Use of natural sorbents as alternative and green extractive materials: A critical review

Highlights

• Recent applications (2009–2019) of the use of natural sorbents for sample preparation are reviewed.

• Green sampling strategies for the extraction of metals are discussed.

• Exhaustive and non-exhaustive approaches for pre-concentration and sampling of organic molecules are presented.

• Functionalization strategies for enhanced extraction efficiency of natural sorbents are proposed.

Abstract

The increasing concern about environmental degradation and resource depletion has inspired the analytical chemistry community to develop analytical methods that comply as much as possible with the principles of Green Analytical Chemistry. Significant progress has been made in greening sample preparation strategies by miniaturizing sampling devices and decreasing the amount of sorptive phase needed for efficient extraction of targeted molecules. In this context, the use of natural sorbents represents an additional and convenient option for green sample preparation. The advantages of using natural sorbents for extraction include their availability from renewable sources, low toxicity and biodegradability. In this review, we describe the use of various natural sorbents for metals and organic molecules extraction, focusing on the most innovative applications within the decade 2009–2019. Particular emphasis is given to the description of commonly used biopolymers – e.g. cellulose, chitin, and lignin – and their use in a variety of sample preparation strategies. We also refer to different functionalization approaches that enhance the extraction efficiency of natural sorbents.

Introduction

With the raising awareness of the importance of developing greener analytical protocols to prevent environmental pollution, the use of biosorbents certainly has caught the interest of many researchers across the globe. One task separation scientists have been very successful at, over the last two decades, is the elimination of excessive amounts of organic solvents for extraction [[1], [2], [3]]. This task is achieved by i) the development of sorbent-based extraction techniques - Solid Phase Extraction (SPE) [4] and Solid Phase Microextraction (SPME) [5] among others-, ii) the reduction of organic solvents needed for an effective extraction - e.g., single-drop microextraction (SDME), dispersive liquid-liquid microextraction (DLLME) [6]– or iii) the use of alternative eco-friendly extraction fluids such as supercritical CO₂ and ionic liquids [7,8]. In the case of sorbent-based extraction approaches, many research efforts are focused on the development of synthetic procedures that comply as much as possible with the principles of Green Chemistry [9], such as in the case of metal-organic frameworks (MOFs) and nanoparticles [10]. However, considering the sorption capacity of many biomaterials, often obtainable from the food and manufacture industry waste, numerous research efforts focused on testing their sorptive properties for inorganic and organic species. The use of biosorbents for sample preparation improves the greenness of analytical protocols and complies to several of the 12 principles of Green Analytical Chemistry [3,[11], [12], [13]], such as:

• eliminating the use of hazardous substances during the production of the sorbents

• minimizing the amount of hazardous waste

• guaranteeing safer procedures for accident prevention during fabrication of the sorbents: due to the greenness of the starting materials used, that are non-toxic, neither corrosive nor explosive, there is no major risk for users

• using materials easily degradable after use

• using renewable raw materials

Biosorbents have been primarily used for the remediation and purification of both metal ions and organic contaminants from water samples [14]. These applications often involve the irreversible extraction of target analytes, thus the sorbent use is limited by its saturation threshold (unless regeneration is possible). The mode of use of these biosorbents changes when they are intended for sample preparation purposes, as the interaction between the sorbent and the analyte must be reversible to guarantee quantitative desorption prior to instrumental analysis. Moreover, the biosorbent should be able to perform consecutive extraction/desorption cycles to minimize the production of waste (unless single-use extraction devices are needed). The extraction selectivity of many biosorbents can be easily tuned by functionalizing their native functional groups, making them very versatile starting materials for a broad range of extraction strategies.

In this review, we survey recent applications of different biosorbents used for a variety of sample preparation techniques and describe strategies for their functionalization as well as applicability for metals and organic molecules extraction.

