Figure 2 Wastewater groups and treatment schematic From

Chromium can be precipitated only in the trivalent state. The process is effective in removing other soluble and particulate pollutants, often by the aid of organic and inorganic coagulants.

Hydroxide precipitation has the advantage of removing many of the pollutant parameters existing in metal finishing wastewaters without pretreatment. The process operates at ambient conditions and its operation is easy and suited to automatic control. The most important advantage of the process is its low cost, particularly when lime is used for pH adjustment. Common use of the process has provided accumulation of data and experience on which precipitation strategies can be based and prediction of performance can be made within a certain range of precision. Existing information about the theoretical basis of the process as well as ongoing research in the field help modeling and planning of the process.

However, there are disadvantages to hydroxide precipitation. The first and most important one is the relatively high quantity of sludge produced. Secondly, the optimum pH values of the different metals, the pH at which a certain metal reaches minimum solubility, are different. Maximizing the removal of a certain metal, generally, results in significant reductions in the removal of other metals. Sometimes a two-stage treatment may be needed to provide high efficiencies for all the metals existing in the wastewater. On the other hand, solubilities of the metal hydroxides are not low enough to remove complexed metals. Furthermore, as with all precipitation-sedimentation processes, the performance of the process is determined by the solid separation step. Another drawback is the use of lime for pH adjustment. Lime is used as slurry, which is difficult to handle in terms of pumping, piping, and feeding. However, it is very cheap compared to other alkalis and, more importantly, it proves quite beneficial for further process steps.

Solubility of hydroxides is dependent on pH. This dependency is not only due to the hydroxide ion concentration that determines the solubility of metal hydroxides, but also to a larger extent due to the formation of metal hydroxo complexes. Concentration of hydroxo complexes is also determined by pH. Hydroxo complexes may be positively or negatively charged or neutral. Positively charged ones dominate at lower pH, namely the pH values lower than the minimum solubility of the metal. Negatively charged species prevail at higher pH values. Concentration of neutral species is, however, independent of pH. Polymer (polynuclear) type hydroxo complexes that have more than one metal atom also exist. Metal solubility is the sum of free metal ion and hydroxo complex concentrations. Figure 3 shows the solubility diagram of cadmium hydroxide [7]. As seen from the figure, solubility exhibits a shape similar to a parabola or an upside-down bell shape. This shape sometimes approximates to a triangle. In this shape, solubility decreases with increasing pH up to a minimum, then it increases with further increase in pH. The point of minimum solubility is termed "optimum pH." Some metals such as Fe3+ do not exhibit such a solubility diagram due to very low solubility product, and solubility monotonously decreases as the pH is increased (Fig. 4) [3]. For such metals, there is no optimum pH. In Figure 3, cadmium solubility is seen to be determined by Cd2+ and major

Aluminum Hydroxy Solubility
Figure 3 Cadmium solubility diagram. (From Ref. 7.)

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