Selective Removal Of Heavy Metal Ions From Strong Acid Wastewater By Novel Pyridine Chelate Resins:behavior And Mechanism

Considerable acid wastewater containing heavy metal ions is generated in industrial processes,such as non-ferrous metal metallurgy,metal mining and so on.Meanwhile,the coexisted salts further increase the treatment difficulty.The acid wastewater has to be neutralized before using traditional technologies,resulting in considerable alkali consumption and secondary pollution.In addition,conventional adsorbents are sensitive to acidity and salinity.Therefore,it is of great significance to develop novel adsorbents and techniques to remove and to recover heavy metal ions selectively from highly acidic and salty wastewater.Pyridine chelate adsorbents have unique advantages of acid resisitance and high selectivity.In this article,amine and pyridine were successively introduced into methacrylate-di vinyl benzene copolymer,getting series of pyridine chelate adsorbents(PAPY,PBPY,PCPY,PDPY).PAPY was screened out as the optimum resin due to its best adsorptive properties.Characterization of resins involving SEM,EA,PSD,FTIR and XPS,proved that all the resins were successfully prepared.The adsorption and separation behaviors of heavy metal ions onto pyridine chelate resin with or without salts were comparably investigated by carrying out the sole-/binary-component static and dynamic experiments under high-acidity condition.The principles of selective separation and removal of heavy metal ions as well as the micro-interface interaction mechanisms could be revealed.This findings will inspire important theoretical guidance for the optimized design and preparation of pyridine chelate resins and for the recycling of highly acidic and salty heavy metal wastewater.In high-acidity systems,as pH values rised,the adsorption capacities of PAPY toward Cu(Ⅱ),Ni(Ⅱ)and Zn(Ⅱ)first increased and then decreased,with the optimum pH values of 1,2 and 3 for Cu(Ⅱ),Ni(Ⅱ)and Zn(Ⅱ),respectively.This phenomenon could be attributed to the pore structures of PAPY as well as ionic properties like hydrated ionic radius,electronegativity,polarization force,etc.The adsorption isotherms could be well fitted by Langmuir model with the maximum adsorption capacities of 1.463 mmol/g,1.317 mmol/g and 1.051 mmol/g for Cu(Ⅱ),Ni(Ⅱ)and Zn(Ⅱ)at 298 K,respectively,which were 11.0%,69.5%and 75.8%higher than those of commercial pyridine chelate resin DM4195.The adsorption processes were endothermic and spontaneous and impelled by entropy.The kinetic data could be well fitted by Lagergren-second-order equation,and the adsorption capacity as well as the initial adsorption rate followed the order of Cu(Ⅱ)>Ni(Ⅱ)>Zn(Ⅱ).The binary adsorption isothermal surface could be well fitted by Extended Langmuir model with the selective order of Cu(Ⅱ)>Ni(Ⅱ).In binary adsorption kinetics of Cu(Ⅱ)/Ni(Ⅱ),compared with sole Cu(Ⅱ)system,the adsorption capacity and the initial adsorption rate of Cu(Ⅱ)decreased by 13.2%and 55.8%,respectively,while those of Ni(Ⅱ)decreased sharply due to the competition of Cu(Ⅱ).In dynamic adsorption,the breakthrough point was 90 BV for Cu(Ⅱ)in sole system,which changed to 75 BV in binary system,while the breakthrough points were 0 BV for Ni(Ⅱ)in both systems.The regeneration rate could be almost 100%by using 25%HNO3 as the regenerant.The adsorptive properties of the resin remained the same after six times of adsorption and desorption experiments.The adsorption mechanism was demonstrated by the analysis of FTIR and XPS spectra and the calculation of Density Functional Theory(DFT).The most stable configuration was that heavy metal ions interacted with one tertiary amine group and two pyridyl groups to form two five-member ring structure.In high-acidity and high-salinity systems,NaCl could increase the adsorption capacities of PAPY toward Cu(Ⅱ),Ni(Ⅱ)and Zn(Ⅱ)with the largest enhancement ratio of 22.4%,48.1%and 507.0%.In NO3-and Cl-systems,the adsorption capacity of heavy metal ions increased with the increase of salinity,while those in SO42-system first increased and then decreased.Results calculated by Visual MINTEQ illustrated that the above laws were bound up with the speciation of heavy metal ions in different anion systems.Moreover,effects of electrostatic shielding and compressing double electrical layers played a primary role in the enhancement of adsorption capacity.The binary adsorption isothermal surface with co-existed salts could be well fitted by Extended Langmuir model.The value of KCu/KNi as well as the selective separation ratio both increased with the increase of salinity,indicating the competition enhancement of Cu(Ⅱ)toward Ni(Ⅱ)with co-existed salts.When the concentrations of NaCl and NaNO3 were 250 mmol/L or that of Na2SO4 was 125 mmol/L,the maximum selective separation ratios were 80.93,49.77 and 114.56,169.3%,65.6%and 281.2%larger than those of salt-free system,respectively.In binary system with 500 mmol/L NaCl,the maximum promoting ratios of initial adsorption rate of Cu(Ⅱ)reached 233.3%,while strong competition of Cu(Ⅱ)resulting in the replacement of Ni(Ⅱ).The results of dynamic adsorption revealed that the dynamic separation properties of PAPY were superior to those of DM4195 in high-acidity and high-salinity systems.Consequently,PAPY resin was proved to be a novel inexpensive adsorbent with strong acid and salt resistance as well as high selectivity and reproducibility.The preparation of this novel resin could provide a new method to remove and to recover heavy metal ions from highly acidic and salty wastewater.