Solar Photo-Assisted Degradation of Bipyridinium Herbicides at Circumneutral pH: A Life Cycle Assessment Approach

Alejandra Teutli-Sequeira,Jorge G Ibanez,Ruben Vasquez-Medrano,Dorian Prato-Garcia

doi:10.3390/pr8091117

Abstract

This study investigated the degradation of the herbicides diquat (DQ) and paraquat (PQ) by a solar photo-Fenton process that is mediated by Fe(III)-oxalate complexes at circumneutral pH = 6.5 in compound parabolic collectors (CPC)-type reactors. The photo-Fenton process operates efficiently at acidic pH; however, circumneutral operation was key to overcome drawbacks, such as acidification and neutralization steps, reagent costs, and the environmental footprint of chemical auxiliaries. This work revealed a remarkable reduction of total organic carbon for PQ (87%) and DQ (80%) after 300 min (at ca. 875 kJ L−1). Phytotoxicity assays confirmed that the treatment led to a considerable increase in the germination index for DQ (i.e., from 4.7% to 55.8%) and PQ (i.e., from 16.5% to 59.7%) using Cucumis sativus seeds. Importantly, treatment costs (DQ = USD$8.05 and PQ = USD$7.72) and the carbon footprint of the process (DQ = 7.37 and PQ = 6.29 kg CO2-Eqv/m3) were within the ranges that were reported for the treatment of recalcitrant substances at acidic conditions in CPC-type reactors. Life cycle assessment (LCA) evidenced that H2O2 and electricity consumption are the variables with the highest environmental impact because they contribute with ca. 70% of the carbon footprint of the process. Under the studied conditions, a further reduction in H2O2 use is counterproductive, because it could impact process performance and effluent quality. On the other hand, the main drawback of the process (i.e., energy consumption) can be reduced by using renewable energies. The sensitivity study evidenced that carbon footprint is dependent on the energy share of the local electricity mix; therefore, the use of more renewable electrical energy sources, such as wind-power and photovoltaic, can reduce greenhouse gases emissions of the process an average of 26.4% (DQ = 5.57 and PQ = 4.51 kg CO2-Eqv/m3) and 78.4% (DQ = 3.72 and PQ = 2.65 kg CO2-Eqv/m3), respectively. Finally, from the economic and environmental points of view, the experimental results evidenced that photo-assisted treatment at circumneutral pH is an efficient alternative to deal with quaternary bipyridinium compounds.

Highlights

Diquat (DQ: C12 H12 Br2 N2, 6,7-dihydrodipyrido[1,2-a:20,10 -c]pyrazinediium dibromide) (Figure 1a) and paraquat (PQ: C12 H14 Cl2 N2, 1,10 -dimethyl-4,40 -bipyridinium dichloride) (Figure 1b) are quaternary bipyridinium compounds that are extensively used in agriculture for post-emergence weed application, and as desiccants because of their potent herbicide action [1,2]
The sensitivity study evidenced that carbon footprint is dependent on the energy share of the local electricity mix; the use of more renewable electrical energy sources, such as wind-power and photovoltaic, can reduce greenhouse gases emissions of the process an average of 26.4% (DQ = 5.57 and PQ = 4.51 kg CO2 -Eqv/m3 ) and 78.4% (DQ = 3.72 and PQ = 2.65 kg CO2 -Eqv/m3 ), respectively
Concentration according to a series of factors to influence the yield of the photo-Fenton process mediated with Fe(III) oxalate, such as oxalate doses, ferric ion, hydrogen peroxide, the initial concentration of the herbicide, and pH

Summary

Introduction

Diquat (DQ: C12 H12 Br2 N2 , 6,7-dihydrodipyrido[1,2-a:20 ,10 -c]pyrazinediium dibromide) (Figure 1a) and paraquat (PQ: C12 H14 Cl2 N2 , 1,10 -dimethyl-4,40 -bipyridinium dichloride) (Figure 1b) are quaternary bipyridinium compounds that are extensively used in agriculture for post-emergence weed application, and as desiccants because of their potent herbicide action [1,2]. The degradation of PQ and DQ in the soil takes place through several mechanisms, such as photodegradation, chemical degradation, and microbial metabolism. Photodegradation reactions take place on the soil surface [3] and they depend on the intensity of UV light in the wavelength range of 285 nm to 310 nm [4,5]. Various groups of microorganisms, such as bacteria and actinomycetes, which use paraquat as a source of nitrogen [6]. Microbial degradation can be limited by rapid soil adsorption and slows down when PQ becomes fixed in a clay network [7]. PQ is completely degraded by soil microorganisms within six years into ammonia, carbon dioxide, and water [8]. PQ is not readily degraded by microbes in river water samples, since more than 80% of the initial concentration remained unchanged, even after

Methods

Results

Conclusion