A novel way to facilely degrade organic pollutants with the tail-gas derived from PHP (phosphoric acid plus hydrogen peroxide) pretreatment of lignocellulose

Highlights

• A new way to degrade organic pollutants by tail-gas of lignocellulose pretreatment.

• The tail-gas efficiently degraded 7 organic pollutants by 73.8–99.5% within 6–7 min.

• The tail-gas from 6 lignocellulose achieved 68.0–98.3% methylene blue degradation.

• Self-exothermic process drove volatilization of produced peroxy acids in tail-gas.

• Thermally activating peroxy acids into free radicals for the pollutant degradation.

Abstract

The abundantly released tail-gas from lignocellulose pretreatment with phosphoric acid plus hydrogen peroxide (PHP) was found to accelerate the aging of latex/silicone textural accessories of the pretreatment device. Inspired by this, tail-gas was utilized to control organic pollutants. Methylene blue (MB), as a model pollutant, was rapidly decolorized by the tail-gas, and oxidative degradation was substantially proven by full-wavelength scanning with a UV–visible spectrometer. The tail-gas from six typical lignocellulosic feedstocks produced 68.0–98.3% MB degradation, suggesting its wide feedstock compatibility. Three other dyes, including rhodamine B, methyl orange and malachite green, obtained 97.5–99.5% degradation; moreover, tetracycline, resorcinol and hexachlorobenzene achieved 73.8–93.7% degradation, suggesting a superior pollutant compatibility. In a cytotoxicity assessment, the survival rate of the degraded MB was 103.5% compared with 80.4% for the untreated MB, implying almost no cytotoxicity after MB degradation. Mechanism investigations indicated that the self-exothermic reaction in PHP pretreatment drove the self-generated peroxy acids into tail-gas. Moreover, it heated the pollutant solution and thermally activated peroxy acids as free radicals for efficient pollutant degradation. Here, a brand-new technique for degrading organic pollutants with a “Win–Win–Win” concept was purposed for lignocellulose valorization, pollutant control by waste tail-gas, and biofuel production.

Introduction

In the field of lignocellulose biorefining, pretreatment is a crucial step to deconstruct the rigid structure and remove the recalcitrant fractions of lignocellulose, from which the subsequent bioconversion to biofuel/bioproduct (for example, bioethanol) will be made much easier (Rajendran et al., 2018). A pretreatment using phosphoric acid plus hydrogen peroxide (PHP) was invented by our group in 2014; this can be regarded as an oxidative method. By contrast, PHP pretreatment can work efficiently at mild conditions (40–55 °Ϲ) to achieve over 95% hemicellulose removal and 71–96% delignification using various typical lignocellulosic feedstocks. Correspondingly, more than 90% enzymatic hydrolysis for glucose can be achieved (Wang et al., 2014). When the pretreatment and simultaneous saccharification and fermentation were both performed at the optimal levels, approximately 15.5 g bioethanol was yielded from 100 g wheat straw (Qiu et al., 2018). Moreover, the main chemical, phosphoric acid, can be recycled at least 11 times for lignocellulose pretreatment (Yao et al., 2019). More recent results indicated that a multi-product platform for bioethanol, lignin and ultra-high surface area carbon was successfully integrated based on the PHP pretreatment for lignocellulose (Liu et al., 2021). Therefore, the PHP pretreatment technique is promising for efficient lignocellulose refining according to these results. Unexpectedly, a large amount of tail-gas was observed when the lignocellulosic feedstocks were pretreated with PHP; moreover, some accessories of the pretreatment device, such as the latex tailpipe and the silicone sealing ring, were rapidly aged by the derived tail-gas (Supplementary Fig. S1). These observations triggered us to valorize the released tail-gas in a reasonable way to avoid the potential corrosion risk as much as possible and build a more integrated biorefinery platform.

Organic contaminants, especially the refractory organic pollutants, are growing dramatically with the rapid development of modern industry and society. Their persistent accumulation in aquatic environments causes serious damage to human health and ecological security. Efficient treatment of these organic contaminants is strongly required in the real world, and this topic is widely focused on by the scientific community (Ahmed et al., 2021, Show et al., 2021). A widely investigated technique, the advanced oxidation process (AOP), realizes the effective degradation of refractory organic pollutants with non-selectivity via the highly reactive radicals, such as HO·, O₂·^- and SO₄·^- (Chen et al., 2021). Compared with other AOPs, the oxidative gas involved in AOPs, such as ozone, H₂O₂/O₃, UV/O₃ (Gomes et al., 2017) and UV/Cl₂ (Tian et al., 2020), can simplify the operational process, reduce the sludge and promote efficient degradation (Malik et al., 2020). According to a critical comparison of the energy efficiency of various established and emerging AOPs based on electrical energy per order values, the electrical energy per order value of the oxidative gases involved in AOPs was estimated to be < 1 kWh/m³, which was lower than that of other AOPs (Miklos et al., 2018). However, the large amount of energy consumed by the oxidative gas supply is challenging the application (Srivastav et al., 2018). Therefore, it is essential to seek a more cost-efficient method in the area of oxidative gas-based AOPs.

Considering this application challenge of the oxidative gas involved in AOPs, the valorization of the derived tail-gas offers the possibility to achieve a gas-based AOP. This will create a brand-new technology to control organic pollutants based on the lignocellulose pretreatment platform once this cross-border attempt becomes feasible. However, to verify the proposed method, scientific investigations should be performed to smooth some knowledge gaps, mainly including the feasibility of organic pollutant degradation by the derived tail-gas and the technical compatibility with lignocellulosic feedstocks and organic pollutants, as well as the underlying mechanisms.

To achieve these investigation aims, first of all, the tail-gas from PHP pretreatment was introduced into a solution of methylene blue (MB) to check the technical feasibility of organic pollutant degradation by the derived tail-gas. Based on the MB solution, the technical mechanisms of organic pollutant degradation by the derived tail-gas were investigated. Afterwards, in addition to MB, other six pollutants, namely rhodamine B (RhB), methyl orange (MO), malachite green (MG), resorcinol (RC), tetracycline (TC) and hexachlorobenzene (HCB), were also selected to check the technical compatibility to pollutants. Moreover, PHP pretreatment of six typical lignocellulosic biomasses was tested for MB degradation to display the feedstock compatibility of this method. Besides, the analysis of potential byproducts and the cytotoxicity assessment of the degraded MB were performed to evaluate the environmental risks of the effluent after degradation by the tail-gas. To the best of our knowledge, this is the first report to valorize the tail-gas from lignocellulose PHP pretreatment for organic pollutant degradation. Furthermore, a novel “Win–Win–Win” concept of waste control and waste valorization for biofuels/bioproducts will be potentially integrated to form a more sustainable lignocellulose biorefinery process.