Biodegradation of a mixture of the herbicides ametryn, and 2,4-dichlorophenoxyacetic acid (2,4-D) in a compartmentalized biofilm reactor

Abstract

In this work, an efficient degradation process for the removal of 2,4-D and ametryn, together with organic and inorganic adjuvants used in the commercial formulations of both herbicides, was developed. Although both compounds are toxic for microbial communities, ametryn is markedly more toxic than 2,4-D. In spite of this, the microbial consortium used could resist loading rates up to 31.5 mg L⁻¹ d⁻¹ of ametryn, with removal efficiencies up to 97% for both herbicides. Thus, an alternative use of this consortium could be bioaugmentation, as a tool to protect the structure and function of an activated-sludge biota against ametryn or 2,4-D shock loads. The process was carried out in a lab-scale prototype of aerobic biobarrier constructed as a compartmentalized fixed film reactor with airlift recirculation of oxygenated liquid.

Introduction

The biotreatment of agricultural wastewater involves highly complex microbial communities and multiple agrochemical pollutants such as fertilizers and diverse kinds of pesticides. In México, the combination of the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and ametryn (N-ethyl-N′-(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine) has been widely used for controlling weeds in crops of maize and sugar cane. The use of mixtures of herbicides with different modes of action is considered a successful practice to control weeds resistant to triazine herbicides. Unfortunately, the effects of potentially interacting mixtures of contaminants in receiving environments and the chronic effects of long-term exposure of relevant biota to low pollutant concentrations are scarcely known.

Many commercially available herbicides act by inhibiting the chloroplast electron transport chain, among them ametryn, which is considered an extremely phytotoxic PSII herbicide, exerting ecotoxicological effects at exceptionally low concentrations. This fact, joined to its relatively high solubility in water, and its reduced sorption coefficients account for the low affinity of ametryn for soil colloids (Kasozi et al., 2012), making the herbicide potentially leachable and increasing threat to the aquatic environment (Carraro-Borges et al., 2009).

With regard to 2,4-dichlorophenoxyacetic acid (2,4-D); since 1940, it has been one of the most widely used phenoxy acid herbicides in agriculture and gardening. However, 2,4-D exhibits serious ecological effects. In addition to its toxic effects on birds, beneficial insects, soil annelids and non-target plants, it also negatively impacts aquatic life, affecting algae, small invertebrates, amphibians, and fishes, especially in their juvenile stages (Tomlin, 2006). These facts, joined to their persistence and mobility in the environment, make that both compounds are considered harmful to aquatic ecosystems. Physicochemical and biological alternatives to prevent contamination of water bodies by this class of agrochemicals, including biological permeable barriers, can be used. This last option requires microorganisms capable of degrading ametryn and 2,4-D; however, information about microbial degradation of ametryn, alone or mixed with other herbicides, is scarce.

Because both herbicides are of ecological concern, the purpose of this research was to examine a reliable biological treatment method for the removal of ametryn and 2,4-D from contaminated water using an acclimated microbial consortium attached to an aerobic biofilm reactor that mimics a permeable biological biobarrier. Biobarriers are engineered systems designed to remove contaminants from water. Usually, these systems operate in anoxic or anaerobic conditions, however, with some modifications in design or operational conditions, biobarriers; conceptualized as non conventional three phase fixed-bed reactors (FBR), can be used for the aerobic biotreatment of pollutants originating from different contamination sources, avoiding or minimizing impacts to water resources. Frequently, conventional FBRs are subject to liquid maldistribution, partial catalyst wetting, residual liquid holdup, and slow reaction rates. FBR’s performance can also be limited by mass transfer; therefore, high liquid circulation and oxygen supply rates are needed to deliver substrates, keeping the immobilized biomass active and uniformly distributed within the reactor. To overcome oxygen mass transfer limitations in submerged fixed-bed systems, a laboratory-scale biobarrier equipped with two draft channel risers for oxygenation, and axial and transversal recirculation of the oxygenated liquid, was constructed.