Metal inhibition on the reactivity of manganese dioxide toward organic contaminant oxidation in relation to metal adsorption and ionic potential

Selective recovery of Cu(II) from strongly acidic wastewater by zinc dimethyldithiocarbamate: Affecting factors, efficiency and mechanism

The selective recovery of copper from strongly acidic wastewater containing mixed metal ions remains a significant challenge. In this study, a novel reagent zinc dimethyldithiocarbamate (Zn(DMDC)₂) was developed for the selective removal of Cu(II). The removal efficiency of Cu(II) reached 99.6% after 120 min reaction at 30°C when the mole ratio Zn(DMDC)₂/Cu(II) was 1:1. The mechanism investigation indicates that the Cu(DMDC)₂ products formed as a result of the displacement of Zn(II) from the added Zn(DMDC)₂ by Cu(II) in wastewater, due to the formation of stronger coordination bonds between Cu(II) and the dithiocarbamate groups of Zn(DMDC)₂. Subsequently, we put forward an innovative process of resource recovery for strongly acidic wastewater. Firstly, the selective removal of Cu(II) from actual wastewater using Zn(DMDC)₂, with a removal efficiency of 99.7%. Secondly, high-value CuO was recovered by calcining the Cu(DMDC)₂ at 800°C, with a copper recovery efficiency of 98.3%. Moreover, the residual As(III) and Cd(II) were removed by introducing H₂S gas, and the purified acidic wastewater was used to dissolve ZnO for preparation of valuable ZnSO₄·H₂O. The total economic benefit of resource recovery is estimated to be 11.54 $/m³. Accordingly, this study provides a new route for the resource recovery of the treatment of copper-containing acidic wastewater.

Water with low ionic strength recovers the passivated birnessite-coated sand reactivity towards lincomycin removal

The ionic strength of infiltration water changes with the seasonal alternation of irrigation sources. In this study, reactivity changes of birnessite-coated sand with the fluctuations of ionic strength of infiltration water (i.e. from groundwater to rainwater) and the involved mechanism were investigated through column experiments. Birnessite-coated sand was less reactive in groundwater than in rainwater because of the higher cation content and higher pH of groundwater. The cations in the groundwater were adsorbed on birnessite-coated sand and then desorbed in presence of a dilute aqueous solution represented by rainwater. The reactivity of the passivated birnessite-coated sand was recovered instantaneously, and approximately one-third of the pristine reactivity was restored. During recovery, Na⁺ desorption and lincomycin (LIN) removal both exhibited a two-stage reaction pattern. The LIN removal correlated with Na⁺ desorption (r = 0.99) so that the reactive sites that were binding 5.602 μmol of Na⁺ became available for 1 μmol of LIN removal. These results suggest that the reactivity of manganese oxides toward organic contaminant is associated with the ionic strength of infiltration water and indicate that the partial reactivity can be naturally restored.

Effect of humic acid on phenanthrene removal by constructed wetlands using birnessite as a substrate

The binding of polycyclic aromatic hydrocarbons (PAHs) to humic acid (HA) can boost the complexation–flocculation process and promote pollutant oxidation through the role of HA as an electron shuttle. HA-coated biochar (BA) was added to study the effects of HA on phenanthrene (PHE) removal by constructed wetlands (CWs) using birnessite as a substrate. HA reduced the average PHE concentration of effluent by 26.58% due to its role as a complexing agent, based on Fourier-transform infrared spectroscopy analysis. For CWs with birnessite, the PHE removal performance was further enhanced due to the role of electron shuttles. X-ray photoelectron spectroscopy and illumina high-throughput analysis revealed an enhanced Mn–Fe cycle. The total relative proportions of Mn-oxidizing bacteria and iron-oxidizing bacteria in VFBCW-HA/BA were 2.33 and 5.50 times as high as those in VFBCW-BA and VFCW-HA/BA. Humic acid also accelerated the biodegradation of PAHs and the quantity of PAH degradative bacteria in VFBCW-HA/BA was 6.29 times greater than in VFBCW-BA.

An enhanced birnessite constructed wetland for phenanthrene removal is proposed based on HA coated biochar and the strengthening mechanism is reported.

Phenol driven changes onto MnO2 surface for efficient removal of methyl parathion: The role of adsorption

Manganese oxides (MnO₂), important environmental oxides, have drawn significant attention in areas such as detoxification of micro-hazardous organic contaminants with electron-donating functional groups such as -OH. However, studies on whether these oxidized processes might further impact the fate of some esters like organophosphorus pesticide (OPPs) remain poorly understood. Herein, we propose a new mechanism involved in the enhanced removal of methyl parathion in mixtures of MnO₂ and phenol. Specifically, the removal of methyl parathion (up to 73.7%) was significantly higher for a binary system than for MnO₂ alone (approximately 9.3%) and was primarily due to adsorption rather than degradation. The extent of methyl parathion adsorption was dependent significantly on pH, reactant loading and metal ion co-solutes (such as Ca²⁺, Mg²⁺, Fe³⁺ and Mn²⁺). Both spectroscopic (FT-IR, SEM-EDX and XPS) and chromatographic (LC/HRMS) analyses showed that the remarkable increase in the number of organics (e.g., polymers) onto the MnO₂ surface dominated methyl parathion adsorption via hydrogen bonding, n-π and π-π interactions, van der Waals forces and pore-diffusion. The results from this study provided evidence for the role of manganese oxides in adsorption of methyl parathion in soil-aquatic environments involving phenolic compounds.

