Optimization of thermally activated persulfate pretreatment of corn straw and its effect on anaerobic digestion performance and stability

Highlights

• The optimum condition was 50 °C, 48 h, 0.5% Na₂S₂O₈, pH 5, S: L = 1:10.

• The lignin of straw was degraded by 77.18% after thermal activated SPS pretreatment.

• The yields of RS and VFA were 2.38 and 0.67 times higher than control group.

• Cumulative methane production pretreated by SPS reached 271.6 mL·g⁻¹ VS

Abstract

The effect of thermally activated persulfate on anaerobic digestion is incompletely understood. An orthogonal test was used to optimize the pretreatment process of corn straw using thermally activated persulfate, a lab-scale batch anaerobic digester was used for exploring the effect of pretreatment on anaerobic digestion. The optimal pretreatment conditions were 50 °C, 16 h, 0.5% Na₂S₂O₈, pH = 5, and solid–liquid mass ratio (S:L) = 1:10, which was the most crucial factor. Lignin pretreatment with thermally activated sodium persulfate (SPS) considerably decreased. The yield of reducing sugar (RS) and VFAs were observably higher than the untreated group. This trend occurred because the thermal activation of persulfate primarily destroys the benzene ring in lignin by generating and releasing active sulfate radical (SO₄^·−); moreover, hydroxyl radical (·OH) and SO₄⁻ work together during thermal activation. Furthermore, the daily biogas production of the groups pretreated for 16 and 48 h without elution were 22.78% and 26.80% higher than that of the group untreated, respectively. The degradation rates of total and volatile solids increased to 60% and 70% after the pretreatment. Therefore, thermally activated SPS could improve the performance and stability of anaerobic digestion, and the impact of dissolved organic matter loss exceeded that of residual Na⁺ and SO₄^{2 −}.

Introduction

Many types of straws are generated in high yields from agricultural activities in most areas of the world. Straw is considered a valuable agricultural resource. As per preliminary statistics, China generated 900 million tons of straw in 2017. On an average, 580 million tons of processed byproducts are annually produced, of which <40% are utilized. Therefore, it is important to promote the comprehensive utilization of agricultural products and byproducts. The straw-burning ban does not address the fundamental problem, and these resources are still being wasted [1].

Straw treatment methods primarily include combustion, pyrolysis, gasification, and anaerobic digestion (AD), which is an important method to achieve the resource utilization of straw [2]. The direct use of straw for anaerobic digestion to generate biogas is inefficient and leads to a slow start, extended time, and low conversion efficiency because of the high crystallinity and polymerization degree of the lignocellulose in straw. Straw has a high lignin content with structural complexity and stability, which is attributed to its irregular structure with methoxyl-substituted phenyl and phenolic subunits; thus, straw requires pretreatment. The lignin model is cleaved to aromatic aldehydes by selective C–C cleavage in the presence of persulfate [3]. Corn straw should be pretreated before anaerobic digestion to destroy the crystalline structure and reduce the cellulose polymerization degree to enhance its digestibility [4], improve the cellulose hydrolysis rate, and increase methane production.

Oxidation pretreatment methods using different oxidants, such as hydrogen peroxide, ozone, and oxygen, have been reported [[5], [6], [7], [8], [9]]. In H₂O₂ pretreatment, 50% of lignin can be removed, and cellulose hydrolysis rate reaches 95% [10]. Ozone pretreatment can considerably degrade lignin without affecting carbohydrates. Song et al. [11] reported that combining H₂O₂ presoaking with steam explosion degumming can effectively remove hemicellulose and lignin and increase the crystallinity while retaining cellulose.

Persulfates are stable at room temperature and can be activated by multiple methods such as heating, transition metals, and ultraviolet (UV) rays. The active sulfate radical (SO₄^·−) acts as the oxidant in activated persulfates [12]. Persulfates form two highly reactive sulfate groups by absorbing heat, which breaks the peroxide bond. SO₄^·− has a high oxidation potential (2.60 V), is nonselective [13], and has a longer half-life than hydroxyl radical (·OH). Moreover, SO₄^·− tends to react through electron transfer mechanisms, while OH reacts through addition, hydrogen atom abstraction, and electron transfer mechanisms. SO₄^·− is selective [14] as the reaction rate of the electron-donating group with the sulfate group is faster than that of the electron-absorbing group because sulfate is electrophilic [13]. Compared to OH, which is extensively used in traditional advanced oxidation technologies, SO₄^·− can efficiently work over a wider pH range (2–8), which indicates that the persulfate anion is more stable than the peroxide-based hydroxyl radicals, and hence it is increasingly favored [15]. Both direct (through persulfates) and indirect (through free radicals) reactions oxidize and decompose organic components. Moreover, the stability of persulfate is of considerable interest because of its ability to treat large contaminated areas [16]. However, the oxidation of persulfate may reduce the pH and cause sulfate overload. Wang et al. [17] examined the oxidation mechanism of persulfate by studying a thermally activated persulfate oxidation anaerobic digester in which high temperature promotes the direct fission of persulfate. The resulting SO₄^·− triggers OH formation. These highly reactive radicals react with organic components, which leads to partial mineralization or structural damage. Ahmed et al. pretreated straw with thermally activated potassium persulfate and reported that the untreated biomass exhibited a characteristic smooth, continuous, and flat surface [18]. The surface of the pretreated straw was destroyed, thus exposing the silica in the original material's structure. This oxidation method can be used as an economic alternative to the Fenton reaction. Moreover, thermally activated persulfate pretreatment of wheat straw in which ·OH, SO₄^·−, and O₂^·− are produced considerably increased its sugar yield. The maximum sugar yield is 49.4% under heat-activated persulfate conditions, while the glucan yield of untreated material is only 3.7% [19]. By comparing different persulfates with different temperature and time conditions, lignin has different degradation effects. When the reaction was conducted in 2.0 mL of MeCN/H₂O (3:1, v/v) with 0.1 mmol of lignin β-O-4 model and 0.2 mmol of sodium persulfate at 100 °C for 12 h [3], the best result was obtained with a 68% isolated yield. Moreover, the effect of persulfate pretreatment of corn straw on its anaerobic digestion biogas production and stability has hardly been studied.

In this study, the effect of corn straw pretreatment using thermally activated SPS was investigated, and then the pretreatment conditions of thermally activated SPS were optimized. The mechanism of SPS pretreatment using thermal activation was explored via changes in structure and physicochemical properties of corn straw as well as the pretreatment liquid before and after pretreatment. Moreover, the effects of thermoactivated SPS on anaerobic digestion were evaluated by recording the daily biogas production and its methane content. Finally, the performance and stability of anaerobic digestion were evaluated.