All KOC values derived from the column experiments are given in Figure 2.1 and the corresponding breakthrough curves are shown in Figure 2.2. The comparison of measured with correlated data after Franco and Trapp (2008) is given in Figure A1 (see Appendix A).
2.4.1 Acidic compounds
Two acidic compounds (naproxen, phenobarbital) with their anionic species occurring in the investigated pH range were investigated. A general low retardation was measured for phenobarbital. R ranges between 1.6 at pH = 4 and 1.1 at pH = 8.
The appearing weak pH-dependence is caused by a hydroxyl group due to a lactam-lactim tautomerization in the barbituric substructure. This group leads to the resulting pKa and becomes deprotonated with increasing pH. The increasing anionic character makes the molecule more polar, thus resulting in a decrease in retardation.
The comparison of the measured and calculated KOC values shows moderate differences. At this point, it is obvious that the relative error of the measured KOC is considerable at small R around 1 (see Eq. 2.2).
A higher retardation and pH-dependence was observed for naproxen. R decreases from 7.7 at pH = 4 to 1.4 at pH = 8. The increasing deprotonation of the carboxyl group with increasing pH changes the molecule to the hydrophilic anionic form.
Except at pH = 8, where the log D seems underestimated, accordance between experimental and calculated KOC was found. In general, the two investigated acidic compounds seem to confirm the expected transport behavior received from the modified KOC concept as well as from the regression of Franco and Trapp (2008).
Fig. 2.1 Average from column experiments derived log KOC values (uniform columns) compared with estimated log KOC values (hatched columns) calculated with a modified logKOW -logKOC correlation after Sabljic et al. (1995) for all compounds at pH = 4, pH = 6, and pH = 8. Error bars indicate the range of experimentally derived log KOC for at least three measurements, except for TRI at pH = 8.
2.4.2 Neutral compounds
The validity of the KOC concept together with no pH-dependence of the retardation was expected for the two neutral compounds primidone and carbamazepine because of their pKa values being far beyond the investigated pH range.
Primidone has shown low retardation as well as its main metabolite phenobarbital.
For all considered pH levels, R remains around 1.2. No dependence from pH could be detected. A theoretically possible deprotonation of the hydroxyl group due to a
lactam-lactim tautomerization cannot occur at the experimental pH values. So, the molecule stays in its neutral form and is very mobile because of its low log KOW.
According to the higher log KOW, R between 5.3 at pH = 4 and 3.6 at pH = 8 were found for carbamazepine. Because of its very low pKa, only the neutral form of carbamazepine is the clearly dominating species at all investigated pH levels. Despite this, a weak pH-dependence was registered. The weak decrease in retardation for carbamazepine with increasing pH originates from the significantly higher log KOW
compared to primidone. The probability for hydrophobic interactions with the sediment decreases with rising pH due to the increasing deprotonation of the functional groups (mainly carboxyl and hydroxyl groups) on the sediment organic matter with increasing pH. As these interactions dominate the sorption process for carbamazepine (Scheytt et al., 2005) and other neutral compounds with higher log KOW their retardation decreases, too. In contrast, the influence of the pH-dependent sediment charge on the sorption of the very polar compound primidone seems to be negligible. As expected, the two applied correlations gave similar and acceptable results for neutral compounds.
2.4.3 Alkaline compounds
Strongest retardations and therefore highest log KOC values were observed for the cationic compounds atenolol, diazepam, and trimethoprim. In contrast to organic acids, the nitrogen bearing functional groups in these compounds are protonated and thus ionic below their pKa. An increase of pH leads to a shift of the dissociation equilibrium in favor of the neutral species. Hence, the hydrophobicity and log D values increase with rising pH. However, the pH-dependent retardation of the compounds investigated in this study shows an inverted image. R decreased from 23.3 to 15.8 for atenolol, from 29.4 to 8.8 for diazepam and from 219.6 to 47.0 for trimethoprim with increasing pH from 4 to 8. Despite of the non-protonated, neutral species becoming the dominating species the retardation decreases with increasing pH. Therefore, the modified KOC concept fails to predict the sorption behavior of these alkaline substances. Even worse, the implementation of log D intended to correct for neutral/charged species distribution further enhances the differences between estimated and measured log KOC data. Much better results were obtained by
using the specific correlation for bases from Franco and Trapp (2008). In contrast to the modified KOC concept, at least the trend of decreasing sorption with increasing pH could be reproduced. Nevertheless, absolute KOC values are still underestimated.
An additional sorption process has to be responsible for the increasing sorption of the cationic species. The existence and dominating role of cation exchange processes on the retardation was earlier reported for the beta-blocker atenolol (Yamamoto et al., 2009) as well as for the antibiotic trimethoprim (Bekçi et al., 2006; Qtaitat, 2004).
This is plausible since the composition of the sediment (TOC, minerals) gives evidence for the presence of negatively charged surfaces at all here investigated pH levels (see Chapter 2.3.2). Also, Figueroa et al. (2004), Carmosini and Lee (2009), and Vasudevan et al. (2009) could highlight the importance of cation exchange processes for the cationic species of tetracycline antibiotics and the fluoroquinoline ciprofloxacin on negatively charged surfaces. Therefore, the shown similar behavior is likely for the further investigated basic compound diazepam. Atenolol at pH = 8 is the only substance which was noticeably biodegraded (to 80% of c0) during the experiments (Fig. 2.2).
2.4.4 Zwitter-ionic compounds
The zwitter-ionic character of cetirizine in the considered pH range is caused by two basic reacting tertiary amine groups with pKa of 2.2 and 8.0, respectively and one acidic carboxyl group with pKa of 2.9. At pH values close to 4 and 8, cetirizine occurs as a monovalent cation and a monovalent anion, respectively. The log D value changes minimally between pH = 4 and pH = 8 (van Balen et al., 2001) and only a weak pH-dependence may be expected. But in contrast to this presumption, a strong pH-influence associated with a much higher retardation was observed. The reason for such behavior might be explained by a combination of the above-mentioned effects. At low pH the cationic species with positive partial charges are dominating.
Therefore, the affinity to negative charged sediment surfaces is high and the molecules are able to sorb due to cation exchange. With increasing pH the positive charges of cetirizine becomes weaker because of an increased deprotonation. As a result, the sorption due to cation exchange is reduced and the retardation factors decrease drastically from 130 at pH 4 to 16 at pH 8.
Fig. 2.2 Experimental and modeled breakthrough curves for eight pharmaceuticals at pH = 4, pH = 6, and pH = 8 showing the pH-dependence for different compounds and a wide spectrum of pKa. Concentrations c of the breakthrough curves were normalized with the initial concentration c0. Pore volumes were calculated by normalizing the experiment duration to the breakthrough time of the Cl− tracer.