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Essay: Chemical Fate and Transport of 2,4-Dichlorophenoxyacetic Acid

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The chemical compound 2,4-Dichlorophenoxyacetic Acid (2,4-D) is the active ingredient in the majority of herbicides used around the world. The reason this controversial compound was of interest to me is because I used to work at a golf course as a greenskeeper where we sprayed herbicides, pesticides, insecticides, fungicides, and dye all along the course on almost a daily basis. The course was located through a neighborhood and had multiple ponds and streams runnning through it. This made me question the fate and transport of all these potentially dangerous chemicals being sprayed. Do they run off into these residential ponds and streams where the dissolve in the water? Do they evaporate into the air? Will they end up in the soil? To answer these questions, and I chose to study the commonly used herbicide, 2,4-Dichlorophenoxyacetic Acid. To answer these questions I will be using equations and methods learned in class to find its physiochemical properties, comparing these properties to any literature values available and then applying these properties to decide the fate and transport in the environment.

2,4-D was patented by Dr. Frank Jones in 1945. 2,4-D is a synthetic version of indole acetic acid, which is a natural plant auxin that Dr. Jones had studied. As the name suggest, 2,4-D is a benzene ring with 2 chlorines in the ortho and para position in relation to an oxygen with acetic acid attached to it. The structure of 2,4-D can be seen below in Figure 1.

Figure 1: Chemical structure of 2,4-Dichlorophenoxyacetic Acid

2-4,D is produced by starting with 2,4-Dichlorphenol.1 It is then mixed with NaOH to produce sodium dichlorophenate. 1 The sodium dichlorophenate then reacted with sodium monochloroacetate to create sodium 2,4-dichlorophenoxyacetate. 1 The sodium can then either be removed to get 2,4-D or it can undergo further reactions to create one of many derivatives of 2,4,D. 1 The general manufacturing process of 2,4-D is shown below in Figure 2.

Figure 2: General manufacturing process of 2,4-D6

As previously mentioned, the main usage of 2,4-D is as a herbicide. It is very proficient at killing broadleaf weeds, while not harming grass. It does so by attacking the tissue that is responsible for carrying water and nutrients and making it divide and grow uncontrollably until it kills itself.2 This allows for it to be used as a fertilizer in low doses, though this is not very common because there are better fertilizers with less risks associated. Roughly 46 million pounds of 2,4-D are used per year.3 Of that, roughly 66% is agricultural use and 33% non-agricultural.3 2,4-D is used on a variety of food/feed sites, such as fields for fruit and vegetable crops. Human exposure to 2,4-D can occur through inhalation, skin absorption, ingestion or eye contact. Once absorbed into the body only a small percentage of 2,4-D is transformed into conjugates with sugars or amino acids. It takes a few days for it be excreted and it is mostly removed in through urine and bile of feces.(EPA website)

Health and Environmental Concerns

The negative effects of 2,4-D are mostly seen in the endocrine system of humans, mostly the thyroid gland and the gonads. A study in the Netherlands found that 2.4-D displaces sex hormones from the proteins in blood that carries them (toxipediea). In a later study done at the University of Missouri there was a strong correlation found between low sperm count, high numbers of abnormal sperm and atrophy of testes with high levels of 2,4-D found in urine (toxipedia). It was also found that 2,4-D can act as estrogen in breast cancer cells increasing the activity of these cells.

Acute toxicity of 2.4-D can also be seen in humans. There are formula variants of this herbicide that can cause slightly different effects. For example: the acid and salt formula is a severe eye irritant where as the ester formula does not cause eye irritation. The most common toxicity seen with this chemical in high doses is central nervous system depression, which causes stiffness in arms and legs. There have also been cases of incoordination, lethargy, and even coma. Along with central nervous system depression, respiratory irritation has been reported, this includes difficulty breathing, burning, coughing and temporary loss of coordination. The most common sites of acute toxicity in humans is lungs, eyes, and the CNS.

2,4-D has been studied in animals and has been confirmed to be a teratogen. This means that observed effects during gestation have occurred. This includes skeletal abnormalities, and effects on the thyroid and gonads.

Although 2,4-D has been shown to causes health issued in humans it is not currently listed as a carcinogen. Some studies have shown it to be carcinogenic in rat studies where the rats were fed high doses of the substance.  The EPA in 2007 sited an association with exposure to 2,4-D and tumor growth. This has not been confirmed because the findings have not been confirmed as a true association between tumor growth and 2,4-D exposure.

