Roles of the mineral constituents in sludge-derived biochar in persulfate activation for phenol degradation

Highlights

• Demineralized biochar possessed the better catalytic ability than the original one.

• Different minerals in biochar exhibited varying effects on phenol degradation.

• Fe minerals had positive effect while negative one was for Ca minerals in biochar.

• Mg and K had no effect on the phenol degradation.

Abstract

Biochar as an environmental-friendly and low-cost catalyst has gained increasing attention in the catalytic degradation of organic pollutants. However, the roles of endogenous mineral constituents in biochar in the catalytic degradation are still unclear. In this study, the mineral-rich biochar produced from sewage sludge at 400 °C (SS400) and 700 °C (SS700) and their corresponding demineralized biochar (DSS400 and DSS700) were used to be the persulfate (PS) activator for phenol degradation. Results showed that the mineral-rich biochar + PS system had negligible phenol degradation (≤12.6 %), whereas distinct degradation of phenol were obtained in the demineralized biochar + PS system where DSS400 + PS and DSS700 + PS exhibited 36.3 % and 57.8 % degradation, respectively. Different minerals in mineral-rich biochar exhibited varying functions on phenol degradation. Mg and K in biochar had less effect on the phenol degradation, while Fe-containing minerals favored the phenol degradation. However, Ca-containing minerals more greatly reduced the formation of hydroxyl radical, resulting in more inhibited degradation of phenol. Thus, the overall degradation of phenol was reduced by the mineral-rich biochar. The findings indicated that the inherent minerals in biochar were not favorable for the phenol degradation, which guides us the application of biochar containing different minerals in the remediation of organic pollutants.

Introduction

Advanced oxidation processes, such as hydrogen peroxide (H₂O₂)- and persulfate (PS)-based oxidation are the efficient methods for the degradation of pollutants. Compared to the traditional H₂O₂-based Fenton oxidization, PS-based oxidization was more extensively used for the wastewater treatment, due to the higher redox potential of energetic sulfate radical (2.5–3.1 V) and wider pH range application (2–10) (Duan et al., 2015; Hu and Long, 2016). Furthermore, the low cost and stable chemical nature give PS additional advantages over other oxidants, making it more favorable for the pollutants degradation (Wacławek et al., 2017).

Carbonaceous materials, such as graphene oxide and nanotube, are commonly used as catalyst assistance to the PS for enhancing pollutants degradation (Duan et al., 2018a; Chen et al., 2018). However, the relatively high cost of graphene oxide and nanotube restricted their practical application in wastewater treatment. Biochar, which was produced from the waste biomass via pyrolysis process, has been regarded as a possible catalytic material due to its easy access, low cost, sustainable development requirement, and excellent catalytic performance (Fang et al., 2014). It has been reported that biochar derived from plant-based waste (e.g. pine needle, wheat straw, maize straw) which contains low mineral content (e.g. 0.108 %-0.141 % Fe and 0.008 %-0.015 % Mn) possessed the excellent catalytic performance for the degradation of 2-chlorobiphenyl and 1,4-dioxane (Fang et al., 2014; Zhao et al., 2013a; Ouyang et al., 2019). The great catalytic degradation of pollutants was attributed to the roles of carbon or carbon-related fraction in biochar, including the persistent free radicals or conjugated π-electron system or defect structures (Fang et al., 2014; Ouyang et al., 2019; Liang et al., 2019).

Several studies showed that the sludge-derived biochar rich with Fe minerals also had more excellent catalytic performance for the degradation of pollutant, implying the positive role of Fe (Wang and Wang, 2019; Huang et al., 2018; Zhu et al., 2019a; Yin et al., 2019). Addition of Fe minerals including ferrous (Liu et al., 2012), zero-valent iron (Kim et al., 2018), and iron oxides (Wu et al., 2017) enhanced the degradation of pollutant by PS, compared to the oxidation by PS alone, which further demonstrated the positive roles of Fe in the catalytic degradation process. The similar positive role of Mn in the catalytic oxidation process was also studied (Anipsitakis and Dionysiou, 2004; Zhu et al., 2019b; Shah et al., 2019). However, the negative role of Ca-containing minerals was reported by Teel et al. (2011) since calcite, main constituent of Ca-containing minerals inhibited the hydroxyl radical generation and correspondingly reduced nitrobenzene degradation in the calcite-persulfate system. In addition, Ca-containing minerals may block the pores in biochar, accordingly lowered the specific surface area, and then reduced the catalytic performance (You et al., 2017). If those minerals co-existed in the biochar, what about the comprehensive catalytic abilities of this biochar should be observed?

In this study, the mineral-rich biochar was produced from sewage sludge. In order to explore the role of mineral constituents, the mineral-rich biochar was demineralized through the acid-washed treatment. Both mineral-rich and demineralized biochar were compared for the pollutant catalytic performance and characterization analysis. Phenol was selected as representative of pollutant for its comparable degradation performance by carbon-based materials (Duan et al., 2015). The objectives of this study were (1) to reveal the catalytic performance of mineral-rich and demineralized biochar in the presence of PS for the degradation of phenol and corresponding degradation products; (2) to explore the possible reactive oxygen species between mineral-rich biochar + PS and demineralized biochar + PS system; (3) to evaluate the different roles of endogenous minerals in biochar in the persulfate catalytic degradation of phenol.