Low Humic Acid Concentration Research Articles

Interplay of humic acid and Cr(VI) on green microalgae: Metabolic responses and chromium enrichment

Dissolved organic matter (DOM) present in aquatic environments can significantly influence microalgal metabolism and the enrichment of heavy metals. However, the specific mechanism through which typical DOM affects the enrichment of the heavy metal chromium (Cr) in green algae remains unclear. This study investigates the impacts of varying concentrations of humic acid (HA), selected as a representative DOM in water, on the growth, metabolism, and Cr enrichment in Chlorella vulgaris, a typical green alga. The results indicated that low concentrations of HA were capable of enhancing Cr enrichment in C. vulgaris, with the highest Cr enrichment rate recorded at 41.50 % at TOC = 10 mg/L. The enrichment of Cr in algal cells primarily occurred through cell proliferation and complexation reduction of extracellular polymeric substances (EPS). In the presence of HA, C. vulgaris predominantly removed Cr through extracellular adsorption, accounting for 79.76–85.88 % of the total Cr removal. Furthermore, carboxyl complexation and hydroxyl reduction within EPS facilitated both the enrichment of Cr (18.72–21.49 %) and the reduction of Cr(VI) (63.93–74.10 %). These findings provide valuable insights into strategies for mitigating heavy metal pollution and managing associated risks in aquatic environments.

Efficient removal of Cr(VI) containing tunnel wastewater by sludge biochar loaded with ferrous sulfate

The ease of agglomeration reduced the removal of pollutants by FeS particles. Hence, sludge biochar was employed as a support to produce FeS-loaded biochar composites (FeS@BC) using a solvothermal method, for removing Cr(VI) from wastewater. The performance and mechanism of Cr(VI) removal on FeS@BC were investigated through adsorption experiments and characterization techniques. The adsorption tests revealed that under optimal conditions of solution pH of 3 and dosage of 40 mg, FeS@BC achieved a removal efficiency of over 97 % for 50 mg/L Cr(VI). The presence of Cl- and NO-3 did not affect Cr(VI) removal, whereas sulfate, phosphate, and carbonate inhibited the removal process. Low concentrations of humic acid (1–5 mg/L) held a negligible influence on removing, but elevated concentrations of humic acid inhibited Cr(VI) removal. The behavior of FeS@BC in removing Cr(VI) complied with to the pseudo-second-order and the Langmuir model. At 308.15 K, the largest removal capability of FeS@BC for Cr(VI) reached 185.874 mg/g. Thermodynamic studies confirmed that the Cr(VI) removing processes on FeS@BC were a spontaneous, entropy-driven, endothermic chemical adsorption. The primary mechanism for removing Cr(VI) on FeS@BC included reduction, electrostatic adsorption, complexation, and precipitation, with the reduction process playing a crucial role. This indicated that sludge biochar loaded with FeS was an ideal material for removing Cr(VI) from wastewater.

Biostimulation of humic acids on Lepidium sativum L. regulated by their content of stable phenolic O⋅ radicals

