Biochars intended for water filtration: A comparative study with activated carbons of their physicochemical properties and removal efficiency towards neutral and anionic organic pollutants

Highlights

• Seven biochars (BCs) and three activated carbons (ACs) are characterized in depth.

• Physicochemical and adsorption parameters of BCs and ACs were critically compared.

• Ash, and metals and PAHs release of BCs are below the UNI EN 12915-1 Standard.

• Multivariate chemometrics analyses are suitable for selecting the best BCs.

• Hundreds ng/g diiodoacetic acid and low μg/g benzene/dichlorobenzene are removed.

Abstract

Seven biochars (BCs) obtained from pyrolysis or gasification of different vegetal feedstocks were thoroughly characterized in comparison with three commercial activated carbons (ACs) routinely used in drinking water treatment plants. BCs and ACs characterization included the determinations of ash, iodine and methylene blue adsorption indexes, and the release of metals and polycyclic aromatic hydrocarbons, which were performed according to international standards applied for adsorption media to be used in drinking waters. Total specific surface area, micropore and mesopore specific surface area, pH of the point of zero charge, and the release of polychlorinated biphenyls were also determined in all chars. Principal component analysis and cluster analysis were performed in order to summarize the complex set of information deriving from the aforementioned characterizations, highlighting the BC most similar (BC6 from high temperature gasification of woody biomass) and most different (BC7 from low-temperature pyrolysis of corn cob) from ACs. These BCs were studied for their adsorption in ultrapure water towards diiodoacetic acid (an emergent disinfection by-product), benzene, and 1.2-dichlorobenzene, in comparison with ACs, and results obtained were fitted by linearized Freundlich equation. Overall, BC6 showed higher sorption performances compared to BC7, even though both BCs were less performing sorbents than ACs. However, the sorption properties of BCs were maintained also in real water samples collected from drinking water treatment plants.

Introduction

Pollution by anthropic activities still affects the quality of water resources. According to the European Water Framework Directive (2000/60/EU), waters must achieve good ecological and chemical status, to protect human health, water supply, natural ecosystems and biodiversity (European Commission, 2010).

The accomplishment of the environmental objectives set by the European policies can be achieved through proper integrated water management policies and technological approaches, which must be sustainable from both environmental and economic points of view. Within this framework, water treatments play significant and crucial roles in the supply of safe waters intended for human consumption.

Adsorption is an effective and economically feasible approach routinely integrated in drinking water production (Ali and Gupta, 2006) for the removal of micropollutants occurring per se in raw waters and/or as a result of disinfection processes, thereby eliminating the toxicity induced by these molecules in treated water (Han and Zhang, 2018; Han et al., 2021). Within this context, adsorption is generally based on the use of activated carbon (Jiang et al., 2017, 2020; Perrich, 2018), even though innovative sorbents, such as mesoporous silica (Kyzas and Matis, 2015; Rivoira et al., 2016) or other waste-derived ceramic materials (Jana et al., 2016; Bruzzoniti et al., 2018) have been proposed as alternative adsorption media. Low-cost materials like biochar (BC) have recently received attention for their physicochemical characteristics including the porous structure, which is similar to that of activated carbons. Biochar is the solid by-product of the thermal conversion of a wide range of feedstocks, such as agricultural wastes (Ali and Gupta, 2006; Colantoni et al., 2016), wood residues (Wang et al., 2013), manure (Cao and Harris, 2010), and sludge (Méndez et al., 2017).

Due to its properties, biochar has found application as animal feed additives (McHenry, 2010) and soil amenders (Singh et al., 2010), as well as for the adsorption of micropollutants from aqueous matrices (Palansooriya et al., 2020).

Internationally recognized standards detailing the physicochemical characteristics of biochars to be used for agricultural applications have been recently released (European Biochar Foundation (EBC)). Similar standards have not yet been established on the characteristics required for biochars to be used in water purification. However, international standards are available that require compliance with specific limits for certain physical and chemical parameters in adsorbent materials (Comite Europeen de Normalisation (CEN), 2004), and particularly in activated carbons (Comite Europeen de Normalisation (CEN), 2009), used for the treatment of drinking water.

In the last years, biochars obtained from a very wide range of experimental conditions (e.g. feedstock, thermo-chemical process and pretreatment of biomass and/or post-treatment of biochar) were investigated as sorbent media for water purification issues, highlighting their promising adsorption properties towards a large variety of organic and inorganic contaminants (Gwenzi et al., 2017; Wang et al., 2020). However, it should be emphasized that, with few exceptions (Del Bubba et al., 2020) it is not verified whether the biochars prepared in the various experimental conditions comply with the requirements set out in the aforementioned standards, relating to the absorbent materials intended to be used for the filtration of drinking water. Moreover, in most cases, ultrapure water is used to investigate sorption capabilities of biochars, whilst it would be advisable to perform these studies using real aqueous matrices. Last but not least, except in rare cases (Del Bubba et al., 2020), no comparison has been made with the adsorption capacity of standard activated carbons (ACs). All these aspects represent obvious limitations in the reliable evaluation of the applicability of biochars for water treatment (Castiglioni et al., 2021).

Based on the considerations mentioned above, the aim of this research was to investigate the physicochemical properties, the regulated leachable substances, and the removal performances of seven biochars (commercially available or synthesized for the purpose), obtained from pyrolysis or gasification of vegetal biomass, in comparison with three commercially available vegetal ACs used in an Italian drinking water facility, at different age of operation. Data obtained were chemometrically treated through principal component analysis, allowing for selecting the most promising biochars to be further investigated by adsorption tests. In a first phase of this study, adsorption capabilities were tested in ultrapure water, whereas afterwards the sorption capacity was evaluated on a restricted group of biochars in water samples collected at intermediate treatment stages of a potabilization plant. In all cases ACs were also tested as reference comparative materials.

Diiodoacetic acid (DIAA), benzene, and 1,2 dichlorobenzene, were selected as model pollutants commonly monitored in drinking water facilities. Specifically, DIAA is a model emerging disinfection by-product (Bruzzoniti et al., 2019b) never investigated before for its sorption by biochars. Moreover, 1,2 dichlorobenzene can also originate from disinfection treatments during the potabilization process (Lahaniatis et al., 1994; Hou et al., 2012) and its monitoring in tap water is recommended by the World Health Organization guidelines (taste threshold value 1 μg L⁻¹) (World Health Organization, 2017), while benzene is regulated by the Directive 2020/2184 regarding the quality of water intended for human consumption (1 μg L⁻¹). It should also be noted that benzene and 1,2 dichlorobenzene are volatile organic carbons (VOCs) still detected in some industrial districts (Martı́nez et al., 2002) and are therefore also important from the wastewater treatment viewpoint.