Bioaccumulation of Polar and Ionizable Compounds in Plants Stefan Trapp Abstract The uptake of neutral and ionizable organic compounds from soil into plants is studied using mathematical models. The phase equilibrium between soil and plant cells of neutral compounds is calculated from partition coefficients, while for ionizable compounds, the steady state of the Fick–Nernst–Planck flux equa- tion is applied. The calculations indicate biomagnification of neutral, polar, and nonvolatile compounds in leaves and fruits of plants. For electrolytes, several ad- ditional effects impact bioaccumulation, namely dissociation, ion trap effect, and electrical attraction or repulsion. For ionizable compounds, the effects of pKa and pH partitioning are more important than lipophilicity. Generally, dissociation leads to reduced bioaccumulation in plants, but the calculations also predict a high poten- tial for some combinations of environmental and physicochemical properties. Weak acids (pKa 2–6) may accumulate in leaves and fruits of plants when the soil is acidic due to the ion trap effect. Weak bases (pKa 6–10) have a very high potential for accu- mulation when the soil is alkaline. The model predictions are supported by various experimental findings. However, the bioaccumulation of weak bases from alkaline soils has not yet been validated by field studies. Keywords Acids Bases Bioaccumulation Ionic pH Plants Model 1 Introduction Living organisms are exposed to chemicals in the environment and may take up and concentrate them in their body. An example is the bioconcentration factor (BCF) of fish, which is the concentration of a chemical in fish divided by the concentration of the chemical in surrounding water: Concentration of fish .mg=kg/ BCF D : (1) Concentration in water .mg=L/ S. Trapp () DTU Environment, Department of Environmental Engineering, Technical University of Denmark, Miljoevej, Building 113, DK – 2800 Kgs. Lyngby, Denmark e-mail: [email protected] J. Devillers (ed.), Ecotoxicology Modeling, Emerging Topics in Ecotoxicology: 299 Principles, Approaches and Perspectives 2, DOI 10.1007/978-1-4419-0197-2 11, c Springer Science+Business Media, LLC 2009 300 S. Trapp Similar is the bioaccumulation factor (BAF), which is the concentration in an or- ganism divided by the concentration in the surrounding medium: Concentration in organism .mg=kg/ BAF D : (2) Concentration in surrounding medium .mg=L/ The BCF was defined as the process by which chemical substances are adsorbed only through surfaces, whereas the BAF is due to all routes of exposure and in- cludes dietary uptake [1]. Accordingly, “biomagnification” is a process in which the thermodynamic activity of the chemical in the body exceeds the activity in the diet. Bioaccumulation in the food chain may lead to high doses of compounds in the diet of top predators and humans [2, 3] and is a highly undesired property of com- pounds [4]. In the European regulatory framework for chemical risk assessment, compounds with a BAF above 2,000 are considered as bioaccumulative and those with BAF above 5,000 as very bioaccumulative [4]. The same criterion (BAF of 5,000) is also used by other governments [5]. It is generally accepted that bioaccumulation is closely related to lipophilicity of a compound, measured as the partition coefficient between n-octanol and water, KOW, or the partition coefficient between n-octanol and air, KOA [6, 7]. Accord- ingly, a theoretical relation for aquatic biota was suggested [6, 8]. The lipid phase accumulates the compound similar to n-octanol, and therefore BCF D L KOW; (3) where L is the volumetric lipid content of an organism (L/L) and KOW is the partition coefficient between n-octanol and water (mg/L octanol:mg/L water D L octanol/L water). In general, most BAF estimation approaches describe the bioaccumulation be- havior of organic substances solely by the octanol–water partitioning coefficient .log KOW/. This may be correct for neutral lipophilic compounds. But there are other mechanisms that can lead to bioaccumulation, which are not connected to lipophilicity. One example is the accumulation due to uptake of water by plants from soil, with subsequent transport to leaves with the water stream, and subsequent accu- mulation in leaves when the water evaporates. Another example is the accumulation of weak electrolytes in living cells. Investigations show that for these dissociating compounds, other processes, such as pH-dependent speciation and electrical attrac- tion, can be the decisive processes determining the accumulation in cells [9–11]. The log KOW approach alone may lead to an under- or overestimation of the ac- cumulation of ionizable substances. In a review using fish with 5,317 BCF values, about 20% of compounds had the potential for ionization. But for less than 40% of the tests, the pH of the water during the experiment was reported [1]. It seems that the critical role of pKa and pH for the BCF of ionizable compounds is sometimes not sufficiently highlighted. Bioaccumulation of Polar and Ionizable Compounds in Plants 301 A mechanistic model described in this chapter will identify accumulation pro- cesses that are not related to lipophilic partitioning. The focus is on accumulation of compounds from soil in plants and, in particular, on ionizable compounds. 