| The heavy metal chromium(Cr)is a carcinogen that is extremely harmful to human health.Excessive chromium in aquatic ecosystem can also cause great harm to aquatic organisms.China is a country with poor chromium mineral resources.Leather tanning factories produce a large amount of chromium containing wastewater and solid hazardous waste every year,causing serious waste of chromium resources and environmental pollution.China’s Hebei Province began to implement the "Discharge standard of water pollutants for leather and fur making industry GB 30486-2013",which reduces the total chromium discharge standard from 1.5 mg/L to 0.5 mg/L.At present,it is difficult for leather tanning factories to treat Cr(Ⅲ)containing wastewater to make the outlet total chromium concentration of the treated wastewater less than 0.5 mg/L.At the same time,a large amount of chromium containing hazardous waste is generated,and chromium resources cannot be recycled.This paper proposed a new mechanism for the treatment of Cr(Ⅲ)containing tannery wastewater with bare magnetic Fe3O4 nanoparticles.Designed and manufactured an innovative neodymium iron boron(Nd-Fe-B)high strength magnetic bar separator.The process of treating Cr(Ⅲ)containing tannery wastewater with magnetic Fe3O4 nanoparticles was systematically studied.The laboratory and pilot scale Cr(Ⅲ)containing tannery wastewater treatment experiments were carried out.The demonstration project for the treatment of Cr(Ⅲ)containing tannery wastewater was completed.An innovative "Fe3O4-Cr(OH)3" cluster mechanism for removing Cr Ⅲ)in chromium containing tannery wastewater was proposed.This mechanism is essentially different from the traditional adsorption mechanism,and it has an ultra high selective capture capacity for Cr(Ⅲ).Magnetic Fe3O4 nanoparticles and"hydrated Cr(OH)3 colloidal clusters" are bound together to form "nanoclusters".The number and size of "nanoclusters" are not limited by the specific surface area of magnetic nanoparticles.Therefore,theoretically,the capture capacity of magnetic Fe3O4 nanoparticles for "hydrated Cr(OH)3 colloidal clusters" is infinite,and its separation efficiency for Cr(Ⅲ)is huge,so it can achieve such a high capture capacity for Cr(Ⅲ)in wastewater.At pH 8,the maximum capture capacity of the bare magnetic Fe3O4 nanoparticles for Cr(Ⅲ)is 452.6 mg/L.Designed and manufactured an innovative neodymium iron boron(Nd-Fe-B)high-strength magnetic bar type magnetic separator.The magnetic separator was composed of 4 vertically aligned magnetic bars in equilateral triangle.The magnetic separator with the advantages of small size,large processing capacity,low energy consumption,and the ability to clean the magnetic bar in situ.The magnetic Fe3O4 nanoparticles were added to water as a simulation solution,and the capture efficiency of the magnetic separator for magnetic Fe3O4 nanoparticles was studied.When the concentration of magnetic Fe3O4 nanoparticles is less than 400 mg/L,the flow rate is less than 18 L/h,and the distance between the magnetic bars is less than 30 mm,the capture efficiency of magnetic Fe3O4 nanoparticles by the magnetic separator is greater than 99.4%within 3 hours.Based on this,the pilot and demonstration project scale magnetic separator with processing capacity of 2-5 m3/h and 10-15 m3/h was designed and manufactured.The laboratory scale continuous flow treatment experiment of Cr(Ⅲ)containing tannery wastewater was carried out under optimal conditions,the magnetic bar distance was 30 mm,and the inlet flow rate was 18 L/h.The outlet total chromium concentration was reduced to about 0.2 mg/L after the secondary treatment,which meets the latest discharge standards.Sodium hypochlorite(NaCIO)solution(10%)was used to regenerate the spent magnetic Fe3O4 nanoparticles,which can achieve close to 100%regeneration of magnetic Fe3O4 nanoparticles,and realized the recovery of more than 99%of Cr(Ⅲ)in wastewater,and no hazardous waste was generated.The regeneration magnetic Fe3O4 nanoparticles were recycled for the treatment of Cr(Ⅲ)containing tannery wastewater,and the outlet total chromium concentration was stable to less than 0.5 mg/L.The pilot experiment was run at flow rate of 2 m3/h for 15 days,working for 8 hours per day,and the in-situ cleaning device was started every 4 hours to clean the magnetic bars.The outlet total chromium concentration in the pilot experiment can be stabilized to less than 0.5 mg/L.The demonstration project operated intermittently and continuously for 7 consecutive days at a flow rate of 10 m3/h,and the outlet total chromium concentration was less than 0.5 mg/L.The magnetic Fe3O4 nanoparticle production line with production capacity of 300 kg/day and a magnetic-chromium elution production line with a capacity of 300 kg/day were built.In addition,China’s Liaoning Province began to implement the "Liaoning Province Comprehensive Wastewater Discharge Standard DB21 1627-2008",which reduces the COD discharge standard to 50 mg/L.Traditional wastewater treatment technology is difficult to make the COD value of oil field produced water meet the latest discharge standards(COD<50 mg/L),or the cost is very high.This paper proposed an innovative mechanism for COD removal from oilfield produced water treated by traditional wastewater treatment technology with magnetic Fe3O4-IDA-Cu2+ nanoparticles.The magnetic separator was used in the continuous flow treatment experiment to remove COD,the COD removal efficiency could reach 66.7%,and the outlet COD value was stable and less than 50 mg/L.The spent magnetic Fe3O4-IDA-Cu2+ nanoparticles could be recycled and reused for 6 times,and the outlet COD concentration was stable less than 50 mg/L. |
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