BackgroundHumic acid affects plant growth. Its source and structure may play a central role to its functionality. The relationship between humic acid and plant bioactivity is still unclear. This study investigated the biostimulation effects of two natural humic acids derived from soil (SHA) and lignite (LHA) on Lepidium sativum in comparison to a synthetic humic acid model (HALP) with known structure.ResultsAll humic acids positively affected cress seed germination and root elongation. Greater root hairs density and dry matter, compared to control, were observed using concentration of 5 mg L−1 for HALP, 50 mg L−1 for LHA, and 100 mg L−1 for SHA. The germination index was the largest (698% more effective than control) with 50 mg L−1 of SHA, while it was 528% for LHA, and 493% for HALP at 5 mg L−1. SHA contained the lowest aromatic and phenolic C content, the largest pK2 value of 9.0 (7.7 for LHA and 7.6 for HALP), the least ratio between the aromaticity index and lignin ratio (ARM/LigR) of 0.15 (0.66 for LHA and 129.92 for HALP), and at pH 6.3 the lowest amount of free radicals with a value of 0.567 × 1017 spin g−1 (1.670 × 1017 and 1.780 × 1017 spin g−1 for LHA and HALP, respectively), with the greatest g value of 2.0039 (2.0035 for LHA and 2.0037 for HALP).ConclusionsThe overall chemical structure of humic acids exerted a biostimulation of cress plantlets. The level of the intrinsic stable free radicals identified by EPR in the humic acids resulted well correlated to the ARM/LigR ratio calculated by NMR. Our results suggested that HA biostimulation effect is related to its applied concentration, which is limited by its free radical content. The modulation of the humic supramolecular structure by ROS and organic acids in root exudates can determine the release of bioactive humic molecules. When the content of the intrinsic humic free radicals is high, possible molecular coupling of the bioactive humic molecules may hinder their biostimulation activity. In such cases, a low humic acid concentration appears to be required to achieve the optimum biostimulation effects.Graphical

Efficient and sustainable photocatalytic inactivation of E. coli by an innovative immobilized Ag/TiO2 photocatalyst with peroxymonosulfate (PMS) under visible light

A novel catalytic system for effective photocatalytic inactivation of Escherichia coli (E. coli) was constructed by anchoring Ag nanoparticles (AgNPs) on silane coupling agent (SCA) pretreated TiO2 nano-tube arrays (Ag/SCA/TiO2NTAs). Morphology and structural analyses revealed that SCA could disperse AgNPs evenly on TiO2NTAs, thus inducing a superior surface plasmon resonance (SPR) effect. Ag/SCA/TiO2NTAs catalyst exhibited excellent inactivation performance when in the presence of peroxymonosulfate (PMS) and visible light (VL), with 6-log E. coli was completely inactivated within 60 min, which was 5.3, 12.5 and 13.2 times higher than that of Ag/SCA/TiO2NTAs/VL, PMS/VL and Ag/SCA/TiO2NTAs/PMS/dark systems, respectively. Additionally, the photocatalyst exhibited a highly reusable property, with the inactivation performance almost unchanged after ten cycles of uses with minimal Ag leaching. The inactivation mechanism analysis demonstrated that both radical (SO4•-, OH) and non-radical (h+, 1O2) pathways involved in E. coli inactivation, and SCA played a pivotal role in the production of reactive species. Chloride ions (Cl−) greatly enhanced the inactivation efficiency, while bicarbonate (HCO3−) and phosphate (H2PO4−) showed an inhibitory effect. Humic acid (HA) displayed a dual effect on inactivation performance, where the low concentration of HA facilitated the bacteria inactivation, while the higher dose suppressed bacteria inactivation. Moreover, the system exhibited excellent inactivation performance in tap water. This work first used SCA as the binder to fix AgNPs on TiO2NTAs for VL photocatalytic inactivation of bacteria with the assistance of PMS, which was expected to provide some insights into the practical treatment of drinking water.

Sludge Biochar Modified by B-doped and Its Adsorption Behavior and Mechanism of 1,2-DCA in Water

