Adsorption and coprecipitation

Fig. 15-17 The effect of chloride on adsorption of mercury by hydrous iron oxide at constant total mercury concentration of 3.4 x 10 M. The lines represent the predicted adsorption assuming that Hg-Cl complexes do not sorb at all. (Reprinted with permission from P. V. Avotins, Adsorption and coprecipitation studies of mercury on hydrous iron oxides," 1975, Ph.D. dissertation, Stanford University, Stanford, CA.)...

Adsorption and coprecipitation by hydrous iron and manganese oxides... 

Avnir, D. (1989) The fractal approach to heterogenous chemistry. J. Wiley, New York Avnir, D. Farin, D. Pfeifer, P. (1983) Chemistry in noninteger dimension between two and three. II. Fractal surfaces at adsorbens. J. Chem. Phys. 79 3566-3571 Avotins, P.V. (1975) Adsorption and coprecipitation studies of mercury on hydrous iron oxide. Ph.D. Thesis, Stanford University, California, 124 p. 

Metals other than chromium are removed through a process of adsorption and coprecipitation within insoluble ferrous ion matrix that is formed from the electrodes.38 Since the ferrous ion is a good chelate breaker, the electrochemical unit is also applicable for removal of complexed metals. 

Dyck W. 1967. Adsorption and coprecipitation of silver on hydrous ferric oxides. Can J Chem 46 1441-1444. 

Morse, J.W., and Arakaki, T. (1993) Adsorption and coprecipitation of divalent metals with mackinawite (FeS). Geochim. Cosmochim. Acta 57, 3635-3640. 

Fuller, C. C., J. A. Davis, and G. A. Waychunas. 1993. Surface chemistry of ferrihydrite Part 2. Kinetics of arsenate adsorption and coprecipitation. Geochim. Cosmochim. Acta 57 2271-2282. 

Although formation of arsenic oxoanion minerals may be uncommon in the subsurface, many other minerals in which oxoanions are the fundamental structural unit (most commonly sulfates, phosphates, and carbonates) can assimilate trace amounts of arsenic via adsorption and coprecipitation processes. By virtue of their ubiquitous nature in sediments, these phases... 

Crawford, R.J., Harding, LH., and Mainwaring, D.E., The zeta potential of iron and chromium hydrous oxides during adsorption and coprecipitation of aqueous heavy metals, J. Colloid Interf. Sci., 181, 561, 1996. 

F. Determination of metal ion removal Precipitation, Adsorption, and Coprecipitation Comparison of HCO and HFO as Substrates... 

Industrial companies are increasingly being required to account for the fate of all chemical species, whether deliberately added or present as by-products, at all stages of industrial, mining, and manufacturing processes. The processes of precipitation, adsorption, and coprecipitation, apart from directly controlling the economics of many chemical processes, are also often involved in the cleanup of industrial wastewater [4 6]. Thus there is also a well-founded need to study adsorption-related phenomena in order to understand and predict the behavior of toxic metals in industry. 

The distinction between simple precipitation, coprecipitation, and adsorption of aqueous heavy metal ions is not always clear, and in particular the terms "adsorption" and "coprecipitation" are often used interchangeably. Figure 2a is a schematic diagram illustrating the differences among the three mechanisms, and Fig. 2b illustrates their relative effectiveness. 

All solutions were prepared to give the equivalent of 0.0175 M KNO3 background electrolyte solution, except for those experiments involving adsorption and coprecipitation of metal ions from arnmoniacal solutions, where a 0.5 M NH4NO3 background electrolyte was used. 

The extent to which metal ions were removed from solution was assessed by performing experiments carried out in three modes direct precipitation, adsorption, and coprecipitation. In all cases, the initial pH of the starting metal nitrate solu-tion( as low (i.e., pH 3.5 or less). Atomic absorption analysis confirmed that no appreciable quantities of insoluble hydroxide precipitates were present in the metal nitrate solutions at these low pH values. 

A series of adsorption and coprecipitation experiments were also performed in the presence of an initially 0.5M NH4NO3 solution. The procedures adopted for adsorption and coprecipitation of metal ions under these conditions were the same for those performed in 0.0175 M KNO3 background electrolyte. 

For both adsorption and coprecipitation mechanisms, the expected correlation between hydrolysis and adsorption (or coprecipitation) is seen in Figs. 3-5. Cr(III), which hydrolyzes at the lowest pH of the three metal ions, was also removed by adsorption and coprecipitation at the lowest pH. Conversely, Ni(II), which hydrolyzes at the highest pH, was removed by adsorption and coprecipitation at the highest pH. 

In all three cases, the expected trend is observed Coprecipitation results in removal at a lower pH than does adsorption, which in turn results in removal at a lower pH than simple precipitation. In the case of Zn(H), however, the difference between adsorption and coprecipitation is barely perceptible. For the cases of Ni(II) and Cr(III), the enhancement of coprecipitation over the adsorption is very significant and is worth discussing further. 

The influence of a second or third metal ion on the adsorption and coprecipitation removal of metal ions is given in this section, initially using HFO, and then HCO, as the adsorbing or coprecipitating colloid. 

The following sections detail adsorption and coprecipitation experiments involving HFO and HCO as the substrate and Cr(III), 2n(II), and Ni(II), in their various permutations, as the competing metal ions. 

T 0 help understand the influence of ammonia on adsorption and coprecipitation, it is useful to first look at the influence of ammonia on the solution chemistry of the metal ions to be removed. The concentration of the ammonia ligand is pH-depen-dent, forming the ammonium ion (NH ) at low pH. It is assumed that the ammonium ion does not, itself, complex with positively charged metal ions and serves mainly to limit the amount of free ammonia available at any given pH. 

The adsorption and coprecipitation profiles for Zn(II) removal using HFO are very similar. There is a small but significant difference between adsorption and coprecipitation for Ni(II) and a large difference for Cr(HI) using the same substrate. In all cases, eoprecipitation was as efficient as or more effieient than adsorption, which was in turn more efficient than precipitation alone. 

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