5. Results and discussion

The first batch evaluation of organic and inorganic exchangers and sorbents gave very promising results for four chelating exchangers; which were Amberlite irc 718, Duolite ES 467, Spheron Oxin, and oxine impregnated active carbon. Inorganic sodium titanate exchanger performed also well. Chelating exchangers have three different amines as functional groups (Figure 6).

Figure 6. Functional groups of Amberlite irc 718 (A), Duolite ES 467 (B) and Spheron Oxin/oxine impregnated active carbon (C)

Sodium titanate is a totally inorganic compound prepared from an industrial intermediate product of a titanium dioxide process (Heinonen). It is probably a layered compound (Clearfield) with an ideal composition of Na4Ti9O20• XH2O in which the sodium ions are located between the titanium oxide layers.

These five exchangers removed more than 99% of Ni and Zn from solution, which corresponds to a K_Dvalue higher than 10000 ml/g. The only exception was nickel sorption to Spheron oxin. The separation percentage was only 81% (K_D= 400 ml/g), this was because the practical nickel capacity of the exchanger in this waste liquid was almost exceeded. In column experiments with this exchanger Ni capacity was found to be only 0.1 meq/g with 0.5 mM waste and in the batch experiment the Ni uptake was 0.08meq/g .

These five promising exchangers were tested (I) in mini columns, using typical Ni and Zn bearing waste solutions from plating plants. Sodium titanate was most efficient for the separation of nickel, and Duolite ES467 for the separation of zinc. The metal loadings of exchangers were 0.65 meq/g nickel for sodium titanate and 1.78 meq/g zinc for Duolite ES467, calculated at the 50% breakthrough point. The concentrations of nickel and zinc were respectively 0.1% and 0.5% of the initial metal concentration, prior to breakthrough. Amberlite IRC 718 was the second best exchanger for both Ni and Zn; metal loadings were 0.61 and 0.64 meq/g respectively (Tables 5 and 6 in (I)).

Fiban AK-22 showed undisputably the best separation performance for all Cr species. The exchanger could remove 97.6% of Cr³⁺ (K_D=4400ml/g) and 99.6% of Cr2O7^2- (K_D= 27000ml/g) at pH 3.1 , and at pH 7.4 the removal of CrO₄^2- was 99.7% (K_D= 36000ml/g). Fiban AK-22 has been developed at the Institute of Physical Organic Chemistry in Minsk, Belorussia (Soldatov). This exchanger has two kinds of functional groups on polypropylene fibres (Figure 7).

Figure 7. Functional groups of Fiban AK-22, imidatsoline and carboxylic acid

Since Fiban AK-22 has carboxylic groups, it works as a cation exchanger and due to the protonation of imidatzolene groups it can work as an anion exchanger as well. In addition, the nitrogen atoms in the imidatzoline group also form chelates with transition metal ions.

Column experiments were done with contaminated effluents from rinsing baths. Fiban AK-22 removed chromium rather efficiently from a 1 mM waste effluent (pH 6.1). The level of Cr in the effluent prior to breakthrough was very low, only 0.01% and the chromium loading was 0.47 mmol/g, calculated from the 50% breakthrough value. The pH of effluent remained constant at pH 6.8-7, which indicates that the chromium in this solution was probably in the chromate form. The behaviour of acidic (pH=1.3 , [Cr]=28 mM) solution was rather complicated. Strong changes in effluent pH value points to a varying mixture of several forms of chromium in the effluent.

A third set of evaluations was done with zeolites and activated carbons in waste simulants. Zeolites A and X (Si/Al ratio 1-1.2) gave KD values higher than 70 000 ml/g for both Ni and Zn in all the tested solutions. Zeolite L (Si/Al ratio 3.1) gave K_D values higher than 60 000 ml/g for Ni in all three nickel solutions and ferrierite (Si/Al ratio 6.1) gave K_D values higher than 10 000 ml/g for Zn in both tested solutions. These K_D values are one or two orders of magnitude higher than the values obtained with high-silica zeolite Y (Si/Al ratio 6.5-10), the K_D values were only in the range of 100 – 1000 ml/g (Table 4 in (V)).

