Application of sodium deoxycholate for separation of heavy metals : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering at Massey University, Palmerston North, New Zealand
Sodium deoxycholate (NaDC) and sodium taurodeoxycholate (NaTDC) have been used to bind and subsequently remove nickel, copper and zinc ions from aqueous streams by ultrafiltration. The mixture of metal and NaDC solutions forms a precipitate of insoluble metal deoxycholate. This precipitate can be removed from the solution in most cases by conventional techniques such as filtration and centrifugation. Ultrafiltration membranes made of polyethersulphone can also retain the precipitate effectively, producing an environmentally accepted effluent. The exception is the high copper and NaDC mixture where none of the operations works at room temperature because of the gelatinous nature of copper deoxycholate. The removal of metal ions by precipitating them as metal deoxycholate is affected by such parameters as equilibration time, surfactant-to-metal (S/M) ratio, feed concentration, temperature and pH. An equilibration time of 3 hours or greater is required to have a major part of precipitation completed. A S/M ratio of 2.5 is sufficient, except in the case of low nickel concentration when a higher S/M ratio is necessary. A higher temperature (e.g., >40°C) does not significantly affect the metal removal, but increases the process flux markedly. The high copper and NaDC mixture can also be operated with reasonable flux at a higher temperature. The use of NaDC to precipitate metal ions is inappropriate below or above the neutral pH value because DCA starts to precipitate as the pH is lowered to around 6, and metal ions precipitate as metal hydroxide at a high pH. Because different metal ions have a differential affinity for the deoxycholate ions, NaDC can potentially separate one metal from mixtures of metals. The separation of individual metals from copper/nickel mixture is good compared to poor separations for copper/zinc, zinc/nickel and copper/zinc/nickel mixtures. This is evident from the molar ratios of two metals of 1:67, 1:2.5 and 1:7 respectively in the respective permeates. The mixture of metal and NaTDC solutions is homogeneous and metals are removed by micellar-enhanced ultrafiltration (MEUF). Since NaTDC is expensive and difficult to recover, its application may not be economically feasible. Based on the present research, two NaDC-mediated processes are proposed: (1) a process for removing metal ions from single-metal systems, and (2) a process for separating copper/nickel mixtures.