Characteristics And Mechanisms Of Dyes Removal From Wastewater With Nano Zero Valent Iron(nZVI) Coated Biochar

Over the past few decades,the emission of excessive synthetic dyes from industrial wastewater has become a serious and challenging environmental threat.Synthetic dyes have carcinogenic,teratogenic and mutagenic properties and may cause damage to the human nervous system.At the same time,their complex aromatic structures make them highly physically and chemically stable and difficult to biodegrade.Therefore,synthetic dye wastewaters treatment is widely concerned.Adsorption technology can effectively remove synthetic dyes in wastewater.However,the commercial activated carbons that are commonly used at present have high costs,so there is an urgent need to find efficient,low-cost and environmentally-friendly alternative adsorbents.Biochar(BC)is used as an adsorbent for the adsorption of environmental pollutants such as heavy metals and organics.It has been an active research area in recent years.However,most biochars have a low adsorption capacity.Therefore,the biochar is modified to improve its adsorption capacity.Studies have found that the biochars(BCs)and nanoscale zero-valent metals(nZVMs),in particular,the combination of nanoscale zero-valent iron(nZVI)with biochar can not only improve the adsorption efficiency,but also the nZVI-based adsorbents have a certain potential for degradation of organic pollutants,attenuating their long-term irreversible environmental problems caused by their resistance to the decomposition.However,in the existing researches,the preparation conditions of nZVI/BC composites for the best adsorption and degradation of dyestuffs are still relatively lacking.At the same time,the removal effect of nZVI/BC composites on the co-existence of multiple dyes in waste water are still missing.Moreover,less research on the degradation mechanisms has been conducted.Therefore,in this work,based on the use of response surface methodology(RSM)to optimize the synthesis of nZVI-loaded corn cob biochar,the synthetic adsorbent was studied on two typical dyes,methylene blue(MB)and methyl orange(MO).Adsorption and degradation characteristics in single and composite systems were investigated.BET surface analyzers,scanning electron microscopy(SEM-EDX),X-ray diffractometers(XRD),surface charge zeta potential,Fourier transform infrared spectrometer(FTIR),UV visible spectrophotometer and HPLC-MS system were used to characterize the mechanisms of dye adsorption and degradation.The main results obtained are as follows:(1)The optimal conditions for preparation of nZVI/BC composites by RSM test design are:Fe/BC weight ratio of 1:1,stirring time 84.40 min,stirring speed 400 rpm for MO and Fe/BC weight ratio of 1:5,stirring time of 60 min and stirring speed of 400rpm for MB.These conditions correspond to the final MO and MB removal efficiency of 98.35%and 84.18%respectively.At the same time,the final models obtained under these conditions was statistically significant at a 95%confidence level(p value<0.05).The R~2 value MB was 0.9846 and that for MO was 0.8412,indicating that the response changes of more than 98.46%and 84.12%could be explained by the empirical model based on the experimental BBD.(2)The optimized nZVI/BC adsorbent prepared by the synthesis process can significantly improve the removal of dyes from wastewater.After 24 hours of reaction,the removal efficiency of MB by the nZVI/BC and BC was 84.25%and 53.28%,respectively.Within 3 h of reaction time,the removal rates of MO by the nZVI/BC and BC were 98%and 44.8%,respectively.Kinetic studies showed that the adsorption rates of MB and MO on nZVI/BC corresponded to pseudo-second order kinetics equations.Freundlich and Langmuir models best fitted the experimental data of both MB and MO than Temkin isotherm model based on the R~2 values and straight lines.These findings show that the adsorption behaviors of MB and MO are mainly multi-layer adsorption on heterogeneous adsorption surfaces containing heterogeneous adsorption surfaces with different adsorption energies as well as a monolayer coverage of the two dyes on adsorbent particles and the homogenous distribution of active sites on the biomaterials involving the chemical and physical adsorption.The Langmuir maximum adsorption capacities were found to be 270.27 and 238.09 mg/g for MB and MO respectively.(3)With respect to column adsorption,the breakthrough curve according to the coefficients MB is consistent with the Thomas and Yoon-Nelson models in a single(MB)dye system,but the best-fit Yoon-Nelson model has R~2=0.986.The dynamic absorption capacity(qb)was 15 mg/g,the column-to-MB removal efficiency was89.64%,and the maximum adsorption capacity was 118 mg/g.In the composite(MO+MB)system,the MB antagonistic action toward the MO removal was a little bit high in column adsorption experiments while the coexistence of the two dyes in batch mode has shown a mutual synergistic effect.These effects may be attributed to the higher affinity of the corn cob BC to MB than MO,moderation by the MB of the electrostatic repulsion between the adsorbent negative charges and anionic dye MO and the rise in pH accompanying the removal of MO by the nZVI/BC.(4)Based on UV Visible spectra and HPLC-MS in particular the intermediate products detected,the degradation mechanism of MB and MO using nZVI/BC involved oxido-reduction,complexation,cleavage of the chromophore group the-N=N bond,reduction of the conjugated aromatic structure as well as the cleavage of methyl groups'substituent on the amine groups or successive hydroxylation that may have led to the aromatic ring opening.