Phototransformation of the Herbicide Propanil in Paddy Field Water

Abstract

When irradiated in paddy-field water, propanil (PRP) undergoes photodegradation by direct photolysis, by reactions with OH and CO, and possibly also with the triplet states of chromophoric dissolved organic matter. Irradiation also inhibits the nonphotochemical (probably biological) degradation of PRP. The dark- and light-induced pathways can be easily distinguished because 3,4-dichloroaniline (34DCA, a transformation intermediate of considerable environmental concern) is produced with almost 100% yield in the dark but not at all through photochemical pathways. This issue allows an easy assessment of the dark process(es) under irradiation. In the natural environment, we expect PRP photodegradation to be important only in the presence of elevated nitrate and/or nitrite levels, e.g., [NO] approaching 1 mmol L (corresponding to approximately 60 mg L). Under these circumstances, OH and CO would play a major role in PRP phototransformation. Because flooded paddy fields are efficient denitrification bioreactors that can achieve decontamination of nitrate-rich water used for irrigation, irrigation with such water would both enhance PRP photodegradation and divert PRP dissipation processes away from the production of 34DCA, at least in the daylight hours.

Environmental effects of ozone depletion, UV radiation and interactions with climate change: UNEP Environmental Effects Assessment Panel, update 2017

“…For example, this may occur near sewage treatment plants where indirect photoreactions can be accelerated due to the high concentration of CDOM in the effluent. 407,408 Since our last update, 9 additional research has confirmed that indirect photodegradation is an important mechanism for the breakdown of a wide-range of contaminants, including healthrelated compounds, [409][410][411] carbon nanomaterials, [412][413][414][415] pesticides, [416][417][418] and UV filters. 408 Improved evaluation of these photodegradation processes is now being achieved with readily accessible models of solar UV radiation 320 to improve predictions of degradation of contaminants in the environment.…”

A Model Study of the Photochemical Fate of As(III) in Paddy-Water

“…In the presence of sufficiently reactive 3 CDOM * , favourable pH conditions for As(III) photooxidation could be produced by algal photosynthesis that consumes dissolved CO 2 and may considerably increase the water pH. The reaction rate constants between paddy-field 3 CDOM * and CO 3 2− are presently unknown and the topic deserves further investigation, but the occurrence of humic substances in paddy water [ 24 ] may suggest a non-negligible photoreactivity of the relevant CDOM [ 25 ].…”

“…The chemical and photochemical parameters of paddy water have been set in analogy with the results of previous work [ 24 ], carried out on paddy-water samples from three rice farms located in the province of Vercelli (Piedmont region, northwestern Italy). In particular, the average formation quantum yields of the transient species i from irradiated CDOM ( ) had the following values: = 1.7·10 −5 ; = 2.9·10 −2 , and = 9·10 −3 .…”

A Critical View of the Application of the APEX Software (Aqueous Photochemistry of Environmentally-Occurring Xenobiotics) to Predict Photoreaction Kinetics in Surface Freshwaters

“…Moreover, APEX strongly relies on the input data and, while plentiful information has been provided on how to use the software, little attention has been paid to how to experimentally obtain such data. The required procedures have been developed as well, but they have been outlined in sparse publications [20,25,33] and this has probably discouraged potential users.While waiting for a (hopefully) forthcoming "APEX 2.0" version that will make use of a different, more user-friendly platform (e.g., based on Python), the present paper will hopefully be an appropriate guide to both APEX and the overall ecosystem in which it works. It is unfortunately not possible to install APEX and use it without reading this paper and the guide (Readme.pdf, which can be found in the .zip file of the Supplementary Materials).…”

“…Moreover, APEX strongly relies on the input data and, while plentiful information has been provided on how to use the software, little attention has been paid to how to experimentally obtain such data. The required procedures have been developed as well, but they have been outlined in sparse publications [20,25,33] and this has probably discouraged potential users.…”