Zeolites A and X showed good performance also for Cd, Cu and Co in all the tested solutions. The distribution coefficients for Cd were in the range 18 000 – 33 000 ml/g in a 1mM CdCl₂ solution. The separation performance was even better in solutions which contained 10 mM NH₄NO₃ as the complexing substance for Cd : the K_D values were in the range of 74 000-440 000 ml/g. The lowest K_D values, 230-460 ml/g, for Cd in both solutions were observed with zeolite ZSM5 (Table 5 in (V)). Zeolites having low Si/Al ratios showed good separation performances for all the metals of interest.

In the column tests four zeolites, A , X , L, and ferrierite were tested for Zn, Ni, Cd, and Cu, in five simulants and in the reference solutions (Table 8 in (V)). X zeolite showed best performance in nickel solutions, the uptake values in all three solution were in the range 2-2.4 meq/g at the 50% breakthrough point. A and L zeolites performed corresponding values around 0.6 meq/g . The efficiency in column operations are dependent on the ion exchange equilibrium and kinetic parameters. In this study column parameters, such as flow rate, were not optimized, but this should not have a significant effect on uptake at the 50% breakthrough point in present study. Every exchanger was tested in the same manner, using the same grain size, flow rate and bed dimensions. Thus, the uptake and the cumulative DF value at the 5% breakthrough point can be compared within these experiments. Uptake at the 5% breakthrough point was selected to represent a processing capacity for typical column operations.

5.1.4. Selecting the most promising exchangers

Evaluation showed that sodium titanate, zeolites A and X, aminophosphonic resin (Duolite ES 467), iminodiacetate resin (Amberlite IRC 718), and the combined imidatzolene – carboxylic exchanger (Fiban AK-22) may have potential for new applications in waste treatment. In bench-scale column tests at Finnair’s plating shop sodium titanate (SrTreat) had the best separation performance for Ni but its regeneration properties were not as good as those for iminodiacetate and aminophosphonate resins (VI). For practical applications exchangers must be commonly available and their price compared to the effort involved must be reasonable. All the named exchangers are available commercially and most of them are commonly in use. Another criteria used in the selection is the cost of the cleaning operation, which can be derived from the capacity of exchanger compared to price of exchanger and the operating costs of the ion exchange plant. The facilities and labour used for granular or bead form exchangers are basically the same. The price of the organic resins, including materials from practical grade strong cation exchanger to chelating cation exchanger, is in the range 2-20€/kg and price of the synthetic zeolites and highly selective inorganic exchangers is in range 20-200€/kg. Although inorganic exchangers have a higher capacity, the advantageous price and regeneration properties and chemical stability of chelating organic resins make chelating exchangers preferable for waste treatment.

As stated before, in waste solutions there are several species and phenomena which interact in ion exchange, such as hydrolysis of the ion exchanger, dissolved atmospheric carbon dioxide, competing cations and process chemicals. Usually, these interactions have a negative effect on the performance of an ion exchanger but sometimes certain complexing agents or an excess of co-ion make the conditions more favourable for metal sorption.

It was found that the chelating ion exchangers studied, iminodiacetate and aminophosphonate resins and amphoteric fibrous exchanger, have a maximum KD value in a certain pH range. Iminodiacetate resin with zinc and nickel, and aminophosphonate resin with zinc had a maximum distribution coefficient in the 5-6 pH range (I, III ,IV) . The amphoteric Fiban AK-22 exchanger has a maximum distribution for chromium in the 3-4 pH range(II).

Figure 9 Distribution coefficient (K_D) of Zn on amiphosphonate exchanger as a function of pH, from (¨ ) 0.31 mM ZnSO₄ solution and from (5 ) 1 M NaCl solution which contains 6x10^-7 M Zn tracer

Sodium titanate did not have a similar maximum in the K_D values, but all exchangers have the same trends in K_D’s in the acidic region.

