Design of biomass-based renewable materials for environmental remediation

Abstract

Various materials have been used to remove environmental contaminants for decades and have been an effective strategy for environmental cleanups. The current non-renewable materials used for this purpose could impose secondary hazards and challenges in further downstream treatments. Biomass-based materials present viable, renewable, and sustainable solutions for environmental remediation. Recent biotechnology advancements have developed biomaterials with new capacities such as highly efficient biodegradation and treatment train integration. This review systemically discusses how biotechnology has empowered biomass-derived and bio-inspired materials for environmental remediation sustainably and cost-effectively.

Why use sustainable materials for environmental remediation?

Environmental remediation has remained a fundamental goal for sustainability since industrial evolution. Notably, continuous discharge of synthetic chemicals at large volumes could exceed natural degradation capacity, which leaves persistent pollutants in the environment and negatively impacts the ecosystem and human health [1]. Remediation typically involves concentrating and removing the pollutants from the environmental matrices and sequential treatment to destroy the contaminants. Concentrating environmental contaminants heavily relies on the adsorption capacity of materials to enrich the pollutants, which is achieved through the physical and chemical bonds between the adsorbents and adsorbates [2–4]. Various materials were developed through non-biological routes for adsorbing and remediating environmental contaminations. Such examples include natural mineral clays [5–7], resins [8,9], metal-organic framework (MOF) [10–12], and mesoporous silica [13–15], all of which have been comprehensively reviewed previously. However, the manufacturing process for these adsorption materials often faces economic and environmental challenges. Techno-economic analysis study has pointed out the high energy consumption and labor cost of conventional remediation material manufacturing [16,17]. In addition, some material synthesis, like pyrolysis (see Glossary), is often accompanied by a corrosive waste stream or toxic gas release. Furthermore, the rejuvenation and post-absorption processing present additional costs and potential secondary environmental hazards.

In this regard, biomaterials, especially those developed from lignocellulosic biomass, provide a renewable route with substantial environmental benefits. Biotechnology can further improve the features of these biomaterials for efficient absorption and remediation processing [18,19]. Biochar, chitosan, microbial biomass, and various agricultural wastes have been studied to adsorb heavy metal and persistent organic pollutants (POPs) [20–23]. The strength of biomaterial, especially biomass-based material for environmental remediation, lies in compatibility with the environment, the often lower cost, and the renewable nature [24]. Biotechnology is further exploited to engineer these biomaterials to achieve improved adsorption capacity, efficient pollutant degradation, and prevent the secondary pollution caused by releasing soluble organic compounds from the materials [2–4]. Furthermore, the recent advancement of biodegradable adsorptive materials empowers a novel sustainable route in that the removal and degradation of the contaminants can be processed in the same system to avoid the traditional treatment train approach in environmental cleanups. Herein we first review the biomass-derived material for environmental remediation and then discuss how biotechnology has brought new capacity into these materials. We will finally review recent breakthroughs to enable treatment train integration through design bioremediation compatible biomass-based sorbent.

Carbonaceous materials from renewable biomass resources for environmental remediation

One of the earlier biotechnologies to prepare adsorption materials to remove contaminants involves transforming biomass into carbonaceous materials. Earlier examples in the eighteenth century included an English sugar refinery that successfully processed wood charcoal and applied the charcoal toward sugar syrup decolorization [25]. Since then, more biomass types have been explored to manufacture adsorption materials. The biotechnology advancements in the 20^th century have delivered various adsorption materials from lignocellulosic biomass or its components (Table 1). The syrup decoloration charcoal has inspired different thermal and thermochemical processes to derive carbonaceous materials for environmental applications. Biochar is such an example that various biomass can be utilized to synthesize porous carbon materials for adsorption. Initially, biochar was considered a byproducts of biomass thermal conversion, generated under oxygen-limiting environments [26,27]. Biochar was later recognized as a low-cost material for environmental remediation and broadly applied in water and soil treatments due to their porosity and stability properties [28,29]. Porous carbon materials like biochar can be derived from various wastes (i.e., agriculture residues, food waste, and animal husbandry waste), which contain significant amounts of lignocellulose-based biomass. The characteristics of porous carbon materials (i.e., biochar) heavily depend on the structure and composition of the feedstock [30]. Elements such as nitrogen, oxygen, sulfur, and hydrogen may exist in the derived biochar, and small amounts of metal species could also be retained. Thus, inorganic impurities could impact material properties [31,32]. Feedstocks such as food waste and manure are more heterogeneous, and the derived biochar products often have less carbon content, and the yield is usually low. Lignocellulosic biomass such as agriculture residues and wood chips serve as high quality feedstock for biochar, rendering higher carbon content due to the polymeric structure and six-membered carbon rings in the monomers. In particular, wood-based feedstocks contain a large amount of lignin, which has a higher carbon content. The stable aromatic structure will significantly improve the carbon content in the biochar [33]. Other parameters, such as pyrolysis temperature, treatment time, and production approaches, are important factors that influence biochar's yield, structural and chemical properties [34].