Sludge biochar (BC600) and B-doped sludge biochar (BBC600) were prepared with the boric acid doping modified co-pyrolysis method using municipal sludge as precursors, and the materials were structurally characterized by SEM, BET, FTIR, and Zeta potential and static contact angle to investigate the adsorption behavior, mechanism of BC600 and BBC600 on 1,2-DCA in water, and the influencing factors. The results of structural characterization showed that the B element content, specific surface area, and pore volume of biochar increased by 76%, 48%, and 30%, respectively, after the B doping modification; the effect of B doping modification on the surface charge and hydrophobicity of biochar was not significant. The results of adsorption experiments showed that the adsorption of 1,2-DCA by BBC600 was better than that by BC600 due to the larger specific surface area and higher strength of oxygen-containing functional groups of BBC600; the pseudo-first-order kinetic equation could better describe the adsorption of 1,2-DCA by BC600, and the pseudo-second-order kinetic equation could better fit the adsorption of 1,2-DCA by BBC600. The intraparticle diffusion was not the only rate-limiting step affecting the adsorption rate; the biochar material was more dispersed and stable under alkaline conditions, and its oxygen-containing functional groups were deprotonated and had enhanced electron-donating ability, which was beneficial to the adsorption of 1,2-DCA. Humic acid (HA) showed a low concentration-promoting and high concentration-inhibiting effect on the adsorption of 1,2-DCA by BC600, whereas both low and high concentrations of HA showed an inhibitory effect on the adsorption of 1,2-DCA by BBC600. The adsorption of 1,2-DCA by BC600 was inhibited by both low and high concentrations of HA, and HA competed with 1,2-DCA for adsorption; Cl-, SO42-, and NO3- all inhibited the adsorption of 1,2-DCA by biochar, and the degree of inhibition ordered from small to large was Cl-<SO42-<NO3-. The main mechanisms of 1,2-DCA adsorption by biochar were pore filling and π-π interaction.

New Insights into the Fouling of a Membrane during the Ultrafiltration of Complex Organic–Inorganic Feed Water

This paper presents an analysis of the fouling of a ceramic membrane by a mixture containing high concentrations of humic acid and colloidal silica during cross-flow ultrafiltration under various operating conditions. Two types of feed water were tested: feed water containing humic acid and feed water containing a mixture of humic acid and colloidal silica. The colloidal silica exacerbated the fouling, yielding lower fluxes (109-394 L m-2 h-1) compared to the humic acid feed water (205-850 L m-2 h-1), while the retentions were higher except for the highest cross-flow rate. For the humic acid feed water, the irreversible resistance prevails under the cross-flow rate of 5 L min-1. During the filtration of an organic-inorganic mixture, the reversible resistance due to the formation of a colloidal cake layer prevails under all operating conditions with an exception. The exception is the filtration of the organic-inorganic mixture of a 50 mg L-1 humic acid concentration which resulted in a lower flux than the one of a 150 mg L-1 humic acid concentration under 150 kPa and a cross-flow rate of 5 L min-1. Here, the irreversible fouling is unexpectedly overcome. This is unusual and occurs due to the low agglomeration at low concentrations of humic acid under a high cross-flow rate. Under lower transmembrane pressure and a moderate cross-flow rate, fouling can be mitigated, and relatively high fluxes are yielded with high retentions even in the presence of nanoparticles. In this way, colloidal silica influences the minimization of membrane fouling by organic humic acid contributing to the control of in-pore organic fouling.

Comparison of lead adsorption on the aged conventional microplastics, biodegradable microplastics and environmentally-relevant tire wear particles

In this study, the adsorption behavior of lead (Pb(II)) onto environmentally-relevant tire wear particles (TWP) gained by natural weathering and mechanical abrasion, ultraviolet-aged polylactic acid microplastics (UV-PLA), and ultraviolet-aged polystyrene microplastics (UV-PS) was compared to investigate the difference of representative microplastics between the adsorption capacity and mechanism. Our results indicated that the aging processes significantly improved the adsorption capacity to Pb(II), and TWP had the highest adsorption amount, followed the order by UV-PLA, UV-PS, PLA and PS. Electrostatic interaction and complexation of oxygen-containing functional groups played crucial roles in the adsorption of Pb(II) onto UV-PLA, the adsorption mechanisms of UV-PS involved electrostatic interaction, complexation and cation-π interaction, while adsorption of TWP was might dominated by the complexation of surface functional groups and properties of the complex rubber co-polymer. Interestingly, the low concentration of humic acid (HA) most significantly improved adsorption capacity of aged microplastics to Pb(II), otherwise it was severely suppressed at higher concentration of HA. Besides, the adsorption behavior of different binary systems between aged microplastics and suspended sediment to Pb(II) had complicated synergistic or suppressive effects with a concentration-dependent and polymer-dependent trend. Furthermore, UV-PLA had the far stronger desorption ability of Pb(II) than that of TWP and UV-PS, suggesting that metal-contaminated biodegradable microplastics posed the higher ecological risks to aquatic organisms. Desorption quantity and rates of Pb(II) from aged microplastics in the simulated sediment system were generally higher than those in turbulent water column. Collectively, these findings provide a wide insight into adsorption capacity and mechanisms of metal ions on the representative aged microplastics and the desorption behavior in complex aquatic environments.

