Ultrafiltration of water containing natural organic matter: heavy metal removing in the hybrid complexation–ultrafiltration process

The efficiency of heavy metal removal from water containing humic substances (HS) has been investigated by means of ultrafiltration. The rejection coefficient of Pb (II), Cu (II), Ni (II) and Co (II) in the presence of humic (HA) and fulvic acids (FA) was studied for different composition of feed solution (pH, concentration of metal ions and background electrolytes) and cellulose acetate ultrafiltration membranes of different molecular weight cut-off. HS rejection increases with pH due to conformational transformation of ionizable molecules. The interactions of HS with multi-charged metals do not practically affect the HS retention. Involvement of heavy metals ions into complexation with HS provides effective removal of trace metals from solutions as a result of complex retention. The metal rejection increases with pH growth and in neutral/alkaline solutions it exceeds 80%. Thus, ultrafiltration is an effective method for simultaneous removal of HS and heavy metals harnessing the principle of reagent-reinforced ultrafiltration. In conformity with this principle degree of heavy metal rejection improves with an increase in HS–Me ratio due to increase in completeness of metal binding. HS–Me ratio also is responsible for phase-dispersion state of the solution initiating a transition of intra-molecular complexes to inter-molecular ones at higher concentrations of metal ions with respect to higher molecular weight HA. This leads to an aggregation of HA–Me complexes and decrease in the trans-membrane flux. FA do not tend to form inter-molecular aggregates.