Table 1.

Carbonaceous materials derived from various biomass feedstocks

Main component in biomass feedstock	Processing conditions	Characteristics of derived carbon	Primary mechanisms for removal of contaminants	Refs
Cellulose	Carbonize under N2	Disordered pores with a higher specific surface area	Physical adsorption, such as hydrophobic reaction [35] and electrostatic interaction [36]	Crystalline cellulose [37] Cotton [35] Bamboo [36,38]
Cellulose and their hydrolyzed monomers	Hydrothermal carbonization	Agglomerates of carbonaceous microspheres with higher oxygen content	Physical adsorption, such as electrostatic attractiona and π-π interaction [39] Ion-exchange mechanism interaction [39]	Cellulose [40,41] Bamboo [39] Cotton [42] Corn starch [41,43] Glucose [41,44] Fructose [45] Sucrose [41] Xylan [46]
Hemicellulose	Carbonization	Higher yield than cellulose due to promoted char formation	Physical adsorption based on high porosity	Glucomannan [47] Beechwood [48,49] O-acetyl-preserved hemicellulose [50]
Lignin	Carbonize under N2	Graphitized carbon with ordered regions	Physical adsorption, such as electron donor-acceptor complexes [51]	Indulin AT [37,52] Lignin precipitate from black liquor [51,53]
Lignin	Hydrothermal carbonization	High-ordered crystalline structure Functional groups removed at above 350°C except for OH groups Ether bond cleavage, C-C bond formed, while aromatic rings remained	Physical adsorption based on rich hydroxyl groups	Black liquor from pulping [54] Sugi wood [55] Buna wood [55]
Chitosan	Heat treatment under an inert atmosphere	N-doped carbon with limited pore and surface area	Electrochemical reaction [56] and electro-static interactiona [57]	Chitosan gel after acid-base treatment [56] Crab shell [58] Shrimp shell [57]
Bio-oil and biochar	Polymerize and then carbonize under N2 at 900°C	High yield of dense carbon	Physical adsorption based on high specific surface area	Mallee biomass [59]
Food waste	Pyrolysis and hydrothermal carbonization	Biochar and Hydrochar, with crude oil as byproducts	Physical adsorption based on high specific surface area	Soybean protein [60] Digestate from food waste [61] Horse manure [62] Biosludge [62]

^aElectrostatic attraction and electrostatic interaction are used following the original reference.

The technology for thermal processing of biomass into biochar and activated carbons has been extensively studied for decades. With the solid basis of the development and ongoing improvements and modification, the technology readiness for thermal processing is high, [63] and such materials have been applied in various fields, including soil contamination remediation [29] and water pollution control [64]. For example, biochar production from pyrolysis has been broadly established on a pilot scale (Technology readiness level (TRL) 5–6), with many plants at an industrial scale (TRL 7 or higher) [65]. However, it should be noted that the current pyrolysis processes use mixed biomass as the feedstock. The pyrolysis of specific components in biomass, such as cellulose and lignin, can potentially tune the properties of derived carbon materials finely. These techniques have the potential to produce value-added carbon materials, but many of them are still at the laboratory scale (TRL 2 to 3) [40,41,51,56]. Despite the broad application, the thermal processing of biomass into porous carbon materials has several limitations. First, biochar yield is typically low, often less than 30% [66]. Second, a significant portion of biomass feedstock turned into gases or liquids, which are emitted into the environment and often need additional treatment or upgrading. The high energy inputs and carbon emissions often lead to unfavorable environmental and life cycle impacts [67,68]. Third, biochar is not as amenable to functional improvement via biotechnologies such as enzyme immobilization, as a large portion of oxygen-containing functional groups are removed during the heat treatment [69,70]. Fourth, most of the biochar products are in powder form, which needs further processing [71,72]. Fifth, the carbonaceous materials are relatively stable, so the treatment of the end-of-life material could become an issue. Other biotechnologies to utilize biopolymers have been developed for manufacturing remediation biomaterials.