Combined effect of humic acid and vetiver grass on remediation of cadmium-polluted water.

Effective treatment of water pollution is an economic and social requirement globally. Humic acid (HA) is a popular mitigator for such waters. However, the combined effect of HA and restorative plants on cadmium (Cd) remediation is not well understood. Therefore, we experimented on Cd remediation using HA along with vetiver grass and HA-vetiver grass. We observed that vetiver grass effectively removed Cd at 15～30mg/L. The accumulation capacity of the root was significantly higher than the shoots (P<0.05), and Cd distribution followed the trend: cell wall >organelle >soluble substance (F1> F2> F3). The plant's accumulation capacity against 25mg/L Cd was higher than for other treatments. The root accumulation capacity was much higher (702.3mg/L) than those without added HA. However, upon adding 200 and 250mg/L HA, the phytoremediation of Cd in the root and shoot significantly reduced (P<0.05). Conversely, HA improved the Cd removal efficiency of the plants, notably at a lower HA concentration (150mg/L). In addition, HA (especially at 150mg/L) influences Cd distribution in vetiver cells (P<0.05) and can significantly increase the proportion of Cd in the root cytoplasm. Consequently, a low HA concentration can significantly improve Cd accumulation in the vetiver, shorten the metal's bioremediation cycle, and improve the biological absorption efficiency.

Photocatalytic disinfection for point-of-use water treatment using Ti3+ self-doping TiO2 nanoparticle decorated ceramic disk filter

The challenge from pathogenic infections still threatens the health and life of people in developing areas. An efficient, low-cost, and abundant-resource disinfection method is desired for supplying safe drinking water. This study aims to develop a novel Ti3+ doping TiO2 nanoparticle decorated ceramic disk filter (Ti3+/TiO2@CDF) for point-of-use (POU) disinfection of drinking water. The production of Ti3+/TiO2@CDF was optimized to maximize disinfection efficiency and flow rate. Under optimal conditions, the log reduction value (LRV) could reach up to 7.18 and the flaw rate was 108 mL/h. The influences of environmental factors were also investigated. Natural or slightly alkaline conditions, low turbidity, and low concentration of humic acid were favorable for the disinfection of Ti3+/TiO2@CDF, while co-existing HCO3− ions and diatomic cations (Ca2+ and Mg2+) exhibited the opposite effect. Furthermore, the practicability and stability of Ti3+/TiO2@CDF was demonstrated. Ti3+/TiO2@CDF showed high disinfection efficiency for E. coli and S. aureus under a range of concentrations. Long-term experiment indicated that Ti3+/TiO2@CDF was stable. The underlying disinfection mechanisms were investigated and concluded as the combination of retention, adsorption, and photocatalytic disinfection. The developed Ti3+/TiO2@CDF can provide an effective and reliable disinfection tool for POU water treatment in remote area.

Adsorption of levofloxacin on natural zeolite: effects of ammonia nitrogen and humic acid.

