Programmable synthesis of exfoliated biochar nanosheets for selective and highly efficient adsorption of thallium

Highlights

• Highly efficient exfoliated biochar containing thin layered nanosheets was synthesized.

• Exfoliated biochar showed higher BET surface area 421.24 m²g⁻¹ than pristine biochar 3.81 m²g^−1.

• A higher Q_max of Tl(I) was observed than previously reported adsorbents.

• Exfoliated biochar removed ∼ 90% of T1(I) in complex river water under real scale study.

Abstract

Tailoring the pathways to synthesize efficient biochars for pollutant adsorption has received extensive attention. Here, we synthesized highly efficient exfoliated biochar having thin layered nanosheets in its structure from agricultural wastes (wheat straw) by an innovative method involving biomass pre-treatment, nitrogen pyrolysis, and a flash heat exfoliation for thallium(I) removal from contaminated waters. Exfoliated biochar with nanosheets in its structure (EBNs) exhibited an open porous structure with a BET surface area 421.24 m² g⁻¹ and pore size 3.98 nm, much higher than the 3.81 m² g⁻¹ and 2.05 nm, respectively, of pristine wheat-straw biochar (PB). Using these materials for the adsorption of T1(I) revealed that the EBNs had a maximum adsorption capacity of 382.38 mg g⁻¹ at pH 7.0, over 9 times higher than the PB. The adsorption kinetics and isotherm data were better fitted by pseudo-second order and Langmuir models. Moreover, EBNs retained its selective adsorption capacity for T1(I) in the presence of competing ions (Ca²⁺, Mg²⁺, K⁺, Cu²⁺, Zn²⁺) and organic materials (humic acid, fulvic acid). Also, high regeneration ability (>93%) of EBNs was noted for five consecutive adsorption–desorption cycles. The efficiency of EBNs was also tested in river water (sampled from Bahe river, Xi’an, West China) spiked with T1(I) where it removed ∼ 90% of T1(I). These findings highlight the potential of EBNs for practical water treatment applications by developing the biochar nanosheets from agricultural wastes and provide insights into a new strategy to develop cost-effective carbon-based nanomaterials for wastewater treatment.

Introduction

Thallium (Tl) is a highly toxic rare element which is frequently encountered in natural settings. Since its discovery in 1861 by William Crookes, it has been widely used in pharmaceutical, aerospace, optical, chemical and electronic industries as well as high-energy physics [1]. However, Tl is considered immensely toxic with the lethal dose of 8–10 mg kg⁻¹ for human adults [2]. The chronic exposure of Tl develops within the concentration range of 0.1–100 µg L⁻¹ [3], [4]. Severe toxicity of Tl can cause various human diseases including the failure of the nervous system, kidney, heart, liver, and congenital malformation [5]. Therefore, T1-based compounds are recognized amongst the major hazardous wastes by World Health Organization [6]. They are also enlisted among the technology-critical elements with substantial risks to environmental processes and human health by European COST Action TD1407 [7]. Similarly, the United States Environmental Protection Agency (USEPA) lists Tl among the priority pollutants and has recommended the maximum permissible limit of 2.0 µg L⁻¹ in drinking water with the intent to further decrease it to 0.5 µg L⁻¹ [8]. In Canada, however, a stringent value of 0.8 µg L⁻¹ was implemented [9] while China even suggested a lower limit of 0.1 µg L⁻¹ [10], [11]. In aquatic environments, Tl, being a redox-sensitive element, exists predominantly in two oxidation states, namely Tl(I) and Tl(III) [2], [10]. However, Tl(I) is much more toxic and mobile pollutant that hardly adsorbs to many natural materials [12]. Its removal from aquatic environments has, therefore, received significant attention.

Adsorption is usually considered an effective treatment to remove water pollutants [13]. A great variety of adsorbents has been reported in the literature with varying costs, characteristics, and treatment efficiency [14], [15]. Biochar has been used as a sustainable solution to remove a variety of pollutants from water. Recently, the biochar obtained from the pyrolysis of agricultural wastes has gained significant interest [16], [17]. Relying on natural or waste materials could be highly rewarding to develop sustainable, cost-effective and eco-friendly adsorbents [16]. The use of agricultural wastes as biomass feedstock for the synthesis of biochar can significantly reduce the treatment cost [16]. Similarly, the development of composite adsorbents based on biochar and other adsorbents can further improve treatment efficiency. For example, MnFe₂O₄–biochar composite produced from banana leaves exhibited substantial adsorption capacity for T1(I) (170.55 mg g⁻¹) [18]. In another study, Tl(I) adsorption capacity of 1123 mg g⁻¹ was attained by Fe₃O₄/biochar composite developed through a series of reactions between biochar (pyrolyzed from watermelon rinds) and iron reagents for in-situ synthesis of magnetite, further coupled with hypochlorite oxidation [12]. Despite remarkable adsorption capacity in the above studies, the majority of these materials entail complex or prolonged synthesis processes frequently involving the addition of chemical agents. In addition, micrometric biochars showed limited ability to adsorb toxic metals from aquatic environment [19]. Therefore, the development of innovative and efficient adsorbents is a promising and continuously evolving field. To our knowledge, this is the first study that reports the development of exfoliated biochar having nanosheets in its structure (EBNs) from wheat straw (agricultural wastes) and its application to remove water pollutants. For this, an innovative method has been used to synthesize EBNs for their subsequent use in the removal of Tl from aqueous solution and river water.

The present study was conducted to develop highly efficient biochar with a tuneable structure of porous carbon-nanosheets from wheat straw followed by its use to remove Tl(I) in an aqueous solution. The effect of competing ions (Ca²⁺, Mg²⁺, K⁺, Cu²⁺, Zn²⁺) and organic materials (humic acid, fulvic acid) was also studied. Final experiments were performed in river water to evaluate the efficiency of EBNs under complex conditions for practical applications. Wheat is a major crop around the globe and its residues could be procured at a low price and at a high scale after harvesting. Furthermore, wheat straw consists of heterogeneous porous microstructures with a high carbon content of > 75% by weight. These characteristics make wheat straw a desirable biomass for the economical production of biochar. The EBNs were developed by biomass pre-treatment with dilute acid to enrich the wheat straw precursor in its crystalline cellulose constituent to unveil the natural nano-architectures. This pre-treatment step was coupled with flash-heat treatment to exfoliate the biochar and expose thin layered carbon-nanosheets. The structure and composition of EBNs were thoroughly characterized. The efficiency of the tailored synthesis pathway was assessed by comparing it with pristine wheat straw biochar fabricated without pre or post-treatment modifications.