Research On The Membrane Distillation Of Acidic Heavy Metals Wastewater

This paper reviews the current situation and common treatment methods for heavy metals wastewater, membrane distillation technology and its application in heavy metal wastewater. The literature review shows that the membrane distillation is a new type of membrane separation technology. Membrane material such as PTFE, membrane components such as hollow fiber membrane, operation types such as AGMD and VMD are the current hot spots in the study of membrane distillation. Membrane distillation research on solutions containing metal ions as well as research on the recovery of acids from solution showed that the membrane distillation technology for concentration of acidic heavy metals wastewater has broad application prospects.PTFE hollow fiber membrane and tubular membrane are selected in this paper. First basic performance of two membranes such as the diameter and wall thickness are tested, then study on treatment of acidic heavy metals solution by membrane distillation started. In PTFE hollow fiber membrane, the effects of operation type(DCMD and the VMD), fluid inflow(lumen and shell side), heavy metals(Zn, and Ni, and Cu, and Ca), pH, EDTA on membrane distillation process of heavy metal wastewater were inspected. Experimental results show that DCMD can concentrate the solution containing three metal ions(copper, zinc and nickel) fivefold and it runs stable, but the flux is 3.0 kg·m-2·h-1. Flux of lumen-side VMD is up to 12 kg·m-2·h-1, but the conductivity rises to 800 μS·cm-1 within 6 hours. Flux of shell-side VMD is half of lumen-side, but the conductivity is more stable. The pH shows no difference on membrane flux. Conductivity of produced water increase with the decreasing of pH when pH value between 0 to 2. Ca2+ has no obvious effect on flux and conductivity of water, but saturated precipitation of CaSO4 leads to the flux decline. EDTA will accelerate the process of hydrophilization and affect water quality.In PTFE tubular membrane, AGMD is chosen in the concentration of heavy metals wastewater. First,The saline is used on the performance test of tubular membrane,then the effect of heavy metal ions(Zn, Ni, and Cu, and Ca, Cr(VI), Fe(III)), pH and EDTA on concentration process of membrane distillation are studied. Results show that flux of Y40 tubular membrane(aperture is min and wall thickness is max) is lowest, but it has better performance in acid resistance and process stability. Cr(VI), Fe(III) has no significant regularity effects on the tubular membrane concentration, pH value is proof for the most significant factor further affecting the performance of membrane distillation, EDTA has some effect on membrane wetting.So the best Y40 tubular membrane and suitable operation condition are selected to concentrate electroplating wastewater. Conductivity of the water remains at less than 60 μS·cm-1, and membrane distillation is stable within 150 hours when wastewater is concentrated nearly tenfold.Last, the materials original and used are both characterized and analyse mainly on membrane fouling of membrane surface sediments, changes of membrane pore size, material phase, membrane’s hydrophobicity and mechanical properties. Research results show that the micro-structure differences of internal and external hollow-fiber membrane surface is the cause of the different performance of VMD lumen-side and shell-side. It’s copper sulfate crystals on the internal surface. The sediment is trapped in surface of tubular membrane. Sediments mainly are calcium salts and silicates while treating wastewater. Phase of membrane material doesn’t change, but after the experiment the membrane pore size are increase by different degrees. Infrared spectroscopy showed some new weak hydroxyl absorption peaks appeared but the hydrophobicity of membrane surface not declined. Mechanical properties of materials such as hollow fiber membrane and Y40 tubular membrane remain unchanged. The test results show that PTFE hydrophobic membrane performs stability in the process of membrane distillation for acidic heavy metals solution.