The environmental impacts of 2,4-D are less severe, “under most environmental conditions 2,4-D esters and 2,4-D amines will degrade rapidly to form 2,4-D acid(toxipedia),” The acid form has a low persistence rate which means it will be transformed and degraded quickly. The normal half-life in soil is about seven days and it can be easily degraded under normal aquatic conditions.  The effects seen on animals is most seen in honeybees. 2,4-d does not directly kill honey bees but indirectly effects them by killing their food sources such as flowering weeds like dandelions. this effect is mostly seen during summer months and the impact is seen from home owners using this herbicide on their lawns to eliminate weeds.(ask.extension.org)

Physiochemical Properties and Estimations

Table 1 below shows the literature and calculated physiochemical properties of 2,4-D.

Literature Value Calculated

Molecular Weight 221.04 n/a

Melting Point 138 oC n/a

Boiling Point 457.4oC n/a

Vapor Pressure 1.8×10-10 3.97×10-11

Water Solubility 33,900 mg/L @ 20oC 2.3×104 mg/L @ 20oC

pKa 2.73 2.9

Air Water coefficient 1.76×10-12 3×10-9

Octanol Water Coefficient 102.81 102.61

Table 1: Physiochemical properties of 2,4-D.

Below, Equation 1, shows how vapor pressure was calculated using boiling point, where KF is the correction factor and is set to 1, T is temperature in K, and Tb is boiling point in K at 1atm.

Eq. 1: Log(P*)l=-(1/2.3)(KF)(4.4+Ln(Tb)(1.8((Tb/T)-1) – 0.8Ln(Tb/T)

This equation solves for the vapor pressure of the liquids state, however, after looking at the melting point, it can be seen that 2,4_d is a solid at 25 oC so Equation 2 and 3 has to be used to find the vapor pressure of the solid state.

Eq. 2 Ln((P*)s /(P*)l)=-(∆fusS/R)[(Tm/T)-1]

Eq. 3 ∆fusS= (56.5-19.2(τ)-(19.2log(σ))

This resulted in a vapor pressure of 3.97×10-11 bar. This is a factor lower than the theoretical value, most likely due to an incorrect KF and other calculation error. This very low vapor pressure indicates that 2,4-D is not very volatile and will likely not enter air phase. Linear free energy relationships (LFER) can also be used to estimate vapor pressures, however, there was no LFER literature available for 2,4-D.

The literature value for the air water coefficient is 1.76×10-12. As with vapor pressure, there are multiple methods of estimating the air water coefficient, including LFER, which we do not have the data for. Another method of estimating the air water coefficient is to sum up to contribution of each bond in the molecule. The equation used is shown below and the bond contributions were gathered from literature tables. 11111111111

Eq. 4  Log(Kiaw )=Σ[(Number of Bonds)(Contribution of Bond)]

This resulted in a rather low estimation of 3×10-9. This may be due to lack of a correction factor. As expected from the low vapor pressure, both the theoretical and calculated values are very low, again indicating that 2,4-D is not volatile, and does not like to enter the gas phase.

The Henry consent can be estimated from the air water partition constant, as shown below in Eq. 5.

Eq. 5   KiH=KiawRT where   Kiaw =1.76×10-12

This will produce a KiH=1×10-9 atm L/mol. The Henry Constant is also related to solubility and vapor pressure. Shown below is how to calculate the Henry Constant with using the known solubility and vapor pressure of 2,4-D.

Eq. 6   KiH=P*is/Cis

Using the literature values for vapor pressure and solubility, we get matching answers of  KiH=1×10-9 atm L/mol.

Equation 6 can also be manipulated to solve for water solubility.

Eq. 7  Cis=P*is/KiH

Plugging in the literature values, and noting any necessary change in units, we can calculate that the solubility of 2,4-D to be 3.97×104 mg/L @ 25oC. This is not too far off from the cited value of 3.39×104 mg/L. This small error may be due to discrepancies among sources literature values of vapor pressure and Henry constant. This large solubility indicated 2,4-D likes water and that the majority of it found in natural systems, will be dissolved in water.

Using this high solubility, we can calculate the activity coefficient. The equations used are shown below.

