Study On A Novel Coupling Technology For Wastewater Treatment Contained Low Concentration Of Cr(Ⅵ) And Resource

This paper mainly studied a kind of new coupling technology which was highefficient and low cost for treatment of low concentration wastewater of Cr（Ⅵ）,including ion exchange resin method, electrodialysis （electrodialysis,ED） method andchemical reduction precipitation process.A nd the best technology conditions wereexplored.An exploratory experimentIon of anion exchange resin method was made.717resin and D301resin were compared,the adsorption ability quite,but D301resin’sdesorption was about4time s more t ha n717res in.The D301weak basic anionexchange resin was chosed to remove heavy metal wastewater Cr（Ⅵ）,and thetheoretical and experimental basis for treatment and condensation cycle of Cr（Ⅵ）were provided. The rate of removal was found to speed up with decreasing pH.Theequilibrium adsorption data was fitted well with Frendilch isotherm,indicating themaximum adsorption capacity of91.70mg/g.The adsorption kinetic andthermodynamic data was analyzed and fitted well in particle dispersion equation andthe rate of removal rised with increasing temperature.Activation energy for theadsorption process was found to be16243.06kJ·mol-1.The resin had high repeatedutilization ratio.The results showed that, the wastewater containing Cr（Ⅵ） with theconcentration of100mg/L, pH=5.7flowed through the ion exchange column filledwith D301resins whose diameter was23mm and filling height was150mm.Continuously running for10.5hours with a flow rate of20BV/h, it treated12.7Lwastewater a total,and the effluent was up to standards completely.The adsorptionmass of Cr（Ⅵ） was approximately1254.6mg and the adsorption rate was high upto99.9%.A s5%NaOH solution fo r the eluting agent,the eluent was concentrated byabout70times,and the dynamic elution curve had no trailing phenomena.The resin isparticularly suitable for the treatment of low concentration wastewater containingCr（Ⅵ）（≤100mg/L）,which was not influenced by the velocity.And the dynamicadsorption was less affected by the flow,the adsorption effect was better with the flowincreasing.Th e best dynamic flow rate was80BV/h.717resin was suitable for the treatment of low concentration wastewater containing Cr（Ⅵ）（≤100mg/L）,and theoptimal dynamic flow rate was50⁶0BV/h. The best elution flow rate of D301was35BV/h.Not only the consumption of the eluent was less,but the elution time wasonly5min with high elution efficiency.The results of the continuous ED method treating wastewater of lowconcentration （≤100mg/L） with Cr（Ⅵ） showed that the optimal operating voltagewas40V and the corresponding maximum current was0.17A.The fresh water ofCr（Ⅵ） can reach the national emission standard more quickly.When the initialconcentrations of raw water reduced from150mg/L to10mg/L,the outletconcentration decreased from about4.83mg/L to1mg/L.This method was moresuitable for the treatment of low concentration wastewater containing Cr（Ⅵ）.It canalso quickly reduced concentration of Cr（Ⅵ） for the high concentrationwastewater.When the voltage,current and the concentration of raw water were underthe condition of invariable,the best freshwater operation flow was20L/h.In theoperating conditions of constant pressure of40V and fresh water flow of20L/h,theconcentration of the concentrated water outlet was up to3123us/cm,and concentrated7times,and that more than65%of the fresh water outlet reached the standard.Theminimum energy consumption can reach1.96kWh/molCr（Ⅵ）,and the currentefficiency was up to28.9%.The intermittent ED method can remove Cr（Ⅵ） fromwaste water swiftly.The treatment effect was more obvious for the higherconcentration of Cr（Ⅵ） wastewater,but it would be set to run longer.The processingtime was relatively long for low concentration wastewater containing Cr（Ⅵ）.Take theenergy consumption into consideration,the treatment was optimal when the voltagewas45V,the freshwater flow was20L/h.In the operating conditions of constantpressure of45V and fresh water flow of20L/h,it only needed15min from the initialconcentration of100mg/L Cr（Ⅵ） reducing to the standards.Whatever the current flowand concentration changed,the CMR was similar the effluent concentration Cr （Ⅵ）was close.CMR=0.83was the optimal value of treating wastewater with Cr （Ⅵ）.The chemical reduction precipitation process were as follows.With Na2S·9H2Oas the reducing agent,the experimental results showed that:the discharge optimal process parameters of treating the enriched liquidof100ml,500mg/L with Cr（Ⅵ）were pH=1.6,Na2S·9H2O dosage of0.6g,stirring speed of150r/min, the reactiontimeof90min.Cr（Ⅵ） can be completely discharged when the initial concentration ofwastewater with Cr（Ⅵ） was from100mg/L to1000mg/L.And the total chromiumtreatment effect decreased first and then increased, eventually decreased and reachedthe emission standard.While the initial concentration were higher than800mg/L withCr（Ⅵ）,thetotal chromium removal rate reached more than99.4%.Na₂S·9H₂O wasmoresuitable for the treatment of wastewater containing Cr（Ⅵ） with highconcentration.The optimal precipitation conditions for Cr（III） were as follows.Withthe lime coarse adjustment and lye fine balance,when the pH value was8.6～9.0,theremoval rate of the total chromium not only achieved the country sewage disposalstandard,and the pH value of the filtrate can reach the discharge standard and directemission.FeSO₄·7H₂O reduction precipitation method treated the wastewater of highconcentration of Cr（Ⅵ） after concentration.The optimum reaction conditions are pH≥7,the reaction time of30min,the mixing speed of100r/min,the dosage ofFeSO₄·7H₂O of1.2times the theoretical amount,and its removal rate can be up to99.7%. FeSO₄·7H₂O was more suitable for processing the concentration of500mg/L¹000mg/L of waste water containing Cr（Ⅵ）,the treatment effect was slightlylower for low （≤300mg/L） or high（≥1500mg/L） concentrations.With Na₂SO₃asthe reducing agent,the experimental results showed that the discharge optimal processparameters of treating the enriched liquid of100ml,500mg/L with Cr（Ⅵ） were pH=1,the reaction time of10min,the mixing speed of120r/min,the dosage of Na2S O3of1.2times the theoretical amount,and its removal rates can be about high up to100.0%. Na₂SO₃were more suitable for processing low concentration （300mg/L⁵00mg/L） wastewater with Cr（Ⅵ）,the effect in treatment was lower forhigher（≥800mg/L） concentration.Based on the experimental results,the optimal combinations of new couplingtechnology were D301ion exchange resin method-FeSO₄·7H₂O chemical reductionprecipitation method and continuous electrodialysis-Na₂S·9H₂O/Na₂SO₃chemicalreduction precipitation method.