Adsorption and regeneration on iron-activated biochar for removal of microcystin-LR

Highlights

• FeCl₃ activated biochar (FA-BC) was prepared and used for MC-LR adsorption.

• FeCl₃ activation increased the BC surface area and MC-LR adsorption capacity.

• Adsorption mechanisms included π⁺-π, hydrogen bond, and hydrophobic interactions.

• FA-BC was effectively regenerated by thermal oxidation and persulfate oxidation.

• FA-BC can effectively adsorb MC-LR in real lake water.

Abstract

Novel iron activated biochars (FA-BCs) were prepared via simultaneous pyrolysis and activation of FeCl₃-pretreated bermudagrass (BG) for removing microcystin-LR (MC-LR) in aqueous solution. Compared to the raw BC (without activation), the surface area and adsorption capacity of FA-BC at iron impregnation ratio of 2 (2 g FeCl₃/g BG) were enhanced from 86 m²/g and 0.76 mg/g to 835 m²/g and 9.00 mg/g. Moreover, FA-BC possessed various iron oxides at its surface which provided the catalytic capacity for regeneration of MC-LR spent FA-BC and magnetic separation after the MC-LR adsorption. Possible mechanisms for the MC-LR adsorption onto FA-BC would include electrostatic attraction, π⁺-π, hydrogen bond, and hydrophobic interactions. The detailed adsorption studies indicated mainly chemisorption and intra-particle diffusion limitation would participate in the adsorption process. The thermal regeneration at 300 °C kept high regeneration efficiency (99–100%) for the MC-LR spent FA-BC during four cycles of adsorption-regeneration. In addition, the high regeneration efficiency (close to 100%) was also achieved by persulfate oxidation-driven regeneration. FA-BC also exhibited high adsorption capacity for the MC-LR from the real lake water to meet the MC-LR concentration below 1 μg/L as a safe guideline suggested by WHO.

Introduction

Microcystin-LR (MC-LR), as a major cyanotoxin, is frequently detected from various water bodies, which can create great risk to various ecosystems and impair human health while causing acute liver failure, cancer, and internal hemorrhage owing to its high toxicity (Lee and Walker, 2011; Liu et al., 2018; Zhang et al., 2011a). Given the significant environmental impacts of MC-LR, the World Health Organization (WHO) has stipulated the safe concentration of MC-LR in drinking water to be below 1 μg/L (Park et al., 2018). In addition, MC-LR exhibits high chemical stability, large molecule size, high solubility in water, and complex structure (He et al., 2020). Therefore, traditional wastewater treatment technologies cannot effectively eliminate MC-LR from water (He et al., 2020; Pavagadhi et al., 2013).

To date, many methods have been utilized to effectively treat water containing MC-LR, including ozonation, TiO₂ photocatalysis, photo-Fenton oxidation, zero-valent iron, and UV/H₂O₂ (Bandala et al., 2004; Eke et al., 2018; He et al., 2012; Pinho et al., 2015; Wang et al., 2016). However, these methods usually require substantial amount of chemicals and high capital costs while generating toxic byproducts. Adsorption, among various methods, has been considered as an effective, reliable, and simple technique without generation of toxic intermediates/byproducts for removal of broad ranges of contaminants in water (Jang et al., 2018a). So far, various adsorbents have been used to adsorb MC-LR from water, including graphene oxide, carbon nanotube, activated carbon, iron oxides, Fe₃O₄/chitosan, and kaolinite (He et al., 2020; Huang et al., 2007; Lee and Walker, 2011; Liu et al., 2019; Pavagadhi et al., 2013; Yan et al., 2006). Nonetheless, high costs associated with preparation, regeneration, and potential environmental risks after disposal still limit their practical applications for MC-LR adsorption. Thus, it is essential for developing an inexpensive and eco-friendly adsorbent for elimination of MC-LR.

