Adsorption and degradation of phenoxyalkanoic acid herbicides in soils: A review

Abstract

The primary aim of the present review on phenoxyalkanoic acid herbicides-2-(2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chloro-2-methylphenoxy) acetic acid (MCPA), (2R)-2-(2,4-dichlorophenoxy) propanoic acid (dichlorprop-P), (2R)-2-(4-chloro-2-methylphenoxy) propanoic acid (mecoprop-P), 4-(2,4-dichlorophenoxy) butanoic acid (2,4-DB), and 4-(4-chloro-2-methylphenoxy) butanoic acid (MCPB)-was to compare the extent of their adsorption in soils and degradation rates to assess their potential for groundwater contamination. The authors found that adsorption decreased in the sequence of 2,4-DB > 2,4-D > MCPA > dichlorprop-P > mecoprop-P. Herbicides are predominantly adsorbed as anions-on organic matter and through a water-bridging mechanism with adsorbed Fe cations-and their neutral forms are adsorbed mainly on organic matter. Adsorption of anions of 2,4-D, MCPA, dichlorprop-P, and mecoprop-P is inversely correlated with their lipophilicity values, and modeling of adsorption of the compounds based on this relationship is possible. The predominant dissipation mechanism of herbicides in soils is bacterial degradation. The contribution of other mechanisms, such as degradation by fungi, photodegradation, or volatilization from soils, is much smaller. The rate of bacterial degradation decreased in the following order: 2,4-D > MCPA > mecoprop-P > dichlorprop-P. It was found that 2,4-D and MCPA have the lowest potential for leaching into groundwater and that mecoprop-P and dichlorprop-P have slightly higher potential. Because of limited data on adsorption and degradation of 2,4-DB and MCPB, estimation of their leaching potential was not possible. Environ Toxicol Chem 2016;35:271-286. # 2015 SETAC

Herbicidal Ionic Liquids Containing the Acetylcholine Cation

“…The values of the octanol‐water partition coefficient depend strictly on the concentration, therefore for this study the salts doses were equal to the doses used for studying the herbicidal activity. The obtained results were compared with the literature values for herbicidal acids,, and presented in Figure and ESI – Table S9.…”

“…The values of the octanol-water partition coefficient depend strictly on the concentration, [56] therefore for this study the salts doses were equal to the doses used for studying the herbicidal activity. The obtained results were compared with the literature values for herbicidal acids, [37,[57][58][59][60] and presented in Figure 4 and ESI - Table S9. ILs 1, 3, 6 exhibit higher affinity for octanol whereas ILs 2, 4, 5 for the water phase. The highest value of octanol-water partition coefficient was measured for 6 (1.11 � 0.07) and the lowest value for 5 (À 1.38 � 0.06).…”

A review of the pesticide MCPA in the land‐water environment and emerging research needs

“…The formulation is of particular importance before the pesticide contacts the soil or plant surfaces. Like all phenoxyalkanoic pesticides, MCPA has very low volatility (Paszko et al, 2016), but esters are more volatile than the free acid, which are more volatile than salt forms (Hornsby et al, 1996). However, MCPA is often applied by spraying which creates "spray drift," broadcasting droplets to the atmosphere that can combine with rain or fog (Freiberg & Crosby, 1986), as well as increasing the chances of MCPA entering streams and lakes (Kreuger, 1998).…”

“…An additional complication in predicting the mobility of MCPA is that liming can be used in conjunction with herbicide spraying and cutting to decrease establishment of rushes (Moran, 2015). As liming increases the soil pH (Blake & Goulding, 2002), MCPA will dissociate more toward the anionic than neutral form (Paszko et al, 2016), thus reducing its affinity for OM (Werner, Garratt, & Pigott, 2013), and hence decreasing its adsorption in the soil (Paszko, 2011). This outweighs the slight increase in adsorption caused by Ca 2+ ions from the lime, which create a Ca-bridging mechanism, because the affinity of MCPA in anionic form to OM is 10 times lower than that of MCPA in neutral form as soil OM is negatively charged (Tülp, Fenner, Schwarzenbach, & Goss, 2009).…”

Controlling Stormwater Quality with Filter Soil—Event and Dry Weather Testing

“…The reason for these contradictive results is not known. At the pH conditions present in the filter soil MCPA is primarily found as an anion, known to be poorly sorbed in soils, and sorption is negatively correlated with soil pH and positively correlated with organic matter content [49]. A way to improve the filter soil towards sorption of MCPA could be to add more stable organic matter; however, this has to be considered jointly with the risk of increasing DOC leaching and associated P and Cu.…”