Adding organics to enrich mixotrophic sulfur-oxidizing bacteria under extremely acidic conditions—A novel strategy to enhance hydrogen sulfide removal

Highlights

• Adding organics under extreme acidity is a new strategy for increasing SOB biomass.

• Mixotrophic SOB (>99 %) was enriched while autotrophic SOB was eliminated.

• Enrichment of mixotrophic SOB enhanced the EC-H₂S by 272 % to 464.3 g/m³/h.

• The desulfurization performance of BTF was optimal with biofilm mass at 22 g/L-BTF.

• The major metabolism pathways of sulfur under extreme acidity were first revealed.

Abstract

Biotreatment of high load hydrogen sulfide (H₂S) can lead to rapid acidification of a bioreactor, which greatly challenges the application of bio-desulfurization technology. In this study, the bio-desulfurization performance was improved by enriching acidophilic mixotrophic sulfur-oxidizing bacteria (SOB) by adding organics under extremely acidic conditions (pH < 1.0). A biotrickling filter (BTF) for the removal of H₂S was established and operated under pH < 1.0 for 420 days. In the autotrophic period, the maximum H₂S elimination capacity (EC_max-H₂S) was 135.8 g/m³/h with biofilm mass remaining within 11.1 g/L-BTF. The autotrophic SOB bacterium Acidithiobacillus was dominant (62.1 %). When glucose was added to the BTF system, EC_max-H₂S increased by 272 % to 464.3 g/m³/h as biofilm mass increased to 22.3 g/L-BTF. The acidophilic mixotrophic SOB bacteria Mycobacterium (78.4 %) and Alicyclobacillus (20.7 %) were enriched while Acidithiobacillus was gradually eliminated (<0.1 %). Furthermore, the major sulfur metabolism pathways were identified to explore the desulfurization mechanism under extremely acidic conditions. To maintain optimal desulfurization performance and avoid biofilm overgrowth in the BTF system, biofilm mass should be maintained within 20–22 g/L-BTF. This can be achieved by adding 1.0 g/L-BTF glucose every 20 days under a load rate of H₂S in 50–90 g/m³/h and a trickling liquid velocity of 1.8 m/h. Extremely acidic conditions eliminated non-aciduric microorganisms so that the addition of organics can increase the abundance of acidophilic mixotrophic SOB (>99 %), thus offering a novel strategy for enhancing H₂S removal.

Introduction

Hydrogen sulfide (H₂S) is an extremely hazardous gas with a rotten egg stench. It is a byproduct of many industrial processes, such as organic anaerobic digestion, petroleum refining, rendering, wastewater treatment, livestock farming, and food processing (Jia et al., 2022b; Qiu and Deshusses, 2017; Vikrant et al., 2018). H₂S can damage various systems in the body, especially the nervous system, as well as corrode buildings and equipment. The H₂S concentration in anaerobic digestion for producing biogas (mainly CH4, CO2, and H2S) can range from 1500 to 30,000 mg/m³ (Montebello et al., 2014; Vikrant et al., 2018). The most common H₂S removal method is chemical washing (Pokorna et al., 2015), but this method is expensive. Burning H₂S leads to the formation of sulfur dioxide, which causes acid rain. In contrast, biological desulfurization offers the advantages of being economic, efficient, and free of secondary pollution (Rybarczyk et al., 2019). However, the treatment of high load H₂S, i.e., with a loading rate (LR) > 100 g-H₂S/m³/h, is very challenging for biotrickling filter (BTF) since it has a constantly circulating liquid phase. Biotreatment of high load H₂S results in rapid acidification of the bio-system because of the formation of sulfuric acid (de Rink et al., 2019). Extremely acidic conditions (pH < 1.0) eliminate non-aciduric sulfur-oxidizing bacteria (SOB) and are adverse to the growth of SOB biomass (Jia et al., 2022a). Such conditions make it difficult to further improve the desulfurization performance because of the lack of a sufficient amount of active SOB (Chen et al., 2021). Without enough SOB for the rapid oxidization of dissolved H₂S, the accumulation of dissolved H₂S (a highly toxic substrate) may inhibit or even kill SOB, thus causing the desulfurization system to collapse (Yuan et al., 2020). Continuous maintenance of the pH within a moderate range will significantly increase the operations complexity of high-load H₂S treatment (Kim and Deshusses, 2005). In addition, moderate pH levels lead to an increase in microbial diversity (Montebello et al., 2013), which favors the growth of certain microorganisms not related to desulfurization. The current lack of methods to improve the H₂S removal performance under extremely acidic conditions represents a critical constraint for the application of biotechnology for biogas desulfurization.

Commonly reported trophicity types of SOB are autotroph and heterotroph (Table S1). Heterotrophic SOB generally have a faster growth rate compared with autotrophic SOB, while most of they can only grow under neutral pH conditions (Haaijer et al., 2008; Kuddus et al., 2013; Nakayinga et al., 2021). Addition of organic carbon under neutral conditions with the goal to increase the biomass of heterotrophic SOB will inevitably lead to overgrowth by other heterotrophic microorganisms, thus resulting in a decrease in the H₂S elimination capacity (EC) (Gao et al., 2011; Jin et al., 2007; Khanongnuch et al., 2019; Rene et al., 2009; Sologar et al., 2003). Mixotrophic SOB with a more flexible metabolic capacity can grow autotrophically on inorganic sulfides and can also grow heterotrophically on organic carbohydrate. Certain species of mixotrophic SOB can grow under extremely acidic conditions. For example, Alicyclobacillus disulfidooxidans can grow at pH levels ranging from 0.5 to 6.0 (Dufresne et al., 1996; Karavaiko et al., 2005). Moreover, the elution effect of extremely acidic conditions on non-aciduric microorganisms makes survival difficult for heterotrophic microorganisms (Montebello et al., 2013). Therefore, excessive proliferation of heterotrophic bacteria can be avoided under extremely acidic conditions even under the addition of organics.

Many studies have attempted to enhance the desulfurization performance by improving operating conditions (i.e., oxygen transfer, constant pH adjustments, or biofilm mass control) or inoculating purified SOB (Fernandez et al., 2014; Nguyen et al., 2016; Nisola et al., 2010; Rodriguez et al., 2014; Ryu et al., 2009; Tóth et al., 2015; Yang et al., 2010). Constantly maintaining optimal conditions requires more complex operations and tends to enable the growth of microorganisms not related to desulfurization. Inoculation with purified SOB is costly and not conducive to large-scale industrial applications. Several studies have investigated the performance of H₂S removal under extreme acidic conditions, with low EC-H₂S obtained (Ben Jaber et al., 2016; Jia et al., 2022a). Few studies have focused on enhancing the removal of H₂S under extremely acidic conditions by adding organics to increase the biomass of mixotrophic SOB. Consequently, knowledge on the performance, microbial communities, and sulfur metabolism pathways of bio-desulfurization systems to which organics are added under extremely acidic conditions is insufficient.

The present study explored the strategy of increasing SOB biomass by adding an organic carbon source under extremely acidic conditions for advanced H₂S removal. A biotrickling filter (BTF) under pH < 1.0 was established and operated under autotrophic conditions for 195 days and under mixotrophic conditions (i.e., adding an organic carbon source) for 277 days. The microbial community structure, biofilm mass, and maximum H₂S elimination capacity (EC_max-H₂S) were assessed in each period. The major metabolic pathways of sulfur in the desulfurization process were identified. Suitable external organic carbon sources were evaluated by exploring the organics degradation rate and SOB activity. The dosing strategy of organics and sulfur balance were analyzed to maintain biofilm mass balance and optimal desulfurization performance.