The persistence of antibiotics in sewage treatment plants in recent years has become a serious problem. Meanwhile, humic acid and ammonia nitrogen are widely distributed in natural reservoirs and might influence the sorption, migration and transformation of antibiotics. In this study, natural zeolite (NZ) was evaluated as an adsorbent for the removal of levofloxacin (LEV). The physical and chemical properties of NZ before and after adsorption were characterized by various analytical techniques to develop the mechanism. The effects of ammonia nitrogen and humic acid (HA) on the interfacial behavior of LEV on NZ were explored. Comparative experiments revealed that LEV adsorption on NZ involved electrostatic interactions and ion exchange, and the adsorption processes were well fitted by the Langmuir isotherm model and pseudosecond-order kinetic model. The maximum experimental adsorption capacity of LEV was 22.17 mg·g-1 at pH 6.5. The presence of ammonia nitrogen and HA significantly suppressed the adsorption of LEV due to competitive adsorption, and the adsorption capacity decreased 58 and 46%, respectively. It is obvious that low concentrations of ammonia nitrogen and HA are conducive to improving the treatment effect of sewage. This study demonstrates that NZ is a promising and efficient material for LEV adsorption.

Photoaging of Baby Bottle-Derived Polyethersulfone and Polyphenylsulfone Microplastics and the Resulting Bisphenol S Release.

This study evaluated the release of bisphenol S (BPS) from polyethersulfone (PES) and polyphenylsulfone microplastics (MPs) derived from baby bottles under UV irradiation. Released BPS fluctuates over time because it undergoes photolysis under UV254 irradiation. Under UV365 irradiation, the highest released concentration at 50 °C was 1.7 and 3.2 times that at 35 and 25 °C, respectively, as the activation energy of the photochemical reactions responsible for MP decay was reduced at high temperatures. Low concentrations of humic acid (HA, ≤10 mg·L-1) promote BPS release because HA acts as a photosensitizer. A high concentration of HA (10∼50 mg·L-1) decreases the BPS release because HA shields MPs from light and scavenges reactive radicals that are produced via photochemical reactions. For example, under UV irradiation, hydroxyl radicals (•OH) attack results in the breakage of ether bonds and the formation of phenyl radicals (Ph•) and phenoxy radicals (Ph-O•).The•OH addition and hydrogen extractions further produce BPS from the decayed MPs. A leaching kinetics model was developed and calibrated by the experimental data. The calibrated model predicts the equilibrium level of BPS release from MPs that varies with the surface coverage density of BPS and leaching rate constants. This study provides groundwork that deepens our understanding of environmental aging and the chemical release of MPs.

Experimental and theoretical study on the degradation of Benzophenone-1 by Ferrate(VI): New insights into the oxidation mechanism

The oxidation of Benzophenone-1 (BP-1) by ferrate (Fe(VI)) was systemically investigated in this study. Neutral pH and high oxidant dose were favorable for the reaction, and the second order rate constant was 1.03 × 103 M−1·s−1 at pH = 7.0 and [Fe(VI)]0:[BP-1]0 = 10:1. The removal efficiency of BP-1 was enhanced by cations (K+, Ca2+, Mg2+, Cu2+, and Fe3+), while inhibited by high concentrations of anions (Cl- and HCO3-) and low concentrations of humic acid. Moreover, intermediates were identified by LC-MS, and five dominating reaction pathways were predicted, involving single hydroxylation, dioxygen transfer, bond breaking, polymerization and carboxylation. Theoretical calculations showed the dioxygen transfer could occur by Fe(VI) attacking the CC double-bond in benzene ring of BP-1 to form a five-membered ring intermediate, which was hydrolyzed twice followed by H-abstraction to generate the dihydroxy-added product directly from the parent compound. Dissolved CO2 or HCO3- might be fixed to produce carboxylated products, and Cl- led to the formation of two chlorinated products. In addition, the toxicity assessments showed the reaction reduced the environmental risk of BP-1. This work illustrates Fe(VI) could remove BP-1 in water environments efficiently, and the newly proposed dioxygen transfer mechanism herein may contribute to the development of Fe(VI) chemistry.

Ti2O3/TiO2-Assisted Solar Photocatalytic Degradation of 4-tert-Butylphenol in Water

Colored Ti2O3 and Ti2O3/TiO2 (mTiO) catalysts were prepared by the thermal treatment method. The effects of treatment temperature on the structure, surface area, morphology and optical properties of the as-prepared samples were investigated by XRD, BET, SEM, TEM, Raman and UV–VIS spectroscopies. Phase transformation from Ti2O3 to TiO2 rutile and TiO2 anatase to TiO2 rutile increased with increasing treatment temperatures. The photocatalytic activities of thermally treated Ti2O3 and mTiO were evaluated in the photodegradation of 4-tert-butylphenol (4-t-BP) under solar light irradiation. mTiO heated at 650 °C exhibited the highest photocatalytic activity for the degradation and mineralization of 4-t-BP, being approximately 89.8% and 52.4%, respectively, after 150 min of irradiation. The effects of various water constituents, including anions (CO32−, NO3, Cl and HCO3−) and humic acid (HA), on the photocatalytic activity of mTiO-650 were evaluated. The results showed that the presence of carbonate and nitrate ions inhibited 4-t-BP photodegradation, while chloride and bicarbonate ions enhanced the photodegradation of 4-t-BP. As for HA, its effect on the degradation of 4-t-BP was dependent on the concentration. A low concentration of HA (1 mg/L) promoted the degradation of 4-t-BP from 89.8% to 92.4% by mTiO-650, but higher concentrations of HA (5 mg/L and 10 mg/L) had a negative effect.

Peroxymonosulfate/LaCoO3 system for tetracycline degradation: Performance and effects of co-existing inorganic anions and natural organic matter

The activation of peroxymonosulfate (PMS) has been developed as an efficient method for the degradation of antibiotics by generating reactive oxygen species (ROS), where perovskite oxides have been found as effective catalysts. However, the reaction mechanism, degradation pathways, as well as the effects of co-existing inorganic anions and natural organic matter are largely unknown. Therefore, the degradation efficiency and mechanism of tetracycline (TC) through the activation of PMS by LaCoO3 were investigated. Moreover, the effects of common inorganic anions and natural organic matter (e.g., humic acid (HA)) on degradation efficiency were also compared. Without the presence of coexisting substance, TC removal exceeded 90% within 30 min, while 72% mineralization was achieved in 6 h, under optimal reaction conditions at neutral pH. In addition to the traditional free radicals of OH and SO4−, 1O2 and O2− also contributed the degradation process, where the lattice oxygen of LaCoO3 played a significant role in 1O2 production. The degradation of TC followed the pathways including demethylation, hydroxylation and ring-opening reactions. The presence of inorganic anions (i.e., H2PO4−, Cl− and SO42−) and low-concentration of HA promoted the degradation. Among them, H2PO4− and lower concentration of HA showed the most obvious outcome, while the SO42− can regulate the radical formation, minimize the peak radical level, and therefore promote the overall performance of the process. LaCoO3 exhibited stable reusability with minor decrease in TC removal, likely due to the adsorption of intermediate products from previous stages. These obtained results shed light on the application of perovskite oxides for PMS activation on removing antibiotics.

Enhanced transformation of phenolic compounds by manganese(IV) oxide, manganese(II) and permanganate in the presence of ligands: The determination and role of Mn(III)

In this study, colorimetric methods using diethyl phenylene diamine (DPD), 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), and potassium iodide (KI) were adopted to determine manganese(III)-pyrophosphate complex (Mn(III)-PP) in trace levels (10 µM). An ozonation method was developed to measure the concentration of dissolved Mn including Mn(II) and Mn(III) via quantifying permanganate (Mn(VII)) generation during ozonation of dissolved Mn with PP. The ozonation method combined with DPD method were applied to determine MnO2 dissolution kinetics in the presence of oxalic acid through monitoring MnO2 decay and Mn(III)/Mn(II) formation. PP and oxalic acid could enhance the oxidation of bisphenol A (BPA) by MnO2 at acidic pH, while no promoting effects were found at neutral pH. EDTA had enhancing effect on the oxidation of BPA by MnO2 at neutral pH but inhibiting effect at acidic pH. This result indicated high activities of Mn(III)-PP at acidic pH, Mn(III)-oxalic acid at acidic pH, and Mn(III)-EDTA at neutral pH, respectively. Enhancing effect of low concentration of humic acid (HA) on the oxidation of BPA by MnO2 at acidic pH, however inhibiting effect was found with high dosage of HA due to the occupation of HA on the active sites of MnO2. Furthermore, desferrioxamine B (DFO) was observed to accelerate the transformation of hydroquinone (HQ) with Mn(II) at alkaline pH, ascribed to the formation of Mn(III)-DFO from the reaction of oxygen with Mn(II) rapidly. Finally, a Mn(VII)-reducer-PP system for the transformation of phenolic compounds was evaluated. Two effective reducers (i.e., Mn(III) inducers), Mn(II) and bisulfite, could accelerate Mn(III)-PP formation in the initial step, and then promote the oxidation of phenolic compounds by Mn(VII). Transformation of phenolic compounds were enhanced by MnO2(IV), Mn(II) and Mn(VII) in the presence of ligands due to the generation of Mn(III), which reflected the potential application value of Mn(III) in micro-polluted water treatment.

Mechanism of humic acid adsorption as a flotation separation depressant on the complex silicates and hematite

Humic acid (HA), as an environmentally friendly depressant, absorbed significant attention for possible cleaner production within mineral separation by selective separation, especially for reverse flotation of iron oxides. However, a few systematic studies were addressed its effect on the iron ore reverse cationic flotation in the presence of complex silicates and its adsorption mechanism on the surface of these minerals. This work is going to fill this gap by exploring the depression mechanisms of HA through the reverse cationic flotation (separation of hematite from complex silicates; augite and hornblende). Wettability analyses, micro and batch flotation scale tests were employed for such a purpose. Adsorption test, turbidity measurement, zeta potential measurement, and Fourier Transform Infrared (FTIR) analyses were conducted to understand HA adsorption's mechanism on these minerals' surface. Results relieved that at the low concentration of HA (20 mg/L), adsorption on the examined minerals' surface occurred. Based on the micro-flotation test results, HA's depression impact has the following order: hematite ≫ augite > hornblende. The surface analysis results suggested that HA can interact with the Fe ions on the hematite surface by chemical and hydrogen bonding. The same mechanism was observed for HA adsorption on the considered silicates; however, the adsorption rate was lower, while lower polyvalent cations are available on these silicates' surface.

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.

Facet-controlled activation of persulfate by magnetite nanoparticles for the degradation of tetracycline

Three kinds of magnetite nanoparticles (MNPs) with different exposed crystal facets were synthesized by hydrothermal methods to investigate the effects of exposed facets on the catalytic performance. Under the heterogeneous activation of persulfate (PS), MNPs exposed with {111} facet exhibited better catalytic performance than that exposed with {110} or {100} facet in the degradation of tetracycline, and catalytic performance followed the order of MNPs{111} > MNPs{100} > MNPs{110}. The degradation efficiency of PS/MNPs{111} system reached 74.38% in 4 h, which was much higher than PS/MNPs{110} (19.29%) or PS/MNPs{100} (33.79%) system. Hydroxide, physically adsorbed H2O on the MNPs surface and transformations of Fe(II)/Fe(III) were the main factors to determine the high catalytic performance of MNPs{111} for PS, as illustrated by the X-ray photoelectron spectroscopy (XPS) analysis. The corresponding catalytic oxidation mechanism has been proposed on the basis of attenuated total reflection flourier transformed infrared spectroscopy (ATR-FTIR), electron paramagnetic resonance (EPR) spectroscopy and quenching tests. SO4− and OH were the main radicals in catalytic degradation of tetracycline. Density functional theory (DFT) analysis suggested that the difference of {110}, {111} and {100} facets of magnetite determine the electron density and the density of states on the surface of catalysts. In addition, relatively low concentration of humic acid promoted the removal of tetracycline, and higher concentration of humic acid inhibited the tetracycline removal.

Hetero-aggregation of goethite and ferrihydrite nanoparticles controlled by goethite nanoparticles with elongated morphology

The dispersities of goethite nanoparticles (GTNPs) and ferrihydrite nanoparticles (FHNPs) affect the transport and retention of nanoparticle-associated contaminants. However, the effects of interaction on nanoparticle stability under varying environmental conditions have not been previously investigated. This study utilized settling experiments, a semi-empirical model, and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory to study the homo-aggregation and hetero-aggregation of GTNPs and FHNPs. The pure system of GTNPs tended to aggregate more easily than that of FHNPs, especially under the conditions of high pH (7.0–9.0), high ionic strength (IS, 10 mM), and low concentrations of humic acid (HA) (2 mg L−1). This aggregation was attributed to the elongated morphology of GTNPs, which contributed to surface heterogeneity. The GTNPs and FHNPs mixtures rapidly coagulated, particularly under the surface-charge disequilibrium caused by an increase in negative charges or IS. Hetero-aggregation increased with increase in the GTNPs ratio, indicating that the elongated GTNPs dominated the coagulation of the Fe mineral nanoparticle mixture, which was attributed to the surface heterogeneity and high probability collisions between the GTNPs. Although DLVO neglects the influence of heterogeneity on the nanoparticle surfaces, SEM revealed that hetero-aggregation of GTNPs and FHNPs occurred. The results obtained in this study provide novel and valuable insights into the behaviors of GTNPs and FHNPs mixtures and suggest that during the gradual transformation of FHNPs to GTNPs in soil or aquatic environments, the hetero-aggregation of GTNPs and FHNPs may be enhanced, thus promoting contaminant immobilization.

Exploring sorption behaviors of Se(IV) and Se(VI) on Beishan granite: Batch, ATR-FTIR, and XPS investigations

As the important anions in the high-level radioactive waste (HLRW), and the sorption behaviors of Se(VI) and Se(IV) on Beishan granite are important to the safe evaluation and performance assessment of the deep geological repository for HLRW in China. In this study, the sorption behaviors of Se(IV) and Se(VI) on Beishan granite were investigated under different environmental factors combining batch and spectroscopic approaches, such as X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transformed infrared (ATR-FTIR). The results showed that Se(IV) sorption on Beishan granite was much higher than that of Se(VI) mainly due to their molecule charge densities and speciation in solution. The sorption of Se(IV) was independent of ionic strength, suggesting that the inner-sphere complexes (ISCs) was dominant for Se(IV) sorption on granite. However, Se(VI) sorption on Beishan granite was strongly dependent on ionic strength, which indicated that ion exchange and outer-sphere complexes (OSCs) dominated Se(VI) sorption. ATR-FTIR confirmed that Se(IV) and humic acid (HA) could form a soluble complex of HA-Se(IV) in solution. Therefore, under low pH and HA concentration conditions, the presence of HA could enhance Se(IV) sorption; while inhibited Se(IV) sorption on granite to some extent under high pH and HA concentration conditions. In case of Se(VI), the presence of HA greatly reduced the surface charge density of the Beishan granite, which in turn enhanced the electrostatic repulsion of Se(VI) and granite surfaces, thereby inhibiting Se(VI) sorption on granite surface. When Se(IV) coexisted with Eu (III), the sorption of Se(IV) on granite could be enhanced to a large extent, which might be due to the formation of Eu2(SeO3)3 complex with Se(IV) under alkaline condition (consistent with ATR-FTIR), or surface charge modifications of Beishan granite caused by Eu(III) sorption. Owing to the ISCs of Se(IV) on granite, the presence of anions almost no obvious effect on Se(IV) sorption except for SO32− and HPO42− in terms of the competitive sorption, which inhibited Se(IV) sorption to some extent. It was interesting that CO32− could enhance Se(IV) sorption to a large extent, however the mechanism was still not clear. Furthermore, XPS further confirmed that Se(IV) sorption on granite was dominated by Fe(II) and Fe(III), and the host mineral was biotite.