Development Of Catalyst Derived From Drinking Water Treatment Iron Sludge For Peroxymonosulfate Activation And Mechanisms

Drinking water treatment iron sludge is a by-product produced by using iron salts as coagulant in water treatment plants.The annual water purification plant discharge volume in China is about 1.5 billion cubic meters,and the dry sludge production is over millions of tons.Its main components are iron salt hydrolysis products[Fe(OH)₃],solid particles,humus and algae.It has the advantages of wide source of raw materials,large yield,dispersed active sites,and abundant surface chemical functional groups,etc.However,inappropriate treatment and disposal methods may also cause secondary pollution to the ecological environment,therefore,iron sludge has the typical dual properties of"pollutant"and"resource".The advanced oxidation technology based on sulfate radicals(SO₄^·-)has received a lot of attention as an effective method for the removal of organic contaminants.Iron sludge contains a large amount of iron,which has a great potential for persulfate activation.Therefore,no research has been reported on how to make full use of the active ingredients and morphological structure in iron sludge for harmless treatment and at the same time realize waste resource utilization;and how to develop efficient persulfate catalysts by simple and facile modification methods by using iron sludge as raw material or supports.Based on these above problems,this study takes the actual iron sludge as raw material and makes full use of its active ingredients and morphological structure to realize reclamation of iron sludge through a simple and easy modification method,develops a low-cost and high-efficiency iron sludge-based catalyst,constructs an efficient catalytic oxidation system of persulfate.The typical antibiotic--Ciprofloxacin(CIP)was used as the target organic pollutant,this study investigate the catalytic performance and mechanisms.Firstly,the elemental composition and physicochemical properties of the iron sludge from the water treatment plant were analyzed.Fe accounted for about 18.5%by weight with an amorphous non-crystalline structure.The adsorption and removal effect of iron sludge on ciprofloxacin and the oxidative degradation efficiency of activated persulfate were initially investigated,which showed that the adsorption and catalytic capacity of the original iron sludge were both limited and the removal effect on ciprofloxacin was poor.The magnetic catalyst Fe₃O₄@SiO₂ was successfully prepared by solvent thermal reduction method to modify the iron sludge,which improved the catalytic activity for PMS and increased the removal rate of CIP from 44.7%to 82.8%at 60 min of reaction compared with the original iron sludge.This is due to the solvent heat treatment that reduced part of Fe(?)in the iron sludge to Fe(?),and also increased the catalyst specific surface area,which exposed more active sites and improved the catalytic activity for peroxymonosulfate(PMS)to some extent.In order to further improve the catalytic oxidation rate,cobalt-iron layered double hydroxides(Co-Fe-SiO₂ LDH)were prepared using iron sludge as the iron source and carrier,and loaded with highly catalytic active transition metal cobalt(Co)by the solvothermal method.The microscopic morphology shows a hollow cavity lamellar structure with a large comparative area(145.87 m²/g)and stable Co/Fe-O and Co/Fe-Si functional groups and good magnetic characteristics(10 mu/g).The Co-Fe-SiO² LDH-PMS system has excellent catalytic activity for PMS,with a CIP removal rate of 99.6%at 20 min,while maintaining a low metal loss.The Co-Fe/SiO₂ LDH-PMS system has good degradation effect on organic matter under different p H,coexisting anions,natural organic matter and actual water environment background.In order to make fuller use of the iron and organic carbon components in the iron sludge,an efficient,green and economical Fe⁰/Fe₃C catalyst was prepared by modifying the iron sludge through a one-step carbon thermal reduction method without adding chemical reagents.With the increase of temperature in the carbonization process,the self-reduction reaction between Fe(OH)₃ and organic carbon in the iron sludge,Fe(OH)₃ was gradually reduced to Fe₃O₄,Fe⁰ and Fe⁰/Fe₃C.among them,the Fe⁰/Fe₃C catalyst formed microspheres with a diameter of about 25 nm on the surface,and the nano-Fe⁰ was uniformly encapsulated in the carbon-based material with good magnetic properties(35.3emu/g).Fe⁰/Fe₃C showed the best catalytic activity for PMS,with 98.2%CIP degradation at 20 min.Fe⁰/Fe₃C has good reusability and regeneration ability,and also has good catalytic activity for the degradation of many typical organic pollutants.The degradation of organic matter is achieved in Fe₃O₄@SiO₂ mainly through the activation of PMS by Fe(?)to produce SO₄^·-and^·OH.Meanwhile,Fe(?)can also participate in the activation reaction after consuming PMS to generate Fe(?).In the Co-Fe/SiO₂ LDH-PMS system,Co(?)and Fe(?)can directly activate PMS to produce SSO₄^·-and^·OH.there is a synergistic effect between Co and Fe on the catalyst solid surface,and the Fe(?)and Co(?)electron transfer can promote the regeneration of Co(?).Based on the joint mechanism of the two radicals,a catalytic reaction kinetic model was successfully established to better fit the organic oxidative degradation law.synergy exists between Fe⁰ and graphitized carbon in Fe⁰/Fe₃C,Fe⁰ can directly activate PMS to generate radicals and also accelerate the conversion of Fe(?)to Fe(?)to maintain a high catalytic activity,while graphite carbon in Fe₃C can also activate PMS through electron transfer.Four reactive oxygen species,SO₄^·-,^·OH,O₂^·-and ¹O₂),are produced simultaneously in this system.CIP is degraded to CO₂ and H₂O by piperazine ring cleavage,hydroxylation reactions and fluorine substitution reactions with mechanism of action of deamination,de-aldehyde and decarboxylationThe simple and easy modification of iron sludge has enabled the recycling of iron sludge,which not only reduces the cost of sludge treatment and disposal in water treatment plants,but also convert waste into valuable materials,which saves raw material and synthesis preparation cost,and puts forward the core concept of"treat wastewater by waste sludge".It provides a theoretical basis and scientific solution for the practical application of iron sludge modification in water treatment plants to produce efficient catalysts.It is in line with the national environmental protection policies of green energy,sustainable development and zero discharge of water plants,and has important practical application value.