Functional biomass-based materials for environmental remediation

Besides thermal processing to transform biomass, biopolymer modifications have produced more advanced material structure design and diverse functions for environmental remediation. Besides achieving high specific surface areas through biomaterial designs, three-dimensional (3D) materials have been a recent focus in adsorption biomaterials developments (Table 2). The advantages of functional biomass-based materials include low cost, biodegradability, biocompatibility, and being eco-friendly and renewable. [73–77]. Those recently developed biomaterials have also shown excellent performance properties, such as complexation, flocculation, chelation, separation, and adsorption capacities [78]. For example, the reactive carboxymethyl groups present in the carboxymethyl cellulose render reasonable solubility, chemical reactivity, and strong chelating capacity, which makes it attractive as an adsorbent [79].

Table 2.

Adsorption biomaterials developed through biomass modifications

Main component from biomass	Biomass materials type	Contaminant	Primary mechanisms for removal of contaminants	Refs
Biomass-derived powder/particles
Lignin	Aminated CELF lignin derived powder	Azo dye	Electrostatic interaction, hydrogen bonding and π−π stacking	[73]
Alkali lignin	PEI-graft-alkali lignin with nanoscale lanthanum hydroxide	Phosphate	Surface precipitation and ligand exchange	[74]
Cellulose	PEI modified cellulose microcrystals	PFAS	Electrostatic interaction and physical adsorption	[75]
Three dimensional (3D) porous materials foam
Nanocellulose	Nanocellulose-based foams	Oil	Physical adsorption, such as hydrophobic reaction	[80]
Nanocellulose	Nanocellulose aerogels	Cation dye	Electrostatic attraction	[81]
Nanocellulose	Cellulose and metal-organic-framework aerogels	Organic dyes	Intraparticle diffusion	[82]
Spent coffee	Spent coffee bioelastomeric foam	Pb2+ and Hg2+	Electrostatic attraction	[76]
Loofah plant	Three dimensional functionalized carbon/Tin(IV) sulfide biofoam	Cr6+	Physical adsorption and photocatalytic	[83]
Bio-based hydrogel
Carboxymethylcellulose	Functional hydrogel materials	Heavy metal ions	Metal displacement reaction	[84]
Carboxymethylcellulose	Carboxymethylcellulose-grafted polyvinyl alcohol magnetic hydrogel	Cu2+	Ion exchange	[85]
Carboxymethylcellulose/alginate	Carboxymethylcellulose/alginate hydrogel	Pb2+	Physical, chemical, and electrostatic adsorptions	[77]
Sodium Carboxymethyl Cellulose	Sodium Carboxymethyl Cellulose /Sodium Styrene Sulfonate hydrogels	Metal ions	Electrostatic attraction	[78]
Carboxymethylcellulose	Carboxymethylcellulose grafted-poly(N-isopropyl acrylamide-co-acrylic acid)/montmorillonite hydrogel	Heavy metal ions	Electrostatic interaction and valence forces	[86]
Sodium lignosulfonate	Bentonite/sodium lignosulfonate/acrylamide/maleic anhydride hydrogel	Pb2+	Electrostatic interaction	[87]
Lignin and starch	Lignin/peat/starch/acrylamide-based hydrogel	Cu2+ and Ni2+	Electrostatic interaction	[88]
Lignosulfonate	Lignosulfonate-g-acrylic acid hydrogel	Methylene blue	Ion exchange	[89]
Kraft lignin	Kraft lignin-N-isopropyl acrylamide hydrogel	Methylene blue	Electrostatic interaction	[90]
Lignin peroxidase	Manganese peroxidase/lignin peroxidase/laccase/polyacrylamide/pectin hydrogel	Bisphenol A	Enzymatic reaction	[91]
Lignin	Bentonite/lignosulfonate hydrogel	Pb2+	Chelation and ion exchange mechanism	[92]
Biopolymer processed nanofibers
Nanocellulose	Cellulose nanofibers	Cd2+	Electrostatic interaction	[93]
Cellulose	Esterified cellulose nanofibers	Ciprofloxacin and ofloxacin	Electrostatic interaction, π-π interactions and hydrogen bonding	[94]
Cellulose	Cellulose nanofibers	Cationic and anionic dyes	Electrostatic interaction	[95]
Cellulose	2,2,6,6-tetramethylpiperidine-1-oxyl radical-mediated oxidized cellulose nanofibers	Cu2+	Electrostatic interaction	[96]
Cellulose	2,2,6,6-tetramethylpiperidine-1-oxyl radical-mediated oxidized cellulose nanofibers	Methylene blue dye	Electrostatic interaction	[97]

