Kinetics and mechanisms of hydrogen sulfide adsorption by biochars

Abstract

Three different biochars as cost-effective substitutes for activated carbon (AC) were tested for their hydrogen sulfide (H₂S) adsorption ability. The biochars were produced from camphor (SC), bamboo (SB), and rice hull (SR) at 400 °C by oxygen-limited pyrolysis. The surface area (SA), pH, and Fourier transform infrared spectras of the biochars and AC were compared. The maximum removal rates and the saturation constants were obtained using the Michaelis–Menten-type equation. The three biochars were found to be alkaline, and the SAs of the biochars were much smaller than that of the AC. The H₂S breakthrough capacity was related to the local pH within the pore system of the biochar. The order observed in terms of both biochar and AC adsorption capacity was SR > SB > SC > AC. SR efficiently removed H₂S within the inlet concentration range of 10–50 μL/L. Biochars derived from agricultural/forestry wastes are a promising H₂S adsorbent with distinctive properties.

Introduction

Hydrogen sulfide (H₂S) is one of the most common compounds that can be found in petrochemical plants, coal gasification plants, wastewater treatment plants, man-made fiber paper, and other production processes (Latos et al., 2011, Lebrero et al., 2011). This compound is extremely toxic to the central nervous system even at low doses and corrosive to concrete and steel (Burgess et al., 2001, Lee et al., 2006). H₂S is a major air pollutant and also a cause of rain acidification.

Numerous studies on H₂S adsorption using activated carbon (AC) have been conducted because of the increase in deodorization problems (Bagreev and Bandosz, 2000, Bashkova et al., 2009). However, the manufacture of AC requires high temperature, high pressure, and an activation process (Boehm, 1994). Traditionally, only ACs impregnated with caustics were considered suitable materials. Although caustic-impregnated and catalytic carbons have been proven to work efficiently as H₂S removers, certain disadvantages with the application of such carbons have been observed, including (i) self-ignition at low temperature, (ii) low capacity for physical adsorption attributable to the filling of the pore system with the impregnate, (iii) special precautions are required for use with alkalis, and (iv) difficulties in regeneration after washing with water. All aforementioned factors directed the attention of numerous studies toward unmodified, as-received ACs.

As a precursor of AC, biochars have received considerable attention in the past decades (Azargohar and Dalai, 2006, Hayes, 2006, Renner, 2007, Moussavi and Khosravi, 2012). Biochar is the carbon-rich product of the thermal decomposition of organic material under a limited supply of O₂ and at relatively low temperatures (<700 °C) (Hale et al., 2011). Biochar has been known to act as a super-sorbent for organic contaminants in soil/sediment (Pandey et al., 1997, Sattar et al., 1991, Lou et al., 2012, Inyang et al., 2012). Biochar and AC differ primarily in their preparation method, source material, and the resulting physiochemical properties of the products. In contrast to AC, biochar use could be a cheaper remediation technology as the waste source materials would essentially be free, and the production of biochar at lower temperatures is more energy-efficient and less cost-intensive (Kumar et al., 2006) than AC production. A previous study reported on the potential of biochar derived from camphor to adsorb H₂S at various temperatures (100–500 °C) and demonstrated that the different sizes of biochars and the different pyrolysis temperatures for the camphor particle markedly affect H₂S adsorption. The biochar with particle size ranging from 0.3 to 0.4 mm possesses a maximum sorption capacity at a pyrolysis temperature of 400 °C (Shang et al., 2012). Further studies must be conducted to understand better the mechanisms of biochar H₂S adsorption because biochar characteristics depend not only on the pyrolysis temperature but also on biochar feedstock.

This study aimed to determine the efficiency of H₂S adsorption by three different biochars derived from camphor, bamboo, and rice hull at a pyrolysis temperature of 400 °C. A comparative study with AC was also conducted. Removal kinetics were used to evaluate the H₂S removal rate of biochars.