Treatment Of Etchant Terminal Wastewater Using Single-chamber MECs Under Different Volumes Conditions

The etchant terminal wastewater contains a certain amount of refractory organics and heavy metals.The recovery of heavy metals from etchant terminal wastewater has more significant environmental,ecological,economic and social benefits than mining.Microbial electrolysis cells（MECs）are sustainable,clean and environmentally friendly,which have certain advantages and application potential to treat etchant terminal wastewater.In this paper,different volumes single-chamber MECs（1 L and 40 L）were used to explore the feasibility of treating etchant terminal wastewater and recovering valuable heavy metals and investigated the performance under different conditions of low load and high load,batch and continuous operation mode,the results are as follows:（1）Under the same low load and operating time/HRT of 5 d,the COD removal in effluent under condition of batch and continuous operation of 1 L and 40 L reactor were 73±1-74±7%（p=0.90）and 66±1-69±3%（p=0.44）,respectively.And the heavy metals in the effluent of the two reactors were almost completely removed.The organics and heavy metal in effluent were all lower than the special discharge limits for water pollutants of GB39731-2020,and the effluent met the emission standard.The condition of batch or continuous operation mode had no significant effect on the performance of different volumes of MECs.（2）Under the same high load and operating time/HRT of 5 d,the COD removal in effluent under batch and continuous operation mode of 1 L reactor was 78±1-84±1%（p=0.051）,which showed the condition of batch or continuous operation mode had no significant effect on the performance of different volumes MECs.The COD removal in effluent of 40 L reactor was 68±2%under batch operation mode.Although the heavy metals in effluent of 1L and 40 L reactors were completely removed,the COD in effluent could not reach the emission standard.（3）Under the same high load and operating time/HRT of 7 d（1 L）or 9 d（40 L）,the COD removal in effluent were 89±1-93±1%（1 L）and 89±1-91±0%（40 L）,respectively.The heavy metals in effluent of the two reactors were completely removed and the effluent met the emission standard.（4）The cyclic voltammetry showed that the initial reduction potential of the Cu（Ⅱ）,Zn（Ⅱ）and Ni（Ⅱ）at the cathode of the reactors was more positive and the peak current was higher than that at the anode.In addition,reduction peak current of Cu（Ⅱ）,Ni（Ⅱ）and Zn（Ⅱ）in the high load wastewater was higher than that in the low load wastewater and the initial potential was all positive.The results illustrated that the high concentration of heavy metal ions was conducive to the reduction reaction.Moreover,the higher concentration of organic matter in the high load etchant terminal wastewater could enhance the catalytic activity of electrochemical active microorganisms.The electrochemical impedance spectroscopy showed that the ohmic internal resistance（R_Ω）,charge transfer internal resistance(R_ct)and diffusion internal resistance(R_dif)of the high load wastewater were all lower than those of the low load wastewater.XPS shows that the removal of heavy metals in anode was adsorption,while the removal of heavy metals in cathode was adsorption and reduction,which was related to.reactor volumes and metal types.The research results showed the different volume single-chamber MECs treated etchant terminal wastewater effectively and recovered heavy metals（Cu（Ⅱ）,Zn（Ⅱ）,Ni（Ⅱ）and total chromium）,which was of great significance to make effluent meet emission standard.