5. Discussion
5.1 Experiment A Discussion
5.1.2 Sequential Extraction of Sludge
Inherently, due to the multiple steps in the sequential extraction procedure, multiple and compounding uncertainties in the reported concentrations of metal ion are expected. One of the sources of uncertainty comes from the experimental procedure itself which includes uncertainties relating to the mass of sludge sampled and subjected to sequential extraction, volume of extractants used and the transfer of sludge from one extraction to the next compounding as a single sample is taken through a multiple extraction process. Another source of uncertainty is the dilute sample that undergoes ICP-AES analysis which frequently has metal ion concentrations near or below the detection limit. The dilute sample is as a result of the small mass of sludge sample (1-2 grams) and the relatively large volumes of reagents (between 30 and 45 ml) which are later further diluted to 100 ml as recommended by the procedure (Stover et al., 1976). Therefore, the uncertainties involved in the analysis of metals found in micro quantities are much larger. These difficulties in detection at such a small scale as well as the larger relative magnitude of errors due to the small concentrations are factors contributing to low precision of the reported values for metal concentration in different exchangeable phases. This is shown by the stark difference in the repeatability of the sequential extraction results seen for the three sludge samples in the higher range of metals (mass between 100 and 1400 mg) and the three sludge samples in the lower range
102 of metals (mass between 0 and 100 mg). Despite the large uncertainties in the values obtained from the sequential extraction, it does provide qualitative information on the distribution of metals within the phases.
5.1.2.1 Soluble and exchangeable phases
Although larger errors are expected in the phases that yield small quantities of metal, the results show that for all the metals studied, with the exception of Mg, metals occur in phases mostly associated with solids, with little reporting as free soluble or exchangeable ions. This means that irrespective of soluble metal additions through the feed at each cycle, the amount of metals in the soluble or dissolved phase remains small. This in turn means that the amount of metals that are bioavailable to microorganisms is small and the addition of metals as nutrients through the feed does not alter the amount of bioavailable metal ions. This phenomenon is not peculiar; similar results have been found in different waste water sludge samples where sequential extraction analysis showed that in most cases, less than 1% of metals (including Zn, Fe, Cu, and Mn) were found in the soluble and exchangeable phases even though metal ions were dosed to the anaerobic system (Stover et al., 1976, van Hullebusch et al., 2005, Aquino and Stuckey, 2007) with the exception of Mg where about 9% of the total metal ions reported to the exchangeable phase (Zufiaurre et al., 1998). Nonetheless, an integral point, as displayed by the looking wholly at the sequential extraction, is that in this system, the dissolved metal ions do not exist in isolation. They co-exist with the adsorbed, organically bound and precipitated metal ion phases.
5.1.2.2 Adsorbed phase
Figure 11 illustrates that for all of the metals subjected to sequential extraction, the proportion of total metal ion reporting to the adsorbed phase was small (between 0 and 5%). This is in line with what similar analysis on sludge has indicated for Zn, Fe and Cu where the percentage contribution to the adsorbed phase varied from 0 to 2% of total metal ion content (Stover et al., 1976, van Hullebusch et al., 2005, Aquino and Stuckey, 2007). A possible reason for this is that since the amount of metal ion in the adsorbed phase is controlled by the amount of free metal ion (by an equilibrium isotherm) and there is a relatively small amount of free metal ions that the adsorbed phase is tending towards equilibrium with, the amount of metals in the adsorbed phase is correspondingly small.
5.1.2.3 Organically Bound phase
From Figure 11, it may be deduced that a major portion of metals for both reactor I and II report to the organically bound phase. For Al, Zn, and Fe, when looking at the percentage contribution of
103 this metal phase when compared to the seven other phases, the organically bound phase contributes between 30 and 68% of the total metal ions in the sludge. This is not unexpected as previous experiments involving sequential extraction on different sludge samples indicated that for metal ions Fe, Cu, Zn, Mn, Ni, and Co, the amount of ions within the organically bound, carbonate precipitate and sulphide precipitate phases is significant (Fermoso, 2008, Stover et al., 1976, Filgueras et al., 2002, van Hullebusch et al., 2005, Aquino and Stuckey, 2007). In Figure 11, the average percentage of metal ions extracted to the organically bound phase was calculated to be 48.1% and 44.3% for reactor I and II respectively. This is in the same region as 12 waste water sludge samples analysed in Stover et al., (1976) where the average percentage of Zn extracted into the organically bound phase was 50.3%. Organically bound Fe dominates the trace metal distribution for both reactor I and II by sequestering between 63 and 68% of the total Fe ions. This percentage contribution is higher when compared to metal distributions from other sludge samples in literature where the reported fraction of Fe in the organically bound phase ranged between 19 and 30% with similar or higher contributions to the subsequent precipitate and residual phases (Aquino and Stuckey, 2007, van Hullebusch et al., 2005, Stover et al., 1976).