Biomass derived powder and particles

Biomass-derived components such as cellulose and lignin have been modified to treat environmental contaminants. For example, lignin has been aminated to remove azo dye from aqueous solutions [73]. Similarly, poly(ethyleneimine)-graft-alkali lignin loaded with nanoscale lanthanum hydroxide (AL–PEI–La) has been used to remove phosphate from wastewater [74]. Applications of lignin derivatives to remove heavy metals have been reviewed elsewhere [98]. On the other hand, cellulose microcrystals have been functionalized with polyethyleneimine to remove poly-and per-fluorinated alkyl substances (PFAS) from water [75]. Advances in cellulose derivatives to remove contaminants from aquatic environments are also reviewed elsewhere [99], and the cellulose-derived nanofibers are introduced in a later section “biopolymer processed nanofibers” in this review.

3D porous materials

To date, many 3D porous materials have been developed as adsorbents due to the tunable structures and high specific surface areas. For example, biopolymer-derived foams have high and rapid adsorption capacity and desired hydrophobicity and oleophilicity [80]. Low-cost biomass polymers such as cellulose and other biopolymers like sodium alginate and guar gum have been used as precursors to produce environmentally friendly foams for environmental remediation [100]. Particularly, nanocellulose-based foams have been developed as a promising 3D porous template for oil/water separation [80]. Recent biotechnology advancements and material design have focused on constructing a 3D structure to achieve high specific surface area and utilizing the biopolymer functional groups to generate physical and chemical bonds for functionalization. For example, Liang et al. have designed chemical crosslinking nanocellulose aerogels to remove cation dye [81]. Nanocellulose has also been combined with inorganic materials such as MOF to create high adsorption capacity composites [82]. Spent coffee has been used to prepare composite in a bioelastomeric foam to remove Pb²⁺ and Hg²⁺ from water, with high adsorption capacities at 13.5 and 17.1 mg/g for Pb²⁺ and Hg²⁺ ions, respectively. Loofah plant materials have been used to fabricate 3D functionalized carbon/Tin(IV) sulfide biofoam, which was further used to purify chromium(VI)-containing wastewater [83].

