arsenite, due to the fact that arsenate and arsenite competed for adsorption sites with MTA. ② With an increase in HA concentration (10-50 mg·L-1), the qe of MTA decreased gradually, due to the fact that a large number of functional groups in HA preempted the surface adsorption sites of goethite with MTA. ③ When phosphate was added into the MTA solution, the qe values of MTA, arsenate, and arsenite on goethite were reduced greatly, to 492.802, 815.782, and 303.714 mg·kg-1, respectively, which was caused by the competitive adsorption of P and As. When nitrate was added into the MTA solution, the number of electron receptors and Eh of the solution increased, leading to the qe values of MTA, arsenate, and arsenite on goethite increasing to 2211.030, 3444.023, and 1835.537 mg·kg-1, respectively.

Photochemical Generation of Methyl Chloride from Humic Aicd: Impacts of Precursor Concentration, Solution pH, Solution Salinity and Ferric Ion.

Methyl chloride (CH3Cl) is presently understood to arise from biotic and abiotic processes in marine systems. However, the production of CH3Cl via photochemical processes has not been well studied. Here, we reported the production of CH3Cl from humic acid (HA) in sunlit saline water and the effects of the concentration of HA, chloride ions, ferric ions and pH were investigated. HA in aqueous chloride solutions or natural seawater were irradiated under an artificial light, and the amounts of CH3Cl were determined using a purge-and-trap and gas chromatography-mass spectrometry. CH3Cl was generated upon irradiation and its amount increased with increasing irradiation time and the light intensity. The formation of CH3Cl increased with an increase of HA concentration ranging from 2 mg L−1 to 20 mg L−1 and chloride ion concentration ranging from 0.02 mol L−1 to 0.5 mol L−1. The photochemical production of CH3Cl was pH-dependent, with the highest amount of CH3Cl generating near neutral conditions. Additionally, the generation of CH3Cl was inhibited by ferric ions. Finally, natural coastal seawater was irradiated under artificial light and the concentration of CH3Cl rose significantly. Our results suggest that the photochemical process of HA may be a source of CH3Cl in the marine environment.