The Photochemical Behavior Of Iron-Phenolic Complexes In Fresh Water Drained From Peatland And Its Significance

Iron is an essential element for organisms,and also a nutrient element limiting phytoplankton prosperous.A large number of studies have confirmed that peatland is an essential place controlling the exportation of terrestrial iron into aqueous systems.There are a lot of natural organic matter in peatland with a relatively higher content of phenolic compounds.The solubility and stability of iron can be significantly improved by the formation of iron-phenolic complexes even in alkali condition.However,the photochemical process could probably affect the stability of these complexes and thus the effective output of soluble iron.Considering the photo-stability of these complexes has received little attention thus far.We hope to explore the photochemical behavior of this complex through field investigation and indoor simulation experiments.Jinchuan peatland,located in the Jinchuan Town,Jilin Province,is the focus of this study.Water samples and peat samples in different layers were collected.Some basic physiochemical characteristics of water samples,including pH,total dissolved iron,ferrous iron and total dissolved phenol were investigated.Solid phase extraction-high performance liquid chromatography method was used for the determination of phenolic composition and content in different layers,followed by the extraction with methanol and alkali solutions.The photo reaction equipment was designed and established and the determination method of soluble iron was improved.Gallic acid was used as a structural analog for the simulation experiments.Under weakly alkali condition(pH=8.0±0.1),iron-phenolic complex underwent simulated solar radiation and both ferrous iron and total dissolved iron were detected.Some other experiments were also carried out for the explanation behind them and the photochemical characteristics of the iron-phenolic complex were further explored by changing the composition of the system.Main results are revealed through systematic researches:1.Some basic physiochemical characteristics of water samples were detected,the results showed that similar to other peat areas,Jinchuan peat water were acidic with an average pH of 5.47.The concentration of soluble iron was higher with an average value of 10.50 mg/L.The phenolic/iron ratio was between 0.11-0.57,but there was no significant correlation between them.2.Phenolic acids in free and weakly-bound state were extracted using methanol and NaOH solutions respectively,which showed a maximum difference in concentration of 3 orders of magnitude.Nine phenolic acids,including gallic acid,protocatechuic acid,gentianic acid,p-hydroxybenzoic acid,caffeic acid,vanilla acid,syringic acid,p-coumaric acid and ferulic acid were found in the samples among which p-hydroxybenzoic acid,p-couamric acid,syringic acid and ferulic acid are the richest.There was no significant correlation between phenolic composition in different layers of surface peat within 20 cm depth.3.The photo reaction of gallic acid-iron complex under weakly alkali condition showed that(1)When pH became 8.0±0.1 and gallic acid/iron ratio drops to 0.1:1,the soluble iron has been precipitated under the simulated solar radiation within 3 hours.And we call this experimental condition as threshold system.The concentration of gallic acid during photo reaction showed that the decomposition of gallic acid was accomplished by selfoxidation and direct&indirect photo-degradation.The specific decomposition process of gallic acid is that: self-oxidation of(about 31.76%)during p H adjusting,direct photolysis(about 17.65%),indirect photolysis(about 33.53%)and co-precipitation with iron(about 5.76%).(2)Some co-existing phenolics in peatland have a great influence on the threshold system.Phenolic acids carrying methoxy groups,such as vanillic acid,could accelerate the decomposition of gallic acid by promoting the generation of reactive oxygen species.While the addition of caffeic acid,p-hydroxybenzoic acid and p-coumaric acid etc.can inhibit or remove this reactive oxygen species in the system.(3)the photo-stability of soluble iron in peat extract was relatively high with over 99% iron being protected under simulated solar radiation,which meant that the existence of DOC,such as phenolics,in peat is conducive to the effective output of soluble iron from peatland to the aqueous ecosystem.This study has confirmed that iron-phenolic complexes drained from peatland have a strong photochemical stability.Considering the wide distribution of peatlands globally and their significant impact on primary marine productivity,the role of peatland in the global carbon and iron cycle has become even more important and has profound environmental significance.