Engineered biochar – A sustainable solution for the removal of antibiotics from water

Highlights

• Antibiotics global consumption and their environmental contamination were shown.

• The use biochar for antibiotic sorption yields economic and environmental benefits.

• Biochar-based materials are effective in removing antibiotics from water solutions.

• Trends in the preparation and application of engineered biochar were described.

• Mechanisms involved in the binding and removal of antibiotics were discussed.

Abstract

Antibiotic contamination and the spread of antimicrobial resistant bacteria are global environmental issues. Given the growing consumption of antibiotics, it is crucial to reduce their presence in the environment. Adsorption is one of the most efficient methods for removing contaminants from water and wastewater. For this process to be effective, it is of key importance to identify adsorption mechanisms that allow an efficient and selective adsorbent to be chosen. Carbon-based materials (including activated carbon, biochar and black carbon) are typically used for the adsorptive removal of antibiotics. To enhance the efficiency of adsorption of pharmaceuticals, engineered biochars (physically, chemically and biologically modified biochar) and their composites have attracted increasing interests. Biochar-based sorbents can be produced from various feedstocks, including waste products. The use of “green”, low cost or sustainable biochar for contaminant sorption yields economic and environmental benefits. Moreover, this is in line with global trends in creating a circular economy and sustainable development. This paper collates the most recent data on the consumption of antibiotics, their related environmental contamination, and their removal using biochar-based materials. Special attention is paid to the newly emerging approaches of biochar modification and biochar composites in relation to the antibiotic removal from water.

Introduction

Without doubt, the penicillin discovery by Alexander Fleming in 1928 was not only the beginning of a new era in medicine but also a milestone in the history of mankind [1]. Subsequent research conducted by Howard Florey and Ernest Chain led to the commercialization of penicillin in 1945 [2]. This contributed to the intensive development of work on antibiotics, including their commercial application, during the period 1940–1970; this period was termed the “Golden Age” of antibiotic discovery [1], [2], [3]. Many lives were saved as a result of the use of this new group of drugs. However, with the wide dissemination of antibiotics, problems associated with their excessive use appeared. Penicillin was replaced with methicillin by 1959, and with ampicillin in 1961, due to the appearance of penicillin-resistant bacterial strains [1]. The phenomenon of antimicrobial resistance (AMR) in bacteria has forced the development of ever newer medicinal products; moreover, it also contributed to a continuous increase in the number and cost of antibiotics consumed. AMR is a serious problem, posing a threat to global health. Indeed, many organizations, including the WHO, have set themselves an overriding goal of combating this phenomenon. AMR is associated with the increased use of antibiotics and the associated contamination of the natural environment, which is another serious problem.

Population increases and the related increased demand for food have caused changes in antibiotic use trends. In recent decades (since 1950s) use of such medications on a massive scale in livestock and poultry production as well as in aquaculture has been started [4]. These have been used not as medicinal agents but predominantly as growth promoters and disease prevention additives [5]. In many countries (USA, Canada, Mexico, Israel), antibiotics are also used to grow and protect crops in order to increase yields [6]. Currently, much greater amounts of antibiotics are used to produce food of animal origin (approx. 70% global antibiotic consumption) versus human medicines (approx. 30% global antibiotic consumption) [7]. In 2010, global antibiotic production was 100,000 tons, of which 63,200 tons were drugs for farm animals [5]. By 2030, the usage of veterinary medicines is projected to rise to 105,600 tons per year [5]. These increases in antibiotic consumption are reflected in growing environmental contamination. This is linked to the fact that 30 to 90% of a drug taken by an organism is excreted, not having been metabolized [8]. Presently, many antibiotics are classified as emerging contaminants (ECs) or contaminants of emerging concern [9]. ECs are naturally occurring compounds or anthropogenically introduced compounds whose presence in the environment may pose a threat to flora and fauna, including humans [10], [11]. Globally, residues of antibiotics belonging to various classes, characterized by different properties, biodegradability, or toxicity, are detected not only in wastewater [12], [13], liquid manure [14], surface waters [15], [16], groundwater [9], soil [8], [17], and plants [18], [19], but also in drinking water [20], [21] and food [22], [23]. Many of these compounds are characterized by significant persistence and the ability to accumulate in soil and other solid matrices. Exposure of humans and other organisms to contact with antibiotics may affect their own microbiome, disturbing the microbiological balance of entire ecosystems, impairing resistance, and contributing to the development of antibiotic resistant bacteria (ARB) [15], [24]. Therefore, it is essential to not only monitor and limit the use of antibiotics but also to effectively remove them from various environmental matrices, in particular water and soil.

