Hydrological and hydrochemical controls on mobility of natural organic matter, iron and arsenic

Abstract

This project reviewed several recent studies conducted by the author that investigated the important role of hydrological and hydrochemical conditions in natural water in the mobility and fate of natural organic matter (NOM), iron (Fe) and arsenic (As). NOM is a critical component in aquatic water as it has a strong control on water chemistry and close association with global carbon (C) cycle, mobility of nutrients (Fe)/pollutants (As) and water treatment. Thus, this study aimed to investigate NOM in quantity and quality from peatland runoff and explore how it was changed. Organic carbon (OC) fluxes were monitored under baseflow and stormflow in uneroded and eroded sites during study period. The data showed that OC flux was higher in the eroded site compared to the uneroded site. NOM export from peatland was increased at a great rate under stormflow with higher discharge (Q) in the eroded site. These demonstrated that the loss and mobility of NOM in peatland is expected to be aggravated by changed rainfall patterns and erosion. Considering that increased aquatic NOM flux is the dominant driver of global warming through greenhouse gas emissions (CO2), therefore, this study suggests that increased NOM release have exacerbated the release of CO2 to the environment posing continuous degradation of global warming. In order to reduce the loss of NOM and mitigate its negative impact on global C cycle, a series of management practices should be considered. Fe is found naturally in aquatic environments in diverse forms and is an essential nutrient. The mobility and speciation of Fe relies on oxygen (D.O.) and NOM. This project simulated the interaction between Fe and NOM under different conditions in natural water (such as NOM content, pH, light, temperature and mixing order of Fe, NOM and D.O.) to understand the controls on Fe mobility in natural water. The results of this study showed that NOM mainly affects the mobility of Fe by controlling size and oxidation state of Fe. The mobility of Fe cannot be assumed to be that of a free aqueous ion due to the generation of Fe particulates with multiple speciation (FeII or FeIII bound to NOM or FeIII(hydr)oxide) by association with NOM. The mobility of particulate Fe will be reduced and the extent of which is controlled by the simulation conditions. The association of Fe with NOM is not low, which affects Fe lability, specific surface area, speciation and therefore its evolution, having important implications in nutrient availability. Both NOM and Fe can play an important role in the mobility and fate of As in upland peat catchment, but there is a lack of data on the reliable quantitative As mobilisation and how As mobilisation affected by NOM or Fe. Considering that As is a toxic element, elevated levels of As in water will have a serious negative impact on human health and environment. Therefore, this project investigated the release and mobility of As in upland peat catchments. The results of this study preliminarily determined the release mechanism of As. Detailed stream water sampling indicated that the As release and mobility from peat soil into stream is strongly correlated to NOM and/or Fe. Fe and NOM mainly affect the transport of As in particulate form and dissolved form respectively. In the eroded site, stormflow with high Q was more likely to cause the export of NOM and Fe in peat soil, which indirectly leads to an increase in As concentration in stream. However, the concentration of As was reduced in the uneroded stie, because its dominant factor, Fe, is diluted by the influence of high Q. Understanding release factors of As provides useful conditions for As monitoring and control in upland peat catchment and therefore it is benefit for managing and predicting As.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stephen Boult (Supervisor) & Bart Van Dongen (Supervisor)