Bio-based hydrogel

Biopolymer hydrogel has been used for adsorption due to its simplicity, high adsorption, and easy recovery compared to other sorbents. Biotechnologies to develop 3D hydrogels have focused on water-swollen structures and crosslinking polymeric materials to retain a large amount of water. One particular benefit of biobased hydrogel is the increased hydrophilicity since the biomass functional groups can increase the material swelling ability and enhance contaminant up-taking [101,102]. Furthermore, hydrogels made by biopolymers offer a sustainable alternative to the conventional pollutant removal processes and are biocompatible for broad applications with a low disposal cost. Among various biopolymers, cellulose [103], sodium alginate [104], chitosan [105], starch [106], and guar gum [107] have been successfully applied for hydrogels. The versatility of the biopolymer chemical structures enables diversified modifications toward broad application requirements. Cellulose from lignocellulosic biomass has been widely used as hydrogel component because of the thermodynamically preferable conformation of its linear polymer chain and the abundant hydroxyl groups ready for modifications. As a biocompatible and biodegradable natural anionic polysaccharide, carboxymethylcellulose (CMC) is a water-soluble cellulose derivative widely used for the fabrication of the functional hydrogel materials [85,108,109]. The hydroxyl and carboxyl groups of CMC can form chelates with the heavy metal ions [84]. A CMC-grafted polyvinyl alcohol (PVA) magnetic hydrogel has been reported to effectively remove Cu²⁺ ions when incorporated with ferriferous oxide (Fe₃O₄) [85]. Similarly, the CMC/alginate hydrogel showed an enhanced capacity of Pb²⁺ ion adsorption compared to activated carbon and cation-exchange resin [77]. The CMC/sodium styrene sulfonate hydrogel presented efficient adsorption of the divalent and trivalent heavy metal ions [78]. Furthermore, a CMC grafted-poly(N-isopropyl acrylamide-co-acrylic acid)/montmorillonite hydrogel was prepared and successfully removed heavy metal ions from aqueous solutions [86]. Other polysaccharides such as alginate and chitosan can also be used to manufacture high-capacity hydrogel. Besides polysaccharides, lignin has been explored for manufacturing hydrogel for remediation. Lignosulfonate-g-acrylic acid (LS-g-AA) hydrogels were synthesized by grafting acrylic acid (AA) on the backbone of lignosulfonate for remediation of cationic dye-contaminated effluent. The equilibrium adsorption of the dye molecules reached 2013 mg/g [89]. The lignin-based hydrogel was reported to have good adsorption performance of Cu²⁺ and Ni²⁺ from contaminated simulated water [88]. The metal adsorption capacity could reach nearly 100% at a low swelling percentage. Lignin sulfonate-based hydrogel was prepared and applied to remove malachite green from aqueous solutions [87]. The adsorption rate of malachite green on the lignin sulfonate-based hydrogel is higher than that on lignin sulfonate-based polymer, and the maximum difference reached 37%. Overall, lignocellulosic biomass, its components cellulose, and lignin can all be used for hydrogel manufacturing toward environmental remediation applications.

Biopolymer processed nanofibers

Bio-based nanomaterials are attractive for remediation because nanomaterials have a higher surface-to-volume ratio than traditional adsorbents. Among various nanomaterials, nanofibers have a greater hydro-stability than many other adsorbed nanostructures, which have been applied for highly efficient organic chemical treatment [110–112]. Biomass-based nanofibers are ideal for designing nano-porous adsorbents due to their biodegradable nature and chemical structures [113]. For example, cellulose nanofibers have been broadly applied to prepare various adsorbent materials for wastewater treatment [114]. Sharma et al. used spinifex to synthesize nanocellulose to remove cadmium (II) from water. The large and rapid removal capacity is due to the interactions between the carboxylate groups on the nanocellulose surface and Cd²⁺ ions [93]. Furthermore, esterified cellulose nanofibers have been assembled with functionalized graphene oxide to treat pharmaceutical waste in water, which reached the maximum removal capacity of 45.04 mg/g and 85.30 mg/g for ciprofloxacin and ofloxacin, respectively [94]. Cellulose nanofibers could be blended with polyvinyl alcohol (PVA) to form dye pollutants adsorbents, and the average removal efficiency of both cationic and anionic dye molecules was higher than 60% [95].

Compared to conventional cellulose nanofibers, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-oxidized cellulose nanofibers (TOCNFs) have an ultra-fine structure at the 3–10 nm diameter scale. For the past few years, TOCNF-based adsorbents, nanocomposites, and membranes have been extensively studied and have been used to remove organic pollutants and heavy metals [115]. For example, Liu et al. reported that the adsorption capacity of Cu²⁺ by TOCNFs (with a carboxylate content of 1.5 mmoL/g) could reach 75 mg/g. Due to the carboxylic acid groups on the surface of carbon nanotubes, the adsorption capacity of TOCNFs for Cu²⁺ is significantly higher than that of the original carbon nanotubes [96]. TOCNFs displayed considerably higher adsorption of methylene blue dye because of their high carboxylic (-COOH) content (2.1 mmol/g). The largest adsorption capacities of the original TOCNFs and CNCs for methylene blue were 769 and 118 mg/g, respectively [97].

