مطالعه خوردگی بتن در یک شبکه فاضلاب ترکیبی ناشی از میکروبیولوژیکی

ABSTRACT Microbiologically induced concrete corrosion: A case study from a combined sewer network

Corrosion of concrete-based sewer systems due to the emission of hydrogen sulfide (H2S) from wastewater is a worldwide occurring issue with great economical relevance. The so-called microbially induced concrete corrosion (MICC) [1,2] is considered to significantly reduce the lifespan of concrete structures, from expected 100 down to 30–50 years, in extreme cases even down to 10 years [3]. The required remediation actions are often challenging as individual processes linked to MICC still remain uncertain and thus typically generate high costs. It has been estimated in 2002 that the United States need to spend about 390 billion dollars within the following 20 years in order to keep the existing wastewater infrastructure on life support [1]. In 1996, over 520 million dollars were spent solely in Los Angeles County for restorations of corroded sewer systems, which were directly assigned to MICC [4]. Besides economic aspects, H2S and other volatile components such as ammonia (NH3), which both are common byproducts of MICC, are extremely toxic even in low concentrations and can cause significant health related consequences [5,6]. Additionally, H2S features an extremely low odor threshold of 0.0035 ppm, thus often being subject to community appeals due to emerging odor problems [7]. Although, the earliest studies of MICC date back to 1900 [8] the environmental controls, processes, and corrosion rates of MICC are still in debate. The deterioration of concrete that is subjected to MICC is attributed to a chain of complex biotic and abiotic chemical reactions: (i) Sulfatereducing bacteria (SRB), e.g. Desulfovibrio spp. and Desulfomaculum spp. [9,10], develop in the organic-rich wastewater and colonialize the adjacent sewer walls and sediment layers. The SRB colonize within an anaerobic biofilm, wherein organic matter breaks down and sulfate is progressively reduced to gaseous H2S [11]. (ii) Accordingly, H2S degases into the sewer atmosphere and subsequently accumulates in the pore space of the concrete via gaseous diffusion and subsequent absorption in the interstitial solution. Since concrete made with ordinary Portland cement (OPC) is a strongly alkaline media with an average pH value of ~13.5 in the pore solution [12], chemical acid-base reactions such as carbonation and in particular H2S acidification successively cause the pH to drop. Starting from pH 9.5, progressive colonization of the concrete walls with strains of sulfur-oxidizing bacteria (SOB) occurs. SOB use H2S either directly [13] or via different key precursors, e.g. thiosulfate (S2O3) or elemental sulfur (S0) as an electron donor [14]. These reactions finally result in the formation of sulfuric acid (H2SO4), which attacks the concrete and leads to a variety of dissolution processes, besides enhancing some neo-formation reactions of sulfate mineral phases within the cementitious matrix [15,16]. (iii) The dissociation of H2SO4 goes almost to completion and thus strongly reduces the pH for stimulating the dissolution of silicate, hydroxide and carbonate minerals. In addition, by providing SO4 2- the precipitation of sulfate minerals such as gypsum (CaSO4 · 2H2O) and ettringite ((CaO)3 · Al2O3 · (CaSO4)3 · 32H2O) is promoted [17,18]. The formation of gypsum and ettringite is well known to cause intense microstructural damage by high crystallization pressures [19–21]. (iv) As the interstitial solution pH of the initially damaged concrete decreases progressively, different strains of bacteria colonize the pores of the concrete, with Acidithiobacillus thiooxidans being the most aggressive one [10]