Biochar (BC), made from pyrolysis of various wastes under oxygen-limited conditions, has been a promising adsorbent to eliminate various contaminants due to its cheap feedstock and unique properties including high carbon content and porosity with rich functional groups (Ahmad et al., 2014; Jang and Kan, 2019; Jang et al., 2018a; Taheran et al., 2016). Nonetheless, the previous studies for MC-LR adsorption onto BCs were quite limited (Li et al., 2014, 2018; Liu et al., 2018). Li et al. (2014) reported that the wood chips-derived BCs could effectively adsorb MC-LR by electrostatic interaction and hydrogen bond. Li et al. (2018) found the chicken manure-derived BCs showed higher adsorption of MC-LR than the sawdust-derived and maize straw-derived BCs owing to their higher ash content, mesopore volume, and surface functionality. Liu et al. (2018) applied the giant reed-derived BCs prepared at various pyrolysis temperatures for MC-LR adsorption. They found that higher pyrolysis temperature improved MC-LR adsorption capacity due to the increase of mesopores of BCs. In addition, for the BCs prepared at high temperature (>500 °C), the mineral components played the key role in MC-LR adsorption. However, these works centered around the MC-LR adsorption onto the pristine BCs with the limited surface areas and porous structure. Additionally, the MC-LR adsorption capacities of the pristine BCs ranged below 4.2 mg/g, except for the giant reed BC prepared at 500–600 °C (41–42 mg/g), respectively (Table S1).

Various studies have investigated the activation of BCs using activators including NaOH, ZnCl₂, H₃PO₄, and FeCl₃ for improving surface area and porosity to increase adsorption capacity for target contaminants (Braghiroli et al., 2018; Jang et al., 2018a; Mandal et al., 2017; Pezoti Junior et al., 2014; Yang et al., 2016a; Zeng and Kan, 2020). Nevertheless, to our best knowledge, any applications of activated BCs for removal of MC-LR have not been reported. Among the activation of BC with various activators, FeCl₃ activation can enable BCs to possess great surface area, well-developed pore structure, and iron oxides at the surface of BC for enhancing adsorption and catalytic oxidation of contaminants with excellent magnetic separation after adsorption and oxidation (Yang et al., 2016a; Zeng et al., 2021). More specifically, during the FeCl₃ activation, Fe₂O₃ can be generated due to the FeCl₃ decomposition/dehydrochlorination at low temperatures (<400 °C): FeCl₃ → FeOCl·H₂O → FeOOH → Fe₂O₃ (Rufford et al., 2011; Zeng et al., 2021). In addition, amorphous carbon can react with Fe₂O₃ to produce Fe₃O₄, Fe, CO, and CO₂ at higher temperatures (Zhu et al., 2016). Therefore, carbon oxidation and evolution of CO and CO₂ resulting from FeCl₃ activation can lead to the pore development, resulting in the increase of porosity and surface area. Thus, it is interesting to evaluate the performance of iron (FeCl₃)-activated BC to adsorb MC-LR. Besides, there have been no reports for any regeneration of MC-LR spent BCs and application of BCs for treating real lake water containing MC-LR, which are of prime importance for assessing practical applications of this process.

In this study, novel iron (FeCl₃) activated biochars (FA-BCs) were prepared via one-step pyrolysis and activation of FeCl₃-pretreated bermudagrass (BG). The preparation procedures for FA-BCs used for this study were the same as those reported by Zeng et al. (2021) except no acid washing after the pyrolysis/activation for FA-BCs so that FA-BC kept high contents of iron oxides for excellent catalytic and magnetic separation capacities during the adsorption and regeneration (please see the characterization data of FA-BCs from Supplementary Materials). FA-BC was fully characterized and evaluated for MC-LR adsorption via the detailed adsorption experiments. Based on the studies of pH effect, adsorption kinetics, isotherm, and thermodynamics, possible mechanisms associated with the MC-LR adsorption onto FA-BC were proposed. In addition, various regeneration methods were applied to reuse the MC-LR spent FA-BC while the FA-BC was also applied to the treatment of real lake water containing MC-LR at ppb levels.