High mesoporosity phosphorus-containing biochar fabricated from Camellia oleifera shells: Impressive tetracycline adsorption performance and promotion of pyrophosphate-like surface functional groups (C-O-P bond)

Highlights

• First prepared Camellia oleifera shells into phosphorus-containing biochar.

• PBC attain superior TC adsorption capacity of 451.6 mg/g (C₀ = 50 mg/L).

• Pyrophosphate-like surface functional groups (C-O-P bond) promote TC adsorption.

Abstract

In China, more than 3.5 million tons of Camellia oleifera discarded shells are produced every year. This work first prepared phosphorus-containing biochar (PBC) from C. oleifera shells and was successfully applied to the efficient removal of tetracycline (TC) from solutions. The prepared PBC exhibits superior TC adsorption capacity of 451.5 mg/g, and TC uptake rapidly reached 315.5 mg/g at the first 5 min (C₀ = 50 mg/L). Furthermore, PBC also shows excellent applicability to the broad range pH value (1–9) and superior selective removal in the presence of various high concentration coexisting ions (1 mM). Mechanisms underlying TC adsorption were also put forward, and analysis suggested that pyrophosphate-like surface functional groups (C-O-P bond) played a critical role in this process. Notably, treating pharmaceutical wastewater with PBC can efficiently reduce chemical oxygen demand (COD) and total organic carbon (TOC) concentration below the discharge standard of China (GB21904-2008).

Introduction

Tetracycline (Tetracycline) is an inexpensive, broad-spectrum antimicrobial agent that is diffusely applied to treating bacterial infectious diseases in animals and humans (Wang et al., 2020a). The TC molecule is not entirely biochemical metabolized by animals and humans. Approximately 70–90% of uptakes TC is excreted in the form of original drugs or parent compounds to the environment through the urine, which may cause the proliferation of drug-resistant bacterial and antibiotic resistance genes (Peiris et al., 2017, Wang et al., 2020b). Various methods as biodegradation (Shao et al., 2019), chemical oxidation (Pi et al., 2019), electrochemical (Yang et al., 2020b), membrane (Slipko et al., 2019), photodegradation (Oseghe & Ofomaja, 2018), and adsorption process (Xiang et al., 2020) have been proposed for remove TC from aqueous solution. Among them, adsorption is an extremely competitive technique for TC removal because of its convenient operation, high efficiency, and eco-friendliness (Yu et al., 2018). It is a challenge to exploring adsorbents that integrate the strengths of high capacity, superior affinity, and reusability is of great benefit but challenging yet (Kong et al., 2017). Recently, biochar (BC) as low-cost adsorbents has attracted broad research interest as a result of its conducive characteristic including sustainability, diverse functional groups, and high stability (Wang & Wang, 2019).

C. oleifera is a unique oil woody tree species mainly planted in the south of China, and its seeds reach over an annual yield of 6.5 million tons (Tu et al., 2018). The seeds must be hulled before oil extraction, and the shell accounts for 60% weight of the whole seed (Zhang et al., 2018). Thus, more than 3.5 million tons of C. oleifera shells are yielded annually, resulting in a large amount of the husks are discarded and incinerated. Therefore, how to utilize the C. oleifera shells has become an important ecological and economic issue. The C. oleifera shell consists mainly of cellulose, hemicellulose, and lignin (Yuan et al., 2016), which is an excellent precursor for biochar preparation. These waste C. oleifera shells can be a cheap precursor of biochar adsorbents, which can simultaneously achieve environmental and economic benefits. For all we know, there is no report on the preparation of C. oleifera husks into biochar and its application for TC removal.

However, we found that the TC adsorption performance of C. oleifera-derived biochar without activation was quite limited (adsorption capacity below 25 mg TC/g BC), owning to insufficient surface functional groups and less-developed pore structure. Various activation methods have been proposed to enhance adsorption ability of biochar, such as steam (Kwak et al., 2019), ZnCl₂ (Xia et al., 2016), KOH (Cheng et al., 2020a), while hardly combine the advantage of economy, environmental protection, and well adsorption performance. By contrast, phosphoric acid (H₃PO₄) is an excellent activation with several strengths such as low costs, high yield rate, and minor pollution. H₃PO₄ activation can also effectively improve the specific surface area and surface chemical properties, which are suitable for use in large molecule adsorption (Liou, 2010, Liu et al., 2012a). Previous researches about H₃PO₄ activation hardly improves the adsorption capacity of biochar dramatically and seldom research the relationship between adsorption ability and structure of biochar (Ahmed et al., 2017, Chen et al., 2018). For instance, Yang et al. (Yang et al., 2020a) have prepared H₃PO₄ activated corn straw biochar to remove TC, and the adsorption capacity was 83.2 mg/g.

In this work, biochar and phosphorus-containing biochar were prepared from C. oleifera shells and applied to remove TC from aqueous solution. We achieved these goals: (1) investigated physicochemical properties of prepared sorbent by systematical characterization and determining the optimal pyrolysis temperature for PBC; (2) evaluating and modeling the TC adsorption ability of the optimal PBC; (3) exploring the relationship between physicochemical characteristics and remarkable TC adsorption capacity of PBC.