Estimation of the soil–water partition coefficient normalized to organic carbon for ionizable organic chemicals

Abstract

The sorption of organic electrolytes to soil was investigated. A dataset consisting of 164 electrolytes, composed of 93 acids, 65 bases, and six amphoters, was collected from literature and databases. The partition coefficient log Kow of the neutral molecule and the dissociation constant pKa were calculated by the software ACD/Labs. The Henderson-Hasselbalch equation was applied to calculate dissociation. Regressions were developed to predict separately for the neutral and the ionic molecule species the distribution coefficient (Ka) normalized to organic carbon (Koc) from log Kow and pKa. The log Koc of strong acids (pKa < 4) was not correlated to these parameters. The regressions derived for weak acids and bases (undissociated at environmental pH) were similar. The highest sorption was found for strong bases (pKa > 7.5), probably due to electrical interactions. Nonetheless, their log Koc was highly correlated to log Kow. For bases, a nonlinear regression was developed, too. The new regression equations are applicable in the whole pKa range of acids, bases, and amphoters and are useful in particular for relatively strong bases and amphoters, for which no predictive methods specifically have been developed so far.

Accounting for the dissociating properties of organic chemicals in LCIA: An uncertainty analysis applied to micropollutants in the assessment of freshwater ecotoxicity

“…Otherwise, due to the variability of the fractions of neutral and ionic species, such KOC values obtained at a given environmental pH would not be suitable for the inter-val of environmental pH values considered in this study. For these 0.65 compounds the soil-water partitioning coefficients are correlated to KOW,n, pKa and pH by using the Franco and Trapp regression equations [1]:…”

“…K OC = f n · 10 (0.54 logKOW,n+1.11) + (1 − fn) · 10 (0.11 log K OW,n +1.54) for acids (1 ) present in the inventory. The USEtox model does not address estimation routines for these mechanisms, therefore the follow-ing models and assumptions were applied in both the alternative approach and the default USEtox to estimate direct and indirect photodegradation rates.…”

“…(4) The extrapolation of parameter values from one compartment to another (biodegradation rates in biosolids) and from other parameter values (KDOC from KOC). The training and validation sets used to derive the regressions adopted in the model were used to derive residual estimation errors between estimated and experimental data [1,11,12]. The training and validation sets of Franco and Trapp [1] used to derive regression to predict KOC of acids and bases were used to generate residual estimation errors of the KOCWIN software applied to dissociat-ing substances in the default USEtox model (parameters a6 and a7 in Table 2).…”

“…The training and validation sets used to derive the regressions adopted in the model were used to derive residual estimation errors between estimated and experimental data [1,11,12]. The training and validation sets of Franco and Trapp [1] used to derive regression to predict KOC of acids and bases were used to generate residual estimation errors of the KOCWIN software applied to dissociat-ing substances in the default USEtox model (parameters a6 and a7 in Table 2). Likewise, the training and validation sets of Fu et al [17] used to derive regression to predict BCF fish of acids and bases were used to generate residual estimation errors of the BCFBAF software applied to dissociating substances in the default USEtox model (parameters a11 and a12 in Table 1).…”

“…The estimation of this parameter in LCIA models, such as IMPACT2002+, USES-LCA, EDIP 2003 or USEtox, is based on conventional non-polar partitioning models correlated only to the octanol-water partition coefficient, KOW, that do not adequately model the mechanism of sorption of dissociating organic chemicals to organic colloids in soil, which consists of organic matter and inorganic clay minerals. These correlations are particularly true for lipophilic compounds, however, the driving force behind the sorption of cations may be electrical attraction to the negatively charged sorption sites in soil since, at the same lipophilicity, the sorption of cations is stronger than of neutral bases, making it unlikely that the process behind is lipophilic sorption [1]. Recently, Droge and Gross [2] cited an ample number of recent studies that have shown that the dominant sorption process for organic cations is cation-exchange at nega-tively charged sorption sites in natural organic matter and whole soils/sediments.…”