Performance And Mechanism Of Iron And Manganese Modified Hydrogel And Hydrochar For Antibiotics And Heavy Metal Removal In Water

Due to rapid developments in industrialization and urbanization around the world,the availability of clean water is adversely affected by the release of various contaminants,which eventually pose detrimental effects on public health and ecological security.To overcome these bottlenecks,effective and efficient water and wastewater remediation techniques are highly required.In this study,we produced hydrous manganese dioxide loaded poly（sodium acrylate）hydrogel（PSA-HMO）and manganese ferrite loaded swine manure hydrochar（MnFe₂O₄-SMHC）via facile approaches to decontaminate antibiotics and heavy metal ions from aqueous systems.The synthesis of PSA-HMO was conducted through sequences of ion exchange and in-situ redox approaches,while MnFe₂O₄-SMHC was produced via hydrothermal and coprecipitation techniques.Tetracycline antibiotics such as tetracycline（TTC）,oxytetracycline（OTC）,and chlortetracycline（CTC）were chosen to represent the antibiotics category.Similarly,cadmium（Cd）and lead（Pb）were considered model heavy metal ions in this study.The chemical and structural features of these materials were intensively investigated.The morphological investigation using the SEM and EDS characterization indicated that the surface structures and the chemical composition of the PSA-HMO were significantly changed after the introduction of HMO.Defect structures were also observed on the surface of PSA-HMO,which may be attributed to the strong oxidative properties of KMn O4 during the material preparation.These observations indicated that the presence of HMO NPs induced the initiation of pore opening and modification of pore structure.The surface topographies of the PSA-HMO composite are sheltered by uncountable small HMO particles.According to the EDS mapping,Mn was significantly detected,indicating the presence of Mn O2 NPs into the network structure of the PSA hydrogel.The XRD analysis indicated that the structure of PSA-HMO exhibited behavior of both amorphous and crystalline nature,which is characteristic of birnessite-type HMO（δ-Mn O2）.The specific surface area,pore-volume,and pore diameter of PSA-HMO were slightly improved after the introduction of HMO.The Mn-OH vibration characteristic peak was detected on the FTIR spectrum of PSA-HMO.The oxygen-containing functional groups were either shifted or disappeared after heavy metal and antibiotics adsorptions.The HMO was also found to be involved in the oxidative degradation of antibiotics,such as OTC.High amounts of electron transfer between PSA-HMO and antibiotics produced ●OH radicals for the subsequent degradation interactions.The surface elemental composition indicated that C,O,and Mn were detected on the PSA-HMO composite.The SEM image of MnFe₂O₄-SMHC revealed loose,distinct,and rough structures decorated with uniformly distributed nanoparticles.The EDS spectra showed that the content of Fe and Mn increased significantly after MnFe₂O₄ loading.The specific surface area and pore volume of the MnFe₂O₄-SMHC were significantly improved after the involvement of MnFe₂O₄,which were essential to reduce the mass transfer resistance during adsorption.The XRD peaks corresponding to the spinel ferrite were notably shown in the MnFe₂O₄-SMHC sample.The saturation magnetization curves of MnFe₂O₄-SMHC exhibited a typical S-shaped hysteresis loop with a saturation magnetization value of 21.41 emu g-1.The FTIR peaks corresponding to Fe–O and Mn–O vibrations were remarkably detected in the MnFe₂O₄-SMHC sample.In the highresolution XPS spectra of MnFe₂O₄-SMHC,the C 1s,O 1s,Mn 2p,and Fe 2p were detected.Batch studies were investigated to gain insights into the characteristics of the antibiotics and heavy metal removal using PSA-HMO.The p H studies showed the removal of antibiotics and heavy metal ions significantly improved as the p H raised from p H 2.0 to p H 6.0.The optimum p H levels for both contaminants were generally found to be in the range between p H 4.0 and 6.0.The p H point of zero charges（p HPZC）of PSA-HMO was measured around p H 6.0.The adsorption kinetics of these contaminants onto PSA-HMO were evaluated using the Pseudo-firstorder,Pseudo-second-order,and intraparticle diffusion models.The adsorption isotherms of antibiotics and heavy metal ions onto PSA-HMO were examined using Langmuir,Freundlich,and Temkin isotherm models.According to the Langmuir isotherm model,the maximum adsorption capacities of TTC,OTC,Pb（Ⅱ）,and Cd（Ⅱ）were 475.8,1,150.4,288.7,and 83.1mg/g,respectively.These adsorption capacities were relatively higher compared to various adsorbents reported in the literature.In a binary system,the uptakes of antibiotics were slightly promoted in the presence of heavy metal ions,which may be due to complexation interactions resulting from the bridging effects.The PSA-HMO was selective for the uptake of antibiotics and heavy metal ions under the presence of common environmental ions and natural dissolved organic matter.The PSA-HMO also exhibited excellent performances for the uptake of antibiotics and heavy metal ions in the real river and tap water.Spectroscopic measures and adsorption models indicated that ion exchange,H-bonding,cation-π bonding,and π-π EDA were the leading mechanisms for the decontamination of antibiotics.On the other hand,Hbonding,electrostatic,and ion exchange adsorption mechanisms were proposed for the uptake of Pb（Ⅱ）and Cd（Ⅱ）by PSA-HMO.In addition to adsorption,the PSA-HMO was found to be an excellent material for the oxidative degradation of antibiotics via hydroxylation,secondary alcohol oxidation,demethylation,decarbonylation,dehydration,and demethylation interaction processes.The electron transfer between the antibiotics and Mn-OH during the interaction process generated ●OH radicals,which then attacked the antibiotics to further decompose into their transformation byproducts.For instance,LC-MS analysis revealed OTC was structurally decomposed into 15 intermediate products.In light of these conditions,PSA-HMO can be considered as environmentally friendly,effective,efficient,economical,and low toxicity to decontaminate antibiotics and heavy metal ions in the practical environment.The removals of antibiotics and heavy metal ions were also studied using the MnFe₂O₄-SMHC composite.CTC and Cd（Ⅱ）were chosen in this study to understand the behavior of antibiotics and heavy metal ion adsorption onto the MnFe₂O₄-SMHC,respectively.The influences of various operational parameters such as solution p H,adsorbent dose,initial concentration were explored using batch experiments.The effects of solution p H showed that the uptake of CTC and Cd（Ⅱ）onto MnFe₂O₄-SMHC were optimized under near-neutral p H conditions.The adsorption of CTC and Cd（Ⅱ）were explained by the Pseudo-second-order model kinetic model,suggesting that the adsorption processes were largely dominated by chemisorption via surface complexation interaction mechanism.The isotherm studies have shown that both the adsorptions of CTC and Cd（Ⅱ）were explained by the Langmuir isotherm model.According to the Langmuir isotherm model,the maximum adsorption capacities of MnFe₂O₄-SMHC for the uptake of CTC and Cd（Ⅱ）were 752.96 and 62.16 mg/g,respectively.The presence of common environmental had no significant influences on the removal of CTC and Cd（Ⅱ）.The presence of CTC or Cd（Ⅱ）in the binary conditions had little among each other,while the presence of small concentrations of Cd（Ⅱ）had a slight promotional effect on the uptake of CTC.The reusability study of the MnFe₂O₄-SMHC indicated that the adsorbent was magnetically separable and recyclable for the removal of both CTC and Cd（Ⅱ）with high stability.To sum up,both PSAHMO and MnFe₂O₄-SMHC exhibited excellent performances for the removal of antibiotics and heavy metal ions in the aqueous system under various conditions...