The chelating resins and inorganic exchangers, which were studied, are weakly acidic in nature and thus very selective for the hydronium ion. In the alkali metal form these exchangers react alkalically in water, the pH value rises when the counter ion exchanges with H⁺ from water. The equilibrium pH value has a very strong effect on the distribution of metals. Metal sorption starts when the pH rises to the range where most acidic ion exchange sites start to exchange hydronium ion for metal and the capacity reaches the maximum value in the pH range where all the ion exchange sites take part in the reaction and the functional group is able to form chelate rings with the metal cations. In Figures 8-10 before the maximum there are linear ranges with upward trends because the degree of dissociation increases and H⁺ concentration in solution decreases (Equation (14)). The same kind of dissociation phenomenon occurs with sodium titanate but with no chelate formation.

The decrease in K_D-values after the maximum in the neutral and alkaline region can be explained by the complex formation with different ligands. The pH has a strong effect on the competing complexation. In waste solutions there are complex forming agents which dissociate and become reactive as hydronium ion concentration decreases. In the neutral region the autoprotolysis of water produces hydroxide ions, and in the alkaline pH range hydrolysis of cations and complexation with dissolved atmospheric carbonate takes place, which interferes with metal sorption at trace concentrations (Equation (15)). The decrease of KD of Zn and Cr in Figures 9 and 10 can be explained rather well by hydrolysis of cations, driven by pH change. In Figure 8 competing complexation with carbonate and hydroxide could not entirely explain the decrease of KD values. It was concluded that very small resin fragments or other colloidal nickel bearing particles released by the exchanger were responsible for this anomaly.

In the batch tests it was observed that aminophosphonate resin may break the zinc cyanide complexes. This has been studied by determining K_D values of zinc as a function of cyanide concentration. Experiments were done at two zinc concentrations, 1 mM and 6x10^-7 M, the cyanide concentration was in the range 10^-5 – 0.1 M.

In these chemical conditions Zn forms several complexes. with cyanide and hydroxyl and experiments showed that aminophosphonate-resin can strip zinc from other cyanide and hydroxide complexes but not from Zn(CN)₄^2-.

Etylenediaminetetra-acetic acid (EDTA) dramatically suppresses metal sorption compared to the effects of citrate or gluconate. The inhibiting effect of EDTA on Zn sorption starts at a concentration of 1m M of the ligand, and a 2 mM solution practically prevents all metal sorption , Figure 6 in (III). Citrate has a much weaker effect, the ligand starts to interfere with Zn sorption only at concentrations higher than 10 mM and gluconate causes no interference at all.

The effect of counter ion concentration was studied in trace nickel and zinc sorption to iminodiacetate and aminophosphonate resins respectively and to Cr sorption to Fiban AK22. Na is a common counter-ion in weakly acidic chelating exchangers and in hydrometallurgical processes additives are often used as sodium salts, therefore in systems with waste effluents it is possible that Na concentrations are several orders of magnitude higher than those of transition metal concentrations. Even the hydrolysis of the sodium form exchanger can produce 10^-4M (Lehto 1994) sodium concentrations in dilute solutions.

The initial nickel concentration was 10^-3 M and the initial Zn concentration was 6x 10^-7M. Metal concentrations were so low that the exchangers were not essentially converted to the transition metal form. The increasing sodium ion concentration in the solution drives the reaction (5) to the left-hand side and thus the increase in sodium concentration brings the pH down and only slightly increases the sodium uptake. In both cases, (Figures 3 in (III) and (IV)) an increase in sodium concentration decreases pH almost linearly and the K_D increases until the sodium reaches a concentration of 0.7 M. At higher sodium concentrations pH still decreases but in the Ni/ iminodiacetate system the distribution coefficient of the metal increases, and in the Zn/ aminophosphonate system the distribution coefficient of metal decreases. This kind of ternary ion exchange equilibrium is quite a complex system. Even though the transition metals and hydronium dominate the equilibrium (IV) it is impossible to make straightforward conclusions from the H/Na/metal equilibrium, because total electrolyte concentration in solution phase varies in a wide scale. However, the equilibrium can be described by plotting binary selectivity coefficients (K_Ni/H , K_Na/H , K_Ni/Na ) vs. Ni loading in exchanger (Figure 11). It can be seen that in case of iminodiacetic exchanger, K_Ni/H has two ranges corresponding to 0.00001-0.001 mmol/g and 1-1.7 mmol/g Ni loadings in which it is practically constant as a function of Ni loading. In the range of 0.00001-0.001 mmol/g Ni-loading only trace nickel exchange takes place and in the range of 1-1.7 mmol/g Ni-loading, equilibrium is practically controlled by binary Ni/H exchange.