Eq. 8   Î³=1/(.018 x Cis)(e^(-∆fusG/RT))

Eq. 9 ∆fus G= ((56.5-19.2(τ)-(19.2log(σ)(Tm-T)

This yield an activity coefficient of 16.357. We can then use this to calculate the Gibbs excess energy, as shown below in equation 10, which gives us the value 6.92 kJ/mol

Eq. 10 Excess Gibbs= RTln(γ)

As, the name suggests, 2,4-Dichlorophenoxyacetic Acid is indeed an acid.  2,4-D has a theoretical pKa of 2.73. This means it will lose the hydrogen in the acetic acid rather easily, as show below.

Figure 3: 2,4-D Ion

This means in natural water systems with pH’s close to 7, the charged form will be dominant.  The pKa of 2,4-D can be approximated by comparing to a similar structure and then accounting for the Hammett Constants of any new atoms. The similar compound used was Phenoxyacetic acid. Its structure is compared to 2,4-D below in Figure 3.


Figure 4: Phenoxyacetic acid on the left and 2,4-D on the right

The only difference between the two compounds is the addition of the ortho and para chlorines. When using this estimation method, substituents in the ortho position have different values due to proximity to acetic acid. The pKa of Phenoxyacetic acid is 3.17111111111. Using that, the pKa for 2,4-D can be estimated as shown below.

Eq. 11 pKa = (pKa similar structure) – (∂ x (Σ(ortho Cl)(para Cl)) where ∂= correction factor of .3 and other Cl=.68 and para Cl= .22

This results in an estimated pKa of 2.9. The slight error can be explained by a non-precise correction factor and this being an estimation method.

The octanol water coefficient was estimated in a similar way to the pKa. Using a similar structure with a known Kow, we can estimate the Kow for 2,4-D. 2,4-Dichlorphenol, shown below has a known Kow of 102.8.

Figure 5: 2,4-Dichlorphenol

Using eq. 12 along with the values associated for each now or old bond added obtained from, the Kow can be estimated to be 102.61.

Eq 12:  Log Kow= (Similar compound Log kow) – (lost bonds constants) + (new formed bonds constants)

Biotic and Abiotic Reactions

The California Department of Pesticide Regulation has done an in-depth study on the fate of 2,4-D in the environment. Figure 6 is a figure from their study that shows the reactions 2,4-D undergoes in biotic and abiotic environments.

Considering the main usage of 2,4-D as an herbicide, the majority of 2,4-D is having biotic reactions with the weeds and crops its being sprayed on. The weeds that die from 2,4-D metabolize it and create amino acids conjugates. Non-susceptible plants still metabolize 2,4-D but it does not affect them and they metabolize it to carbohydrate conjugates. Other Biotic reactions 2,4-D undergoes is the passage through mammals. 2,4-D does not accumulate in mammals and is excreted through the kidneys. While in the body, 2,4-D is in its charged form but undergoes no further reactions. Unlike mammals, fish can metabolize 2,4-D and can have bioaccumulation, however it is not all that likely.

2,4-D undergoes abiotic reactions as well. Biodegradation of 2,4-D is rapid and the fate of the majority of 2,4D in abiotic systems. Biodegradation is very favorable but rates depend many different factors. One of the main products of biodegradation is 2,4-Dichlorphenol, which then is further reduced to aliphatic acids. (Ghassemi et al., 1981) The hydrolysis of 2,4-D creates the cation form of 2,4-D. This step is favorable due to the pKa. The hydrolysis half-life is 39 daysasdf so it will stay in the water for a while allowing plenty of opportunity to be moved along stream, or enter groundwater. 2,4-D can also under go aqueous photolysis and aerobic aqueous metabolism where it will have a half like of 13 and 15days respectively. Anaerobic aquatic metabolism is also possible for 2,4-D, with a half-life of 312 days.

The ultimate fate of the majority of 2,4-D is going to be the plant metabolites. Due to is high Cs and Kh , any 2,4-D that is not metabolized by plants will most likely end up in water. Once in the water, 2,4-D is biodegraded rather quickly to eventually aliphatic acids. Knowing this, if there was a spill, one should try to increase and optimize the microorganism available. Biodegradation would be the fastest and cheapest option. One might consider trying to contain the water in the surrounding areas as well, considering it will be in the aqueous phase

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