The growing demand to introduce high performance, high throughput, and reliable analytical methods in both the academic and industrial sectors has caused the use of synthetic extraction materials to increase exponentially. The development of these materials is critical to better isolate and quantitate trace amounts of both inorganic and organic analytes from complex matrices. In the past few decades, sample preparation has evolved with the almost exclusive use of synthetic solid sorbents providing excellent performances for both ultra-trace level targeted analysis and untargeted screening. Numerous sample preparation methods have been introduced to ensure appropriate analyte pre-concentration while also removing possible interferences from the complex matrices. Examples include SPE methods that use cartridges filled with different sorption materials, Stir Bar Sorptive Extraction (SBSE) consisting of a magnetic bar coated with extraction phase and SPME, a miniaturized extraction technology where micro-volumes of extraction phase are immobilized on supports of different geometry, with fiber-like coated devices being the most used. Among the sample preparation techniques mentioned above, SBSE has only two commercially available extraction phases, polydimethylsiloxane (PDMS) and ethylene glycol – polydimethylsiloxane (EG-PDMS).

Commercially available extraction phases for SPME are more diverse and are classified into two groups: PDMS based sorbents for thermal desorption into GC systems (fiber, arrow and thin-film geometry) and polyacrylonitrile (PA)-based extraction phases for solvent assisted desorption, mainly for LC applications. At this time, SPE is the extraction technique that provides the broadest coverage in terms of commercially available extraction phases. Extractive materials for SPE cover all adsorption extraction modes, including normal phase, reverse phase, ion exchange, immune affinity and mixed-mode [15].

All of the above mentioned commercially available extraction phases are made out of synthetic polymers. Even though these sorbents are commonly applied, many are unable to extract a wide range of analytes classes. In recent years, biopolymers are attracting increased attention due to their indisputable advantages over synthetic polymers, such as sustainability and biodegradability. SPE was first introduced in the late 1970s followed by the disk technology for SPE in 1989 [16]. Various biosorbents were used for SPE since the early 2000s [17]. In the case of SPME, firstly introduced in 1989, the use of a biopolymer to prepare SPME fiber was first reported in 2013, by using bamboo charcoal [18,19].

Biopolymers are polymeric material made out of covalently bonded monomeric units and are categorized according to the sources where they are obtained from, either renewable resources or living organisms. Unlike synthetic polymers, biopolymers have complex and well-defined structures [20]. According to the monomer unit type and the structure of the biopolymer formed, they can be divided into three main categories:

• polynucleotides, which are made out of a monomeric unit called a nucleotide, that replicates to create longer chain polymers that constitute DNA and RNA;

• polysaccharides, constituted by sugar units linked together;

• polypeptides, which are linear organic polymers made of amino acids.

Among these polymers, polysaccharides are commonly used as sorption materials for sample preparation due to their distinctive chemical structure and characteristic properties such as low cost, eco-friendliness, biodegradability, and non-toxicity, which largely differ from synthetic polymers [20]. According to the literature, commonly used polysaccharides for sorptive material synthesis are cellulose, chitin, lignin, and suberin [21]. Among the above-mentioned polysaccharides, the most abundant biopolymer is cellulose, present as the primary constituent of the plant cell wall [21]. Cellulose is a natural polymer made by connecting β-d-glucopyranose units by β-1,4-glycosidic linkages to produce a long-chain polysaccharide. The repeating unit of the cellulose monomer consists of mainly methoxy groups and hydroxyl groups that enhance its ability for adsorption. Moreover, the large number of hydroxyl groups on the outer surface of the polymer chain enhance the hydrophilic nature of the cellulose polymer backbone, also facilitating the surface modification by reacting these groups with various chemical moieties such as amine, carboxyl, nitrile and amidoxime [21]. In applications as a sorbent material, cellulose can be used in its pure form. Cellulose is either isolated from plant tissues or sourced from unrefined plant products (e.g. tree barks, corks). Recently, cellulosic materials have seen substantial use in analytical chemistry as a sorbent for cartridge SPE (c-SPE), disk-SPE, dispersive SPE (d-SPE), magnetic SPE, molecularly imprinted polymer SPE (MIP-SPE), and Thin-Film Solid Phase Microextraction (TF-SPME) [[22], [23], [24]].

