Sustainable Solutions for Environmental Pollution, Volume 2. Группа авторов

or both, all bioremediation techniques involve oxidation-reduction reactions. Indeed, the common denominator for all NBSs applied to bioremediation is the addition of sufficient electron acceptors or donors to oxidize the pollutant or to stimulate the living organisms that will oxidize the pollutant.

      1.2.2 Self-Purification

      Self-purification is a natural biogeochemical process occurring in any ecosystem, and leading to elimination or assimilation of OM, mineral nutrients, or other pollutants by the natural activity of its resident biological communities (Namour, 1999; Marmonier et al., 2012). It is particularly active in the river underflow (hyporheic zone) where large contact surfaces develop and a redox gradient naturally installs (Namour, 1999; Namour and Le Pimpec, 2001). Its effectiveness depends on several factors such as the amount and toxicity of the contaminant, its ability to be degraded or “biodegradability” according to the surrounding physical-chemical conditions.

Biodegradability is the ability of substances to be decomposed into simple chemical elements, by the enzymatic activity of living organisms, mainly microorganisms. In fact, biodegradation is a dynamic balance between more or less complex chemical structures of varying resistance (more or less biodegradable), and the action of physical-chemical and biological agents. The quality and quantity of OM present and the existence of specific enzymes dictate the nature and intensity of biodegradation. Thus, biopolymers such as lignins or geopolymers such as humic substances are refractory to biodegradation due to the absence of specific degradation enzymes and require the prior intervention of redox enzymes that produce free radicals capable of opening aromatic cycles (Lipczynska-Kochany, 2018).

       1.2.2.1 Redox Processes

Sediment is generally oxygen poor (low diffusion and rapid consumption by microorganisms) and overloaded in OM, so microbial metabolism maintains reducing conditions in sediment where the biodegradation reactions take place according to a redox gradient (Figure 1.1) (Bertrand et al., 2011). Biodegradation effectiveness is often limited by the low availability (presence and mobility) in electron acceptors [e.g.,

In such environment, microbial activities maintain reducing conditions in the porous sediment and the biodegradation reactions are gradually moving along the redox gradient (Borch et al., 2010). In addition, environmental conditions affect the microbial metabolic pathway: 1) temperature strongly drives the biological activities; and 2) OM and NO₃^- (exogenous inputs or NH₄⁺ nitrification) availabilities are main reactants for denitrification (Lefebvre et al., 2004). The redox potential is a key element closely related to the pH and the electron acceptor availability (Figure 1.1).

Figure 1.1 Schematic organization of different microbial metabolic pathways in the water systems according the redox potential. The order of terminal electron acceptors (TEA) displayed in an idealized system is Fe(III), Mn(IV), and CO₂. Adapted from Bertrand et al. (2011; p. 573), Diaz and Rosenberg (2008), and Hargrave et al. (2008).

      If aerobic micro-organisms breathe O₂ as terminal electron acceptor (TEA), anaerobic micro-organisms breathe other TEAs, as or OM, producing noxious gases such as CO₂, CH₄, N₂0, H₂S, and NH₃ (Bertrand et al., 2011). These TEAs yield lower energy than O₂, so anaerobic respiration is less energy efficient and anaerobes grow slower than aerobes. Low TEA availability (presence and mobility) often limits self-purification process.

      NBSs of bioremediation aim at optimizing the TEA input needed to on-site self-purification. To do that, they implement three nested logics: action on form or physical structuration of the environment; action on fluxes of water and substances through it; and action on biocenosis living in the ecosystem. Thus, NBSs act on the geomorphology, hydrology, and physical-chemical characteristics of the environment such as in constructed wetlands (CWs) or filtering banks (FBs); on living organisms such as plants in phytoremediation; or on microbial consortia by passive control of the redox potential by supplying electrochemical inexhaustible TEAs in electro-bioremediation.

       1.2.2.2 Photo-Degradation

Photo-degradation is an oxidation of organic compounds induced by solar radiation, mainly by the energetic part of the solar spectrum, i.e., the ultraviolet (UV) radiation. Aquatic OM is photo-chemically unstable under ambient sunlight. In a wetland, according to its chemical structure, OM can be subjected to solar radiation, leading to its photolysis (fragmentation into lower molecular weight molecules, more easily assimilated by microorganisms), photo-bleaching (decrease in UV-visible absorbance due to loss of aromaticity), and photo-mineralization (complete degradation into mineral elements). Organic micropollutants are susceptible to be degraded by photolysis, with a large range of half times, from a few days to several months (Avetta et al., 2016). This degradation is only partial, therefore, the by-products are likely to be other organic molecules, of smaller size and not fully oxidized end-products such as H₂O, CO₂, and NO₃^-. In the case of sulfamethoxazole (SMX), a widely used antibiotic in human healthcare and its human metabolites (86% of the ingested SMX dose), most metabolites were found to be photostable under environmentally relevant conditions. The degradation yields, based on carbon balance, reached 90% for SMX after 4 hours, but only 50% for N-acetyl SMX, the main human metabolite (50% of the ingested SMX dose) (Bonvin et al., 2013). The effect of solar irradiation is both direct and indirect: indirect photolysis refers to the action of hydroxyl and carbonate radicals and excited triplet states of dissolved OM formed in the water phase by solar irradiation.

1.3 Aquatic Bioremediation Structures

      Until the end of the 21st century, urban wastewater was extensively discharged into water bodies such as rivers and lakes, without any treatment: the self-purification taking place in these water bodies was supposed to be sufficient. However, microbiology was still in infancy at that time, and drinking water resources, such as surface waters, were often contaminated by waterborne disease agents, such as protozoa (Giardia intestinalis, first described by Antoni van Leeuwenhoek in 1681, Cryptosporidium) and bacteria (Shigella (discovered in 1897 by K. Shiga and causing dysentery), Salmonella enterica (causing typhoid), Vibrio cholerae (cholera agent discovered by Pacini in 1854 and rediscovered by Koch in 1885).

      One of the first “treatments” of urban wastewater was indeed nature-based as it consists to spread wastewater in agricultural fields where it could act as a fertilizer. This practice has lasted for about 100 years in Paris and Reims (France). The contamination of soils by micropollutants (in particular metals such as lead) has led to the abandoning of vegetable crops for non-food crops.

      Because of rapid urbanization, climate change, and extreme weather events induced, water-related issues, such as flooding, groundwater over-exploitation, water shortages, and wastage, and water pollution, are becoming a global concern (Simperler et al., 2020). The development of the concept of sponge cities (Sun et al., 2020) aiming at improving the response of cities to rainfall should take into account the risks of dissemination of micro-pollutants. Constructed