Various methods have been used for the removal of organic contaminants from environmental matrices. These are typically classified as (i) destructive methods (biological approaches via microbial degradation, and chemical approaches via the processes of oxidation and precipitation, chlorination, ozonation and photocatalysis), and (ii) non-destructive physical methods including filtration, coagulation/flocculation, sedimentation, ion exchange, membrane processes, and adsorption [25], [26]. Among the above-mentioned methods, adsorption is particularly common, especially in the context of antibiotics removal from the environment [13], as confirmed by the huge number of scientific publications in the SCOPUS database on “Antibiotic + Adsorption” (≈4170 during last 20 years). The process of adsorption occurs at the interface of the liquid and solid (adsorbent) phases. The liquid phase (excretions, e.g., urine, wastewater) is usually the primary source of antibiotic pollution in the environment [26]. It is only due to their further cycling that antibiotics are transferred to solid matrices, e.g., soil, sediment, plants. As a process, adsorption is relatively cheap, simple, and efficient; therefore, it is possible to use it on a large scale in wastewater treatment plants [27], [28], [29], [30].

Many types of adsorbents are currently available. These are classified as organic materials (e.g., polymers) and inorganic materials (e.g., silica, clay minerals), which can be of natural (e.g., zeolites, clay minerals) or synthetic (e.g., activated carbon, carbon nanotubes, graphene and graphene oxide, polymer resins, mesoporous silicas) origin [31], [32], [33], [34]. It is extremely important to choose an appropriate adsorbent for removing contaminants from water. When an adsorbent’s physicochemical properties, i.e., sorption parameters (specific surface area (S_BET), pore size), chemical structure, functional groups are known, it can be matched to the contaminant type so that adsorption is as selective and effective as possible [35]. From a practical view point, it is also important for an adsorbent to be sustainable. Activated carbons (ACs) have shown effectiveness in removing inorganic and organic contaminant [36], [37], [38]. However, taking the economic factors into account coupled with wide modification possibilities, biochar (BC) based adsorbents have gained importance in recent years [39], [40], [41], [42], [43]. The number of scientific papers on “Biochar + Antibiotic” in the SCOPUS database witnessed a remarkable jump from only one in 2011 to ≈95 in 2019. Biochar (BC) is a carbon-rich product obtained by heating biomass in presence of little or no air at a relatively low temperature (<700° C) [44]. BCs can be produced from various types of organic feedstocks, including wastes, e.g., waste biomass, municipal waste, agricultural and livestock waste, food production residues [45], [46]. Moreover, BCs are much cheaper than ACs, which further enhances their attractiveness. For example, the price of BCs ranges from $350 to $1,200 per ton, whereas the price of ACs ranges from $1,100 to $1,700 per ton [47].

Owing to their appropriate physicochemical properties and thus adsorption properties, which can be additionally improved through various physical and chemical modifications [48], [49], BCs have been applied to the adsorption of many contaminants [41], [45], [50], [51]. New literature reports continuously appearing regarding the use of BCs of different origins, including engineered BCs (i.e., modified BCs and BC composites) for the adsorption of antibiotics [32], [52], [53], [54], [55]. This is evidence of the relevance of the production and application of new low-cost, but at the same time “green,” adsorbents in the context of environmental pollution by this group of pharmaceuticals.

This paper is a critical review of the most recent literature data regarding the consumption of antibiotics and their associated environmental contamination. It also addresses the use of various BC-based materials (pristine BCs, modified BCs, and BC composites) for the adsorption and removal of this type of contaminant, with special reference to the mechanisms responsible for their adsorption.