Another commonly used biopolymer for extraction, chitin, is structurally similar to cellulose except for the acetyl amine group in the C-2 position in its monomeric structure. This amino group in the polymeric backbone provides unique structural and chemical properties to chitin, such as water solubility [25]. The major sources of chitin are the outer shells of crustaceans such as crabs and shrimp, and cell walls of fungi [26]. Chitin can be converted to chitosan by deacetylation, both chitin and chitosan being considered biofunctional polymers due to their unique property to produce films. Moreover, both are biocompatible, biodegradable, and non-toxic biopolymers. These characteristics make chitin and chitosan ideal green sorptive materials for sample preparation in analytical protocols. Aside from the above-mentioned materials, biosorbents can also be obtained from other sources such as industrial by-products, agricultural waste products and biomass derived recycled materials [27]. Some of these materials are bamboo charcoal and bract, shown to be suitable extraction phases for numerous microextraction techniques [[28], [29], [30]]. Diatomaceous earth, originating from microscopic algae called diatoms being fossilized in sediment, has also been commonly used as a natural sorbent. It is considered a naturally occurring sedimentary rock that contains the amorphous mineral diatomite (fossilized diatoms), the bulk rock being mainly made out of silica [30].

Apart from the approaches mentioned above, biopolymers have been used to prepare analytical columns. As an example, trisphenylcarbamoyl modified cellulose chiral column has been used to separate the enantiomers of omeprazole in normal phase mode [31]. Moreover, a column with peroxidase immobilized chitosan beads was used for the fluorometric determination of hydrogen peroxide in natural water [32].

The structural features of the biopolymers such as type of monosaccharides linkages, substitutes, steric configuration and degree of the substitution play an important role in their physical-chemical properties and consequently affect their extraction efficiency and capacity [33]. Therefore, the functionalization of the biopolymers, to enhance the sorption efficiencies by altering their physicochemical properties, is of high importance [33]. In cellulose polysaccharide, hydroxyl groups are easily accessible to functionalization reactions. The hydroxyl groups of the cellulose could undergo selective oxidation, esterification, etherification, intermolecular crosslinking reactions, and graft copolymerization reactions, without altering the fiber structure of the cellulose [33]. The selective oxidation of the cellulose using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives allows altering the cellulose primary hydroxyl groups into carboxylic groups [33]. The ion exchange capacity of the cellulose, which is in the range of 0.01–0.05 mmol/g for unsubstituted cellulose, is increased by converting the hydroxyl group to carboxylic groups, and this enhanced ion exchange capacity increases the extraction efficiency for metal ions [33,34]. The esterification or the etherification of cellulose occurs either on the whole polymeric chain or only on the surface of the cellulose [35]. In particular, cellulose esterification can be applied in under both heterogeneous and homogenous conditions. Surface modification of cellulose by heterogeneous conditions is the main strategy for chemical modification and isolation of nanocellulose, which is vastly applied in solid based extraction techniques [24,35]. Other chemical modification methods, namely crosslinking and copolymerization, are applied for cellulose to enhance the selectivity towards the analytes by introducing organic moieties that facilitate the specific interaction such as hydrogen bonds, π-π interactions and electrostatic interactions with the analytes [24]. Chitin and chitosan biopolymers consist of not only primary and secondary hydroxyl groups but also free amino groups and acetamido groups. Therefore, chitin and chitosan can be functionalized not only with strategies common for cellulose functionalization but also through Schiff base reactions and quaternization [33]. Moreover, partially crosslinked chitosan with di/polyfunctional reagents enhance its metal complexation efficiency. The adsorption capacity of chitosan depends on the degree of crosslinking and it decreases by increasing the extent of crosslinking. However, the metal adsorption capacity of chitosan can be increased by crosslinking under homogenous conditions compared to heterogeneous conditions due to the enhanced hydrophobicity resulted from partial destruction of crystallinity [36].