In summary, functionalized biomass-based materials can achieve high adsorption capacities due to structural and chemical modifications. Nevertheless, those new materials can also have disadvantages, such as limited utility due to specific chemical modifications. For instance, lignin sulfonate can be an adsorbent to remove cationic dyes and metal ions. However, the application is limited because of its dissolution tendency in the effluent [87]. As discussed earlier, even though the starting biomass materials, such as cellulose, are commercial products, many of those functional materials are still at the research and development stage, and the prototypes have only been validated at the laboratory scale. Most of the examples listed in Table 2 are at the laboratory scale (i.e., TRL value between 3 to 4), where the proof-of-concept is confirmed and validated. In the literature, very limited cost analyses have been performed to estimate those functional biomass-based materials. To calculate the cost, many factors, such as the cost of adsorbent based on the adsorbate removal rate, operating expenditures (OPEX), annual capital expenditure (CAPEX), and adsorbent application costs during adsorption operation need consideration [116]. As most of the studies are at the laboratory scale, the cost analysis remains ambiguous.

Biotechnology modification delivers high-capacity biomaterials for environmental remediation

Biotechnologies have empowered the aforementioned biomaterials with unique capacity for environmental remediation. The traditional contaminant treatment typically involves a treatment train approach, which encompasses pollutant adsorption [117], detoxification, and subsequent material degradation [118]. For example, the emerging contaminant PFAS can be enriched and removed by granular activated carbon, ion-exchange resins, biochar, ash, and absorbents [119]. However, these adsorbents exhaust quickly, and the adsorption medium must be replaced frequently at a high cost. Moreover, incineration is required as the commercial follow-up treatment because those adsorbents do not degrade the PFAS, and adsorbent regeneration may not be cost-effective. The treatment train approach requires multiple steps to remove the contaminants with a low efficiency. Unlike the direct modification of the biopolymer functional groups, coupling macromolecules such as enzymes or microorganisms to the lignocellulose biomass-derived components allows partial treatment train integration, where bioremediation and absorption can be integrated [120,121]. The coupling techniques are typically biocompatible, which retain the original biochemical and biological properties of the protein and the microorganism [122,123] (Figure 1 and Table 3).

Figure 1.

Immobilization technologies for biobased adsorption materials.

(A) Adsorption-based biotechnology immobilizes enzymes, fungi, or bacteria onto the material surface. The physical adsorption involves Van der Waals forces, hydrogen bonding, and ionic bonding. (B) Embedding technology entraps enzymes, fungi, or bacteria into the materials. (C) Enzymes, fungi, or bacteria are immobilized via covalent bonds onto the sorbents.

Table 3.

Adsorption biomaterials developed through enzyme and microbial immobilization

Main component in biomass feedstock	Enzyme type	Contaminant	Refs
Enzyme immobilization
Cellulose	Peroxidase	Undesirable/hazardous water contaminants	[152–154]
Nanocellulose	Laccase	Malachite green and Congo red dyes	[155]
Dialdehyde cellulose	Urease	Urea	[156]
Cellulose/alginate	Laccase	Bisphenol A	[157]
Chitin-lignin	Lipase	Para-nitrophenyl palmitate	[158]
Concanavalin A-cellulose	Tomato peroxidase	Direct dyes	[159]
Cellulose	Turnip peroxidase	Direct dyes	[160]
Microbial immobilization
Bamboo charcoal	Bacillus WHX-1	Cadmium in soils	[163]
Sunflower receptacle	Ultralight and lipophilic microorganism	Petroleum pollutants	[164]
Corn-cobs/alginate	Ganoderma lucidum	Anthracene in soils	[165]
Coconut fiber-derived biochar	mycelia pellets Bacteria Pseudomonas putida	Paraquat	[166]

