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How do water matrices influence QSPR models in wastewater treatment?–A case study on the sonolytic elimination of phenol derivates [1]

['Judith Glienke', 'Institute Of Technical Chemistry', 'Environmental Chemistry', 'Friedrich Schiller University Jena', 'Jena', 'Center Of Energy', 'Ceec Jena', 'Michael Stelter', 'Fraunhofer Ikts', 'Fraunhofer Institute For Ceramic Technologies']

Date: 2023-12

Sonolytic degradation experiments were performed for 32 phenol derivates in three different water matrices (NaCl, glucose, NaCl + glucose). For comparison, kinetic data for the degradation of these substances in ultrapure water (k pure ) was retrieved from our previous study using the same experimental setup [ 18 ]. A full overview of the four datasets with the kinetic constants, standard variation, and the percentage variation to k pure are given in the supplement material in Tables D–G in S1 Text .

The kinetic data of the four datasets are shown in Fig 1

The values for the rate constants k displayed in Fig 1 are sorted by increasing values for k pure . The values of k pure vary between 0.01143 1/min for 2,5-dihydroxybenzoic acid and 0.03356 1/min for 4-hexylbenzene-1,3-diol. For the experiments with added NaCl, the rate constant ranges between 0.06830 1/min for 2,5-dihydroxybenzoic acid to 0.02800 for 4-methylbenzene-1,2-diol. For glucose as water additive, k Glcuose values vary between 0.00912 1/min for 2,5-dihydroxbenzoic acid and 0.02637 for 4-hexylbenzene-1,3-diol. Finally for the water matrix with NaCl and glucose, the range of the rate constants lay between 0.01065 1/min for 2,3-dihydroxybenzoic acid and 0.02733 for 3,5-dichlorobenzene-1,2-diol. Overall, it can be observed that the general trend following the sorting for increasing values of k pure (higher k values from left to right) is still dominant for the single component matrices, but a few major outliers can be perceived. For the two-component water matrix, the sorting gets even more mixed up. Hence, the influence of specific water additives is unique for every chemical structure.

To get a little bit more insight, the change in the kinetic constant due to the addition of water matrices were observed. The variation of the kinetic values for the three water matrices to the values in ultrapure water, calculated as Δk = k matrix -k pure are displayed in the supplement material in Fig B in S1 Text. The percentage variations of the kinetic constants with water additives to the values in ultrapure water are given in Fig 2.

It is noticeable that the influence of matrix compositions on structural related compounds such as phenol derivates is not equal for all substances, but rather large discrepancies can be observed. Qualitatively, a straightforward trend and interpretation for the whole dataset cannot be identified.

The addition of sodium chloride or glucose decreased the kinetic constant for almost all phenol derivates. For the addition of glucose, this goes along with the assumption that an additional hydrophilic organic matrix decreases the degradability as it functions as competitive reactants for reactive oxygen species (ROS) in the bulk liquid, which leads to a decreased probability of a reaction between organic micropollutants and ROS [33]. The addition of sodium chloride has also lead to an inhibition of degradability in previous studies, as chloride ions can react with hydroxyl radicals to form a chloride radical and a hydroxyl anion, scavenging the highly reactive hydroxyl radicals in the liquid [34, 35]. Additionally, chloride and sodium radicals formed in cavitation bubble collapses can also further scavenge other ROS [36]. Formed chloride radicals can subsequently react with chloride anions or hydroxyl radicals to form dichloride anion radicals (Cl 2 ∙-) or ClHO∙-, respectively. As the redox potentials of these oxidizing species are lower than those of hydroxyl radicals, the formation of these radicals can result in a reduced degradation of organic species [37, 38].

Interestingly, the influence of the two-component matrix is not a sum of the single-component influences. For 2,5-dihydroxybenzoic acid for example, the variation to k pure for NaCl+glucose (-15.42%) is in between the influence of the single NaCl and single glucose matrix (-6.52% and -20.20%, respectively). Other than that, the percentage variation for NaCl+glucose (-30.15%) is higher than the influence of glucose (-25.86%) or NaCl (-11.74%), but not as high as the sum of these two single influences. Therefore, the influence of more complex water compositions cannot simply be predict based on experiences of more simple matrices.

Even though a clear trend cannot be seen, a few interesting aspects can be observed. 4-nitrobenzene-1,2-diol for example is among the most influenced compounds in the datasets, as all three investigated water compositions decrease the degradability of this compound drastically. However, for 4-nitrophenol, the addition of NaCl does not have such a large impact, whereas the presence of glucose in the water severely reduces its rate constant. For 2-nitrophenol, the reduction of the kinetic constant is almost equal for all three water compositions. This example shows that small structural differences lead to a completely different impact of water additives on the degradation ability. This can be seen for example also for 4-methylphenol and 4-bromophenol, as the degradability of the first compound is hardly affected of any of the investigated water matrices, whereas the second-named molecule shows high differences in the kinetic constants.

Exceptions in this investigated set of phenol derivates are 4-methylbenzene-1,2-diol and 3,5-dichlorobenzene-1,2-diol. For these two compounds, the degradability increased with the addition of sodium chloride and glucose, respectively. This enhancement with salt was seen in experimental studies before. Seymour and Gupta observed a salt-induced enhancement of the sonolytic degradation of different phenol derivates [39]. On the one hand, this was explained with a salting-out effect, in which the dissolved salt increases the hydrophilicity of the liquid, driving more nonpolar compounds towards the bubble-liquid interface, increasing their sonolytic degradation. This effect should probably be the strongest for hydrophobic compounds. The same applies for the addition of glucose, as glucose is very hydrophilic, the assumption would be that glucose has a lower effect on hydrophobic compounds, as they would be nearer to the bubble interface and therefore would not compete with glucose for ROS as much as hydrophilic compounds in the bulk liquid [34]. In this dataset however, the enhancement effect is not present for the most nonpolar compounds (such as 4-hexylbenzene-1,3-diol) and a salting-out effect is usually observed with much higher salt concentrations as used in this study. Therefore, this explanation appears to be not applicable for the results in this dataset. On the other hand, an enhancement of sonolytic degradability could be explained through a decreased vapor pressure and increased surface tension due to salt ions, but this effect would be very small with such a low salt concentration [39]. A different explanation for these anomalies could be, that 4-methylbenzene-1,2-diol as well as 3,5-dichlorobenzene-1,2-diol could have a higher reactivity towards chloride radicals and degradation intermediates of glucose, introducing another degradation pathway in comparison to an ultrapure water environment.

For the simultaneous addition of sodium chloride and glucose, the kinetic constants increased for four compounds, including benzene-1,2-diol and 4,5-dichlorocatechol in addition to the two substances discussed before. For benzene-1,2-diol however, the percentage increase of the kinetic constant in a water matrix of NaCl and glucose is within the standard variation of the triple determination. Hence, the effect seems negligible for this compound. For all other compounds, the degradability was again decreased with additional water matrices compared to the kinetics in ultrapure water. As seen with the single matrix compound experiments, a qualitative discussion about the reasons behind these differences in enhancement and inhibition is not trivial, as the theoretical background of matrix effects on sonochemistry and sonolytic degradation are not well understood in detail for the used matrix components.

Generally, our data shows that the influence of matrix effects on the degradability of phenol derivates cannot easily be interpretated qualitatively. Additionally, the combination of different matrix compositions does not seem additive. This makes a qualitative prediction of the influence of water matrices on the degradability even more complex, as one cannot simply investigate single component matrices and then make additive assumptions for more complex water compositions. This could also be a problem for the application of QSPR models in real wastewater systems, as most models are based on a dataset derived from experiments using ultrapure water or real wastewater in one specific treatment plant. As seen in Fig 1, the four experimental datasets differ strongly, and the sequence by value of the molecules is mixed up by the addition of water additives. Due to the significantly different values of the experimental endpoint, an influence on the selection of independent variables for the QSPRs is expectable, as descriptor values stay the same with simultaneous changes in endpoint values. However, because of the lack of previous studies, the extend of this impact is still unknown.

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[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000201

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