Complexed metal bearing wastewater treatment

As mentioned earlier, complexed metal bearing wastewaters must be segregated and separately treated to prevent further complexing of metals present in the free

form in the other flows. In these wastewaters, the metals are weakly or strongly tied up or complexed by inorganic or organic ligands (complex formers) whose function is to keep them in solution. When metals are bound by weak complex formers such as succinic acid, acetic acid etc., they are successfully treated by hydroxide precipitation under regular treatment conditions without requiring any pretreatment step (Kabdaşlı 1990; Tünay and Kabdaşlı 1994). On the other hand, since strong complex formers are mostly chelates that bind the metals with more than one hand to form stable structures i.e. strongly complexed, due to their high stabilities these types of complexed metals cannot be efficiently treated under conventional hydroxide precipitation conditions. Therefore, they call for the modified hydroxide precipitation or specialised pretreatment application (Kabdaşlı et al. 2009). The main obstacle of the modified hydroxide precipitation is that complex formers which may be harmful in many ways are simultaneously released into the environment. Via application of a specialised pretreatment the complexing agents can be completely destroyed or converted into forms that do not interfere with subsequent conventional hydroxide precipitation. The specialised pretreatment methods which are applicable for complexed metal bearing wastewaters can be classified as reduction of complexed metals to zero-valent state, electrochemical processes, advanced oxidation processes, sonochemical destruction, and physical separation techniques. These methods, except physical separation techniques, are discussed as follows.

4.2.3.1 Chemical reduction

Reduction of complexed metals to zero-valent metals can be accomplished by electrolytic recovery method or using reducing agents such as sodium borohydride, dithiocarbamate, and hydrazine (US EPA 2003). Since electrolytic recovery method is generally applied to precious metal bearing flows as an in-plant control practice, it will be discussed in the following subsection.

Sodium borohydride (NaBH4) has been widely used to reduce a variety of cations to metallic state to recover precious and heavy metals as well as to remove heavy metals from metal finishing industry wastewaters (Lu et al. 1997).

It has been demonstrated that the process is sensitive to the reaction conditions such as initial metal concentration, and mixing efficiency. The stoichiometry of borohydride depends on the experimental conditions, the electronegativity of the metal and catalysis on borohydride, reaction pH, and the form of the metal salt in solution (Lu et al. 1997). The following independent reactions representing the reduction of the cobalt by sodium borohydride depending on pH are proposed by Shen et al. (1993):

2 0

4 2 2

4Co ⁺+BH^-+8OH^-®4Co +BO^-+6H O (4.24)

2 4 2 2 2

4Co ⁺+2BH^-+6OH^-®2Co B 6H O H+ + (4.25) One of the above reactions takes place depending on working pH. Lu et al. (1997) reported that cobalt ions transform to Co2B according to Eq. (4.25) at a pH range of 5–8. On the other hand, in the related literature this pH range was reported as lower than 10.0–12.0 for Ni2B whose formation can be written similar to Eq.

(4.25) (Tünay et al. 1997; Ying et al.1987).

Applicability of sodium borohydride reduction to EDTA and nickel-acetic acid complexes was also tested at pH range of 10–12 by Tünay and co-workers (1997). Results showed that at the stoichiometric sodium borohydride doses and at pH 12.0 (i) nickel concentration could be reduced from 6000 mg/L to 45.5 mg/L for nickel-EDTA system; (ii) complete nickel removal could be obtained for nickel-acetic acid system; and (iii) bivalent nickel converted to colloidal zero-valent nickel during the process at all tested pH values (10–12).

Dithiocarbamate (DTC) is an effective reducing agent used in the treatment of complexed metal bearing flows. It can reduce complexed metals to zero-valent metals in stoichiometric ratio to the metals present ((M^m+)/(DTC^2–): 1/1 on the molar basis). On the other hand, if used incorrectly, DTC compounds which are a class of pesticides, may create several problems in the aquatic systems. In addition to this disadvantage, large amount of sludge is produced by DTC reduction and the process is expensive because of high cost of DTC (Chen and Lim 2002; US EPA 2003). The capital costs associated with DTC systems are equal to those of hexavalent chromium reduction systems using sodium metabisulphite given in Subsection 4.2.2.1 (US EPA 2003).

Hydrazine (N2H4) can effectively reduce various metal cations (M²⁺) to zero-valent state (M⁰) at alkaline conditions (pH >11) according to the following reaction (Chen and Lim 2002):

2 0

2 4 2

2M ⁺+N H →2M +N +4H⁺ (4.26)

One mole hydrazine is enough to reduce two mole divalent metals. For the case of monovalent cations the reaction stoichiometry is 4 mole M⁺ to 1 mole N2H4. Atmospheric oxygen may decompose dilute solutions of hydrazine into nitrogen.

Chen and Lim (2002) found that a 0.05 M hydrazine solution was decomposed to extent of 20 % at the end of 16 hours exposure to the atmosphere. They also reported that (i) hydrazine was an effective reducing agent for the recovery of silver and copper; (ii) presence of humic acids did not hinder the recovery of copper, but rather, improved process efficiency whereas humic acids thwarted the recovery of silver; and (iii) at high initial metal concentrations, more finer

particles were produced, and the seeding and ageing process had no positive effect on particle size.

The major operational problem faced in the reduction of complexed metals by reducing agent to zero-valent is the separation of fine particles formed upon process. This separation problem can be overcome using suitable separation techniques such as gravity clarification, centrifugation, or coagulation prior to sedimentation/filtration.

4.2.3.2 Other methods

Recently, it has been demonstrated that the electrocoagulation process using stainless steel electrodes has a high capacity in simultaneously removing both heavy metals and organic complex formers from a nickel and zinc plating effluents (Kabdaşlı et al. 2009). Under the optimised operation conditions, complete metal removals (nickel and zinc) and about 66 % TOC abatement were achieved.

The results of another study performed by Kabdaşlı et al. (2010) indicated that combination of electrocoagulation with Fenton’s reagent could enhance organic matter removal which almost equalled complex former removal. In these studies, heavy metal removal mechanism has been described as breaking the structure of complex former via oxidation, followed by hydroxide precipitation, and adsorption or entrapment of the metals on freshly produced ferric hydroxide flocs.

Recently, many researchers have also studied heterogeneous photocatalysis of complexed metals over titanium dioxide as an alternative treatment method.

The process efficiency was tested for various metal-complex systems such as hexavalent chromium-citric acid (Meichry et al. 2007), copper-ferric-EDTA (Park et al. 2006), nickel-EDTA (Madden et al. 1997; Salama and Berk 2005), copper-EDTA (Madden et al. 1997), lead-EDTA (Madden et al. 1997; Vohra and Davis 2000), and cadmium-EDTA (Madden et al. 1997; Davis and Green 1999) at varying reaction conditions in lab-scale. It has been demonstrated that high removal efficiencies in terms of metals and complex formers could be achieved by the process.