Immobilization of enzymes for bioremediation

Enzymes have been broadly used in environmental remediation [124–129]. In designing the enzyme immobilized biomaterials, enzyme stability, increment of enzyme loading volume, biocatalyst recycling, and downstream processes are all important considerations impacting the remediation efficiency. Mainly, enzyme immobilization exploits the functional groups in the enzyme amino acids for the attachment to the supporting materials. In the context of environmental remediation, the supporting material typically has a high adsorption capacity to enrich the contaminant. When immobilized on porous materials with high specific surface areas, enzymes can degrade the pollutants that are concentrated in the material to achieve treatment train integration. The high concentration of the adsorbates can significantly improve the degradation reaction rate. In addition, enzyme immobilization confines and stabilizes the protein structure, which could enhance the enzyme stability [126,130]. Furthermore, various types of porous materials, including carbonaceous materials, silica-based materials [131–133], and biopolymer-based materials [134,135] have been directly utilized for enzyme immobilization. Several enzyme species such as laccase and peroxidase have been immobilized on stable material (i.e., biochar and cellulose etc.) frameworks for the decomposition of environmental wastes.

The enzyme immobilization of cellulose and nanocellulose empowered the integration of adsorption and biodegradation to achieve synergistic and efficient remediation, leveraging the strength of both biomaterial and biocatalysts. Generally speaking, the mainstream methods for enzyme immobilization include physical adsorption [136,137], entrapment [138–140], covalent bond grafting [141–143], and crosslinking [144–146]. Cellulose is hydrophilic but insoluble in water due to its hydroxyl groups, which can support enzyme immobilization and favor hydrogen bond formation, leading to its stability [147]. Cellulose has been immobilized with trypsin [148], pectinase [149], and urease [150], where treated cellulose reacts with enzymes via Schiff base reaction on the aldehyde groups. Modified cellulose with amino anchor groups was also immobilized with peroxidase via glutardialdehyde reaction and diazo coupling [151]. The enzyme immobilized cellulose has been employed for different undesirable/hazardous water contaminants at high efficiencies [152–154]. Nanocellulose was immobilized with laccase for dyes treatment [155]. After laccase immobilization, malachite green and Congo red dyes' removal efficiencies remarkably increased due to the effective enzymatic degradation. The decolorization rate of laccase modified nanocellulose for malachite green and Congo red was 92% and 51%, respectively. The work highlighted the integration of adsorption and biodegradation as unique strategies for efficient environmental remediation. Besides, the laccase-modified nanocellulose showed superior reusability for at least 18 consecutive runs. Urease was immobilized onto the dialdehyde cellulose through a Schiff base reaction between the amine groups of the enzyme and the aldehyde groups of the modified cellulose spheres [156]. The urease-modified cellulose exhibited superior selective adsorption and removal of urea from aqueous solutions with a higher water affinity and better reusability. The percentage attributed to adsorption versus removal can reach 12.54%, and the maximum removal capacity for urea was 276.24 mg/g. Dopamine functionalized cellulose/alginate was immobilized with laccase for bisphenol A removal from polluted water [157]. After the laccase immobilization, the enzymatic activity could reach 462 U/g and stabilities improved dramatically. The laccase immobilized cellulose/alginate could be easily separated and reused, showing 79.6% of its initial activity after 14 cycles of operation. The removal efficiency of bisphenol reached up to 98.7%. The chitin-lignin was immobilized with lipase from Aspergillus niger to hydrolyze para-nitrophenyl palmitate [158]. The lipase immobilized chitin-lignin had a hydrolytic activity of 5.72 mU, and increased thermal and pH stability compared with the native lipase. The lipase immobilized chitin-lignin were also shown to retain approximately 80% of its initial catalytic activity, even after 20 reaction cycles. Tomato peroxidase was immobilized on concanavalin A-cellulose to degrade direct dyes [159]. For two commercial dyes, direct dye 23 (DR 23) and direct dye 80 (DB 80), the maximum removal efficiencies by Tomato peroxidase immobilized cellulose were 93% and 76%, respectively. Cellulose was immobilized with turnip peroxidase for degradation of direct dyes [160], in which the maximum dye degradation rate can reach 93% for Direct Red 23, and the catalytic efficiency remained about 64% of the original efficiency after eight cycles. All of these works have highlighted the effectiveness of biocatalyst immobilization in biomaterial as an effective approach for environmental remediation. The further integration of 3D structure design will allow the materials to further enhance the bioremediation capacities.