Fiban AK22 was used to study the effect of K, Mg, Ca and Fe cations. Alkaline and alkaline earth cations have no effect on the uptake of chromium(III) by Fiban at concentrations below 0.1 M. However, the transition metal, iron, starts to interfere with chromium sorption already at concentrations above 0.001 M, this is as expected since as a transition metal, iron competes with chromium for complex formation with the exchanger. The trivalent ion Fe³⁺ has a stronger interfering effect than Fe²⁺ (Figure 2 in (II)).

In the column tests there were signs of a positive effect of the strong electrolyte background for the sorption of Cd on A type zeolite. Metal sorption from solutions containing a large excess of NH₄NO₃ was considerably higher than metal sorption from pure CdCl₂ (1 mM) solutions (Table 8 in (V)). One should assume that competition between Cd⁺⁺ and NH₄⁺ suppresses the sorption of metal compared to sorption from pure metal chloride solutions, which was not the case. To explain this anomaly there are two alternatives either NH₄⁺/NO₃^- ions or a relatively strong electrolyte background (10 mM) have positive effects on the sorption of Cd. In the NH₄⁺ solution, the exchanger converts to some degree from H⁺/Na⁺-form to NH₄⁺-form , which may be more selective towards Cd. Cl^- forms stronger complexes with Cd⁺⁺ than NO₃^-, so chloride should prevent Cd sorption by the exchanger more strongly than nitrate and thus lower Cd uptake was obtained in CdCl₂ solution. Concerning the electrolyte background, it promotes the sorption of co-ion which enables complex formation between ammonia and Cd in the NH₄⁺-form exchanger.

The pore structure of A type zeolite is a three-dimensional 8-member ring with a pore diameter of 4.1Å. The X type (faujasite) pore structure is a three-dimensional 12-member ring with a pore diameter of 7.4Å. Both exchangers, however, have silicon to aluminium ratios which are very similar, Si/Al =1 for zeolite A and Si/Al = 1.23 for zeolite X. Ni, Co, Zn, Cu, and Cd cations have a hydrated diameter in the range of 5.5 – 6.1 Å (Marcus) and thus they must be stripped at least partly from their hydration shells during sorption to the A type, but all hydrated cations should be able to enter the pores of the X type zeolite. The distribution coefficients of all five metals do not show a significant difference between A and X zeolites (Table 6 in (V)). Thus, we can conclude that the pore structure compared to the stability of hydration shell is not a critical factor in determining the selectivities in these cases.

If the distribution coefficient values of metals are compared with the values of the molar Gibbs energies of hydration of ions, it can be seen that the more negative their free energy of hydration, the higher are the K_D values of the metals (Table 6/V). Thus, we can conclude that in these cases the hydration of cations has a significant effect on selectivities. Because the high charge density in the A and X zeolite can strip water molecules from the shell and then the difference in hydration energies contributes to the overall energy change.

Among the tested zeolite materials there were five zeolites which have identical faujasite framework structures with pore diameters of 7.4 Å but their Si/Al ratio varied between 1.23-10. These exchangers were chosen to study the effect of Si/Al ratios on the ion exchange selectivities. As can be seen from Tables 4 and 5 in (V) the selectivities for these divalent metals increased with the aluminium content of the zeolite. The only exceptions to this rule were Zn and Cd in the reference solutions where Y type zeolites with Si/Al ratios of 4.5 to 6 had a higher selectivity than Y types with Si/Al ratios of 2.7 to 3.3 . Otherwise, sorption from the simulants had the same trends as sorption from the reference solutions (Figure 2-4 in (V)).