Abstract
ABSTRACTThe simultaneous separation of various metal ions (nickel, copper, calcium, and iron) from chelating agents (EDTA and citric acid) in water streams using Nanofiltration membranes is analyzed. Assuming that multiply-charged species are highly rejected, chemical speciation computations reproduce the observed patterns of metal and ligand rejection at different pH values and concentrations. The separation of metal ions from citric acid is achieved in acidic conditions, where multiply-charged free metal ions and neutral or singly charged free chelating species are abundant. Overall, speciation studies help to evaluate the applicability of Nanofiltration for recycling chelating agents used for metal extraction.
Highlights
Relative standard deviation (RSD) of the analysis methods were deter265 mined from duplicates analysis of permeate samples
495 rejection patterns of metals and ligands at different pH values and concentrations can be drawn assuming that only multiply-charged species are highly rejected
The results reveal that knowledge of chemical speciation is a helpful tool to predict Nanofiltration perfor
Summary
Several techniques that include precipitation, adsorption, ion-exchange, electrochemical treatments, and membrane filtration have been proposed to separate or concentrate heavy metals from contaminated waters 20 produced by several industries.[1,2,3,4,5] Among them, Nanofiltration is a membrane process that show high retention of multiply-charged ions but their efficiency is influenced by factors such as other ions (e.g., sodium, calcium, iron, sulfate, nitrate, and chloride), pH, and 25 ionic strength.[6,7] Further, some water streams contain chelating agents because they are used to prevent the formation of insoluble salts incrustations or to extract metal ions from liquid streams or from solids such as contaminated soil, sludge, and sediment. 30 Nanofiltration of solutions containing metal ions such as Cu, Ni, Mn, Sr, Co, rare earth elements, and radionuclides in the presence of various complexing agents (e.g., EDTA, NTA, DTPA, citric acid, NH3, CN–, and phosphates) using membranes with different molecular 35 weight cut-off (MWCO) values (i.e., 150–1000 Da) has been studied by several authors.[7,8,9,10,11,12,13,14,15,16] Most of them reported that the addition of a chelating agent increases metal rejection because the chelate species is larger than the free metal ions. 30 Nanofiltration of solutions containing metal ions such as Cu, Ni, Mn, Sr, Co, rare earth elements, and radionuclides in the presence of various complexing agents (e.g., EDTA, NTA, DTPA, citric acid, NH3, CN–, and phosphates) using membranes with different molecular 35 weight cut-off (MWCO) values (i.e., 150–1000 Da) has been studied by several authors.[7,8,9,10,11,12,13,14,15,16] Most of them reported that the addition of a chelating agent increases metal rejection because the chelate species is larger than the free metal ions. The chelating agents must be separated from the metal ions for their recovery. This process involves fragmentation of the complex, which can be achieved at low pH. In a previous work,[17] the present authors examined 50 the separation of copper ions from citric acid in synthetic solutions using several Nanofiltration membranes of different pore size and nature. Membranes with narrow pore sizes (e.g., SelROR MPF-34, Koch) simultaneously rejected both the copper ions and citrate. 55
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