Microbial immobilized biomass material

Similar to immobilizing enzymes, the whole organism can be immobilized for contaminant remediation purposes. Cell immobilization techniques include adsorption, entrapping, and covalence methods. In brief, adsorption is based on the interactions of non-specific forces (surface tension and adhesion) between the carrier and microbial surface functional groups (e.g., hydroxy group, carboxyl group and carbonyl group) [161]. A recent example used Bacillus WHX-1 immobilized biochar, which improved cadmium remediation performance in soils [162]. Adsorption based on non-specific forces is low-cost, easy to operate and has little influence on cell activities. But the drawback is that cells can easily fall off with unstable immobilizations [163]. Another example includes an ultralight and lipophilic microorganism-loaded sunflower receptacle-based porous foam, which was used to selectively adsorb petroleum pollutants. Subsequently, the immobilized microorganism degraded the petroleum chemicals with a degradation rate as high as 86.65% within eight days [164]. Compared to adsorption, entrapment is an enhancement of cell immobilization that the microbes are entrapped in the natural or synthesized polymer carrier grid [161]. Xie et al. encapsulated the combination of Ganoderma lucidum mycelia pellets with corn-cobs in the hydrophobically modified Ca-alginate (CA), in which poly-E-caprolactone (PCL) was used to modified the CA and remove anthracene from soil without enzyme purification. In the study, the removal of anthracene from soil reached about 96% after incubation under pH 5.0 and 45°C for 20 days [165]. Covalence immobilization is the most stable mobilization technique among all microbial immobilization technologies. The covalent bonding method forms stable chemical bonds between the functional groups on microbial cells and the groups on the surface of the solid phase [166]. Glutaraldehyde was used to crosslink the bacteria Pseudomonas putida and the biochar, which successfully removed pesticides from contaminated water. However, in the literature, covalent bonding is more generally applied to immobilize enzymes rather than microbes. The state-of-art bio-nanomaterial development focuses on immobilizing microbes on nanostructures, where the nanostructures primarily serve as a vehicle or carrier for the microbial physical localization [167,168].

These studies laid down the foundation of biomaterial-microorganism systems for treatment train integration. Overall, in remediation applications, cell immobilization performed better than no cell remediation and free cell remediation [169,170]. The addition of functional groups (e.g., Fe-dopped sludge biochar) typically has better immobilization performance [171]. The covalent immobilization mechanism remains unclear and needs more research efforts. Further studies could be devoted to microbes group immobilization and synergistic effects of different microorganisms in remediation. Despite the progress, the limitation of microbial immobilization or entrapment is that external nutrients need to be added to promote microbial growth. Recent advancements have enabled innovative treatment train integration, where the biomass-based sorbent can serve as microbial nutrients.

Emerging biomass-based materials to enable ultimate treatment train integration

Different from other traditional biotechnologies to design biomaterials, one critical advancement is utilizing the biological material and organism as a model for purposes other than they serve in nature [172]. To this end, novel bio-design of adsorption materials would require a deep understanding of the mechanisms to achieve biomimetic and bioinspired properties. As such, future biotechnologies to engineer novel materials for environment cleanup calls for new bio-design principles. Novel bio-inspired materials could be motivated by natural symbiotic or parasitic biological systems. One recent systematic design of biomaterial reflects a reverse engineering principle, which relied on a multiple component framework, "Renewable Artificial Plant for In-situ Microbial Environmental Remediation", dubbed RAPIMER [173] (Figure 2). RAPIMER is a plant-derived biomimetic nano-framework which achieves a high adsorption efficiency for PFAS and other co-existing contaminants to enable synergistic fungal degradation. Based on the reverse engineering principle, RAPIMER was designed as a biomimetic biodegradable composite that resembles the plant cell wall structure to feed the bioremediation fungus. Considering many PFAS molecules are negatively charged, the lignