# A Study on the Applicability of Bio-Sulfur as a Cement Admixture
## 1 Introduction  
In 2019, a total of 497,238 tons/day of waste was generated in Korea, 89.7% of which was recycled, with the rest incinerated (5.2%) or sent to a landfill (6.1%) [1]. Landfill gas (LFG) is generated when organic substances are decomposed by microorganisms under anaerobic conditions in a landfill. LFG contains CH\({}_{4}\) (40-55%), CO\({}_{2}\) (35-50%), C\({}_{6}\)H\({}_{14}\) (250-3000 ppm), chlorine compounds (30-300 mg/m\({}^{3}\)), and H\({}_{2}\)S (\(\leq\)200 ppm), among other chemicals [2; 3; 4]. As LFG contains methane, it can cause serious global warming when discharged into the atmosphere. If it is captured, it can be used to produce electric power and thermal energy, and, because it can be used as fuel in a gaseous form, there is strong interest in its collection and effective use [5]. The H\({}_{2}\)S contained in LFG is generated by the reduction of sulfate by sulfate-reducing bacteria (SRB) under anaerobic conditions, as the gypsum contained in the cement among food waste and construction waste contains sulfur [5; 6]. The mechanism for generating H\({}_{2}\)S using sulfates in SRB is shown in Figure 1. If the quantity of sulfates in landfills increases, SRB dominates, and most of the electrons generated from the decomposition of organic matter are used by sulfate-reducing bacteria to generate sulfide by means of the sulfate reduction process. As a result, the H\({}_{2}\)S concentration increases and the activity of methane bacteria decreases, resulting in lower methane gas production [7]. Because H\({}_{2}\)S adversely affects health, causes odors, and promotes the corrosion of LFG utilization facilities [7; 8], removing H\({}_{2}\)S is necessary. Physical, chemical, and biological desulfurization technologies are used as desulfurization technologies to remove H\({}_{2}\)S, but biological desulfurization is considered a feasible method in terms of economic and environmental considerations [9; 10]. In particular, the desulfurization technology of Thiopaq, a leading company in the field of biological desulfurization technology, can recover sulfur components (bio-sulfur), while also accomplishing desulfurization, thus giving it an advantage [11; 12]. According to Thiopaq's desulfurization principle (Figure 2), when LFG flowing into the desulfurization facility comes into contact with the cleaning solution (NaOH), H\({}_{2}\)S and CO\({}_{2}\) are converted from the gaseous to the liquid phase. Although the pH of the cleaning solution is decreased, the pH is maintained by continuously adding chemicals to control the alkalinity of the cleaning solution. The gas-to-liquid reaction in the scrubber proceeds as follows [12]:  
\[\mathrm{H_{2}S+NaOH\to NaHS+H_{2}O} \tag{1}\]  
\[\mathrm{CO_{2}+OH^{-}\to HCO^{3-}} \tag{2}\]  
Figure 2: Process diagram of the scheme for H\({}_{2}\)S removal from landfill gas. This system removes the H\({}_{2}\)S contained in Thiopaq’s LFG. When the landfill gas flows in, the refined gas is discharged, and H\({}_{2}\)S reacts with OH\({}^{-}\) and is converted to HS\({}^{-}\) liquid. The converted HS\({}^{-}\) is converted through the oxidation reaction of microorganisms in the bioreactor. The collected landfill gas enters the washer from the bottom of the scrubber, passes through the filled column bed in the bioreactor, and comes into contact with the cleaning liquid. The cleaning liquid then absorbs the H\({}_{2}\)S in the LFG. LFG containing sulfides is converted into sulfur slurry in a biological process. The slurries are transferred to a settling tank through a pump, slowly settle at the bottom of the settling tank, and are finally discharged as sulfur-containing sludge. The sulfur from this process consists mainly of elemental sulfur but is discharged as a sludge containing small amounts of H\({}_{2}\)S\({}_{2}\)O\({}_{3}\), SO\({}_{4}\)\({}^{2-}\), and Na\({}_{2}\)CO\({}_{3}\). The bioreaction for conversion to elemental sulfur in the bioreactor is as follows [12].