5.1.2.4 Carbonate and Sulphide Precipitate phases
The fraction of metal ions in the precipitate phases (carbonate and sulphide together) ranges between 15 and 50% for all the metals in Figure 11. This, when compared to the 0 to 5% fraction in the soluble, exchangeable and adsorbed phases, may be regarded as significant. This is in line with literature suggestions that the carbonate and sulphide phases (as well as the organically bound phase) contribute significantly to the total metal content.
Figure 11 also shows that for some metals (Al and Zn), the contribution of metal ions to the carbonate precipitate phase is less than the contribution to the sulphide precipitate phase while for other metals (Fe) the opposite is observed, however, the total sequestration of metal ions to precipitates is significant. Similar findings were observed in literature where in two sludge samples, the percentage contribution of Zn to the carbonate precipitates was 14% and 23% and to the sulphide precipitates it was 52% and 47%. In those same two samples, the percentage contribution of Fe to the carbonate precipitates was 24% and 60% and to the sulphide precipitates it was 33%
and 4% (van Hullebusch et al., 2005). Therefore, the total contribution of Zn and Fe to the precipitates phase ranged between 57% and 70%.
The formation of precipitates within a system depends on the physical conditions like pH and temperature, as this affects the solubility product Ksp. The Ksp (at those conditions) in turn dictates
104 the minimum required concentrations for both the metal ions and the precipitating ions at which precipitates will start to occur. A key point is that for metals to form precipitates, they are required to exist in conjunction with precipitate counter-ions such that the concentration product of the ions forming the precipitate is equal to or higher than that of the Ksp. It is thus, assuming there are no mass transfer limitations, highly likely that precipitates will be present if the Ksp value for a mineral containing one of the metals of interest is low at the specific solution conditions. Literature indicates that trace metal precipitates have very small Ksp values, especially for sulphide precipitates. In the presence of as little as 0.0003 mg/l of hydrogen sulphide, the predicted solubilities of Fe2+, Co2+ and Ni2+ are 0.0000016, 0.000000006 and 0.000000004 mg/l respectively (Callendar and Barford, 1983).
For precipitates to occur in such significant quantities, the carbonate and sulphide ions with which the metals form precipitates, must also occur in significant quantities. A source of both the sulphide and carbonate ions is from the feed to the reactors where sodium carbonate was dosed as a buffer and sulphate ions (which are reduced to sulphide ions in the reactors) as a nutrient. It would be difficult to try to reduce the concentration of sulphide and carbonate dosed to the reactors in an attempt to reduce the precipitate formation and increase bioavailability of metal ions as both sodium carbonate and the sulphide ions play a role in the anaerobic digestion. Furthermore, the solubility products of metals are very low. An easier alternative would be to use weak complexing or chelating agents as these increases the bioavailability by forming soluble, bioavailable metal complexes which in turn promote the dissolution of metal ions from precipitates (Gonzalez-Gil et al., 2003). The addition of yeast extract is recommended as it was found to increase the solubility of nickel and cobalt (Gonzalez-Gil et al., 2003). Cysteine, a chelating agent as well as a source of sulphur, may provide similar results (Jansen et al., 2007).
The metals within the precipitate phase, together with the metals in the organically bound phase, make up the majority of the total metal content. Although these phases are insoluble in their current form, they may be seen as a large reservoir of metals which may, under different conditions, become soluble. Different ways that change the reactor conditions such that metals from the insoluble phases become bioavailable should be investigated rather than attempting to optimize the amounts of nutrients dosed to a system. The use of weak chelating agents is one such example.
Another possible way could be to make sure that for those metals that have very low solubility limits, they are preferentially precipitated by a precipitate that has faster dissolution kinetics. In this way, after the microorganisms absorb the metal ions in the soluble phase, the precipitates would dissolve at a reasonable rate to replenish those ions that were absorbed.
105 5.1.2.5 The Effectiveness of Sequential Extraction
The first objective is to differentiate between metals in a bioavailable form (soluble) and those that are only potentially bioavailable or not bioavailable, which refers to any metals that are associated in some form with the sludge. Furthermore, the model at this stage only considers a soluble and a precipitate phase. Therefore it is not desirable to differentiate between the different extractable forms associated with the particulates. As such, the potentially bioavailable and non-bioavailable fractions grouped together could be measured by the metals from the acid digestion of sludge minus the metals contained in the supernatant. This avoids additional errors incurred from performing the sequential extraction steps for the different phases, especially for metals that are found in trace quantities.