2 Electrolytes “Electrolytes” is a common term for compounds with electrical charge. Com- mon synonyms are “ionic compounds,” “ionizable compounds,” “dissociable compounds,” “dissociating compounds,” “electrolytic compounds,” and “charged compounds.” Electrolytes may be acids (valency 1; 2, etc.), bases (valency C1; C2, etc.), amphoters (valencies C1 and 1; C1 and 2, etc.), or zwitterions (valencies 0, C1,and1, etc.). Weak electrolytes are compounds with weak acid- and base- groups, which dissociate only partly under usual environmental condi- tions (pH 4–10). Thus, weak electrolytes are commonly present in two or more different forms with very different properties, namely the neutral molecule and the ion, which can rapidly be transferred from one into the other if pH changes. “Very weak” electrolytes are named acids or bases that dissociate only to a minor degree at environmental pH (usually between pH 5 and 9), i.e., bases with pKa <5and acids with pKa >9. Different to the neutral molecule, ions can be attracted or rejected by electri- cal charges. Monovalent bases have a valency of C1 and are thus attracted by negative electrical potentials, while acids have a valency of 1 and would be at- tracted by positive electrical potentials. The neutral compound typically has a far higher lipophilicity than the corresponding ion. In average, log KOW (ion) is equal to log KOW (neutral) 3:5, which means the KOW of ions is 3,162 times lower. For zwitterions, which have a permanent positive and negative charge, but a net charge of 0, the difference is smaller, log KOW .zwitterion/ D log KOW (neutral) 2:3 [12]. There is also a difference in vapor pressure. For ions, it is approximately 0. The vapor pressure of the total compound, p (Pa), can thus be calculated using the va- por pressure of the neutral molecule and multiplying with the fraction of neutral molecules. The applicability domain of most QSAR regressions is limited to neutral com- pounds [4]. For ionizable compounds, the TGD suggests a correction of the physico- chemical properties (log KOW, Henry’s law constant) by the neutral fraction of compound, Fn. For the BCF that means that log BCF D 0:85 log.Fn KOW/ 0:70: (4) The reliability of this method for ionizable compounds was never critically evalu- ated. A recent survey by Ralph K¨uhne revealed that of his database with >10,000 compounds, at least 25% of compounds have structures that may dissociate, such as carboxylic acids, phenols, and amines. In fact, ionizable compounds are frequent 302 S. Trapp and typical for many substance classes. Among pesticides, most herbicides are weak acids. Among pharmaceuticals, weak bases are frequent (“alkaloids”). De- tergents are often anionic, cationic, or amphoteric. Metabolites of phase I reactions (oxidations) are usually acids, while the reduction of nitro-groups leads to amines. Given the widespread occurrence of weak electrolytes, it may surprise that very few models and regressions were developed for ionizable substances. In the following, a dynamic model for plant uptake is developed – first the “stan- dard approach” for neutral compounds. This is then modified to be applicable for electrolytic compounds. 3 Plant Uptake Models for Neutral Compounds In this section, the uptake and accumulation of polar and ionizable compounds in plants is quantified. Based on physiological principles, the mass balance equations for the transport of compounds in the soil–plant–air system are derived and com- bined to mathematical models. Plant uptake models for neutral compounds have been developed by several groups [13–16]. A series of crop-specific uptake models was derived, based on the PlantX model [17] – to mention are the one-compartment analytical solution of the latter [18], and the models for root vegetables [19], potatoes [20], and fruits [21,22]. 3.1 How Plants Function Figure 1 shows schematically how plants function: The large network of roots takes up water and solutes. In the pipe system of the xylem, these are translocated through the stem to the leaves. The leaves take up carbon dioxide from the atmosphere and simultaneously transpire the water. Carbohydrates produced in the leaves by photo- synthesis are translocated in the phloem pipe system to the sinks (all growing parts, fruits, and storage organs). In most ecosystems, plants transpire about two-thirds of the precipitation [23]. For humid conditions, this ranges from 300 to 600-L water per square meter per year. The water, which is taken up by the roots, does not stay there but is translocated in the xylem to the leaves and evaporates. Only 1–2% is taken up into the plant cells.