Removal of lincomycin from aqueous solution by birnessite: kinetics, mechanism, and effect of common ions

The removal of lincomycin (LIN) from aqueous solution by birnessite was investigated by batch experiments. When the dosage of birnessite is 500 mg L^-1 and the initial concentration of LIN is 15.5 μmol L^-1, more than 90% of LIN was removed within 240 min at pH 4.90. Under different conditions, the reactions were well fitted with the second-order model (R² > 0.95). The removal kinetics and the reaction mechanism were described. The presence of cations (e.g., K⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺) inhibited the removal of LIN by birnessite, following the order: Mn²⁺ > Fe²⁺ > Ca²⁺ > Mg²⁺ > K⁺ ≈ Na⁺, due to the sorption of cations on birnessite, companying with the electron transfer and precipitation of oxides (for Mn²⁺ and Fe²⁺). The addition of Cu²⁺, SO₄^2-, or NO₃^- improved the reactions. The presence of Cu²⁺ could oxidize antibiotics, and the repulsion between SO₄^2-or NO₃^- and birnessite might disperse the birnessite suspensions during the reactions. Mn(IV) and Mn(III) were the core Mn species that play an important role in LIN removal. These findings will help to understand the removal process of LIN and illustrate the influence of cations and anions on the removal of similar pollutants by birnessite.

Cotransport of Herbaspirillum chlorophenolicum FA1 and heavy metals in saturated porous media: Effect of ion type and concentration

Predicting the cotransport of functional microorganisms and heavy metals in porous media is essential to both bioremediation and pollutant risk assessment. In this study, batch and column experiments were conducted to explore the cotransport behaviors of functional bacteria (FA1) and heavy metals (Pb²⁺/Cd²⁺) in saturated sand media under different conditions. The sorption capacity of heavy metals on FA1 was much greater than that of the sand, while both FA1 and sand showed stronger affinity to Pb²⁺ than Cd²⁺. The surface properties, especially zeta potential, of the bacteria and sand were altered by metal adsorption. As a result, the co-existence of Pb²⁺ decreased the transport of FA1 more significantly than that of Cd²⁺, and the influence was more significant with higher heavy metal concentration. On the other hand, the co-existence of FA1 inhibited the mobility of Pb²⁺ and Cd²⁺ in most scenarios, except when the cotransport concentration of Pb²⁺ was 5 mg L^-1, and the inhibition was more pronounced for Pb²⁺ than Cd²⁺. Increase in metal concentrations decreased the FA1-associated Pb²⁺/Cd²⁺ in effluents due to the remarkable decrease in FA1 mobility, and free soluble Pb²⁺/Cd²⁺ became the major migration species. In addition, due to stronger attractive forces and affinity between Pb²⁺ and FA1, nearly all presorbed-Pb²⁺ by sand was remobilized by FA1 and transported mainly in FA1-associated form other than soluble Pb²⁺. Findings from this study indicated that the cotransport of biocolloids and heavy metals are highly sensitive to the ion type and concentration, and evaluation of their transport in the subsurface should be carefully carried out to avoid inaccurate estimations.

Oxidation of β-blockers by birnessite: Kinetics, mechanism and effect of metal ions

Manganese dioxides are ubiquitous in natural waters, soils, and sediments and play an important role in oxidative transformation of organic pollutants. This work presents the kinetics of the oxidation of selected β-blockers, betaxolol, metoprolol, and atenolol by birnessite (δ-MnO₂) as a function of concentration of the β-blocker, dosage of δ-MnO₂, and solution pH. The values of pseudo-first-order rate constants (k_obs) of β-blockers decreased in the order betaxolol > atenolol > metoprolol, which was positively correlated with their acid dissociation constants (K_a). Effect of series of metal ions (Fe³⁺, Cr³⁺, Al³⁺, Pb²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Cd²⁺, Mg²⁺, and Ca²⁺) on the degradation of β-blockers by δ-MnO₂ was systematically examined. All of these metal ions inhibited the oxidation reaction under the same constant ionic strength. The inhibition efficiency was positively correlated with the logarithm of stability constant of metal ions in aqueous solution (logK_MeOH). By LC-ESI-MS/MS analyses, the oxidation of β-blockers primarily involved hydroxylation and cleavage of the parent molecules to the short branched chain compounds. An electron transfer mechanism for the oxidation of β-blockers by δ-MnO₂ was proposed. The oxidation was initiated by the electron transfer from the nonbonding electrons on nitrogen (N-electrons) of β-blockers to δ-MnO₂, followed by transformation of radical intermediates. These findings will help to understand the oxidation processes of β-blockers and predict the effect of metal ions on the removal of pollutants by δ-MnO₂ in the environment.

Effect and mechanism analysis of MnO2 on permeable reactive barrier (PRB) system for the removal of tetracycline

Effect of manganese dioxide (MnO₂) on tetracycline (TC) removal/degradation in zero-valent iron (ZVI) based permeable reactive barrier (PRB) system was investigated. To analyze the role of MnO₂, three different PRB columns packed with ZVI, ZVI and a layer of MnO₂, and MnO₂ were set up to investigate the removal effect and reaction mechanism of ZVI coupling with MnO₂ on TC removal, respectively. The results show that the removal efficiencies of three PRB columns are 65%, 85%, and 50%, respectively. MnO₂ could accelerate the transformation of Fe²⁺ into Fe³⁺ and combine with Fe³⁺ to degrade TC in different reaction sites in the ZVI-MnO₂ PRB system. Hydroxyl radicals (·OH) were produced in this process, which contributed to about 58.3% for the TC degradation. The UV-Vis spectrum demonstrated that A ring of TC was the main reaction site for interaction with Fe³⁺ and the BCD rings were crucial for interactions with MnO₂. On the basis of intermediates identified by LC-ESI-MS, the ring structure of TC was opened, and low-molecular-weight compounds were produced in ZVI-MnO₂ PRB system.