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A process for recovery heavy metals — Cu(II), Ni(II), Pb(II) and Cd(II) — from industrial wastewaters has been studied. This process is called PSU (Polymer Supported Ultrafiltration), and includes complexation of metallic ions with polymers (polyethyleneimine — PEI, or poly(acrylic) acid — PAA), in order to make this solution flow through an ultrafiltration ceramic membrane and to obtain two different streams: permeate (nearly free of metal) and retentate (with high complex concentration). In a previous stage, complex stoichiometries and stabilities have been exhaustively studied by UV/visible absorbance spectrophotometry and potentiometric measures. Next, optimum operating conditions and best pH values affecting to permeate flux, complex formation, further polymer regeneration and polymer capacities have been achieved by means of ultrafiltration experiments. Flux and concentration values measured with these processes have been modeled with concentration polarization model, and mass transfer coefficients (k) and polarization concentrations (cm) have been obtained for different operation conditions. These data are not commonly reported in previous literature, although they are important for scale-up to an industrial process.

PSU appears to be a viable method for recovery of heavy metals from industrial effluents. Effects of different operating parameters (membrane state, transmembrane pressure, feed stream velocity, polymer concentration, ionic strength, temperature and pH) on the design parameters (permeate fluxes and rejection coefficients) of this process have been studied. In this way, optimum working conditions ranges have been found for each polymermetal system. It has been found that the key variable is pH. For this, complex stability and coordination index in a wide range of pH has been studied by means of UV/visible absorbance spectrophotometry and potentiometric methods. In this range, polymer capacities for each metal were calculated, and obtained values are very similar to those appearing in bibliography [3–9]. These results were checked Table 4 Typical pH-dependent permeate fluxes and rejection coefficients values for different PEI-metal systems System pHa Rmax pHb Rmin pHc cm, ppm Me k, (m/s)×106 pHd PEI-Cu PEI-Ni PEI-Pb PEI-Cd a Jvmax , l/hm2 78.3 93.6 145.8 73.7 5.75 5.88 5.71 5.59 0.94 0.97 0.90 0.95 5.75 5.88 6.92 5.59 0.13 0.34 0.03 0.01 1.20 1.76 2.34 12.57 10.28 12.34 10.40 5.75 5.88 6.92 5.59 pH for Jvmax; bpH for Rmax; cpH for Rmin; dpH for cm and k by ultrafiltration experiments at different pH values, and coherent conclusions were obtained. Moreover, ultrafiltration experiments of complex in concentration mode have been performed, and pH influence on rejection coefficients and permeate fluxes has been observed again. The results in these experiments have been treated with concentration polarization model, and k and cm values have been calculated. As pH decreases, k rises, except for pH nearly 1, where k decreases again. The cm behavior is totally inverse to that of k, that is to say, that one rises when this one decreases, and vice versa. Finally, k w as evaluated by dimensional analysis in the operating conditions with equation [4] for our system. This k by dimensional analysis (12.41·106 m/s) is in the order of k calculated by ultrafiltration experiments in concentration mode, as can be observed in Table 4. Before developing a practical PSU process, industrial effluents should be considered to study the effects of other metals and substances on the separation of target metal ions. Membrane fouling characterization, thermal and chemical regeneration of the polymer, polymer behavior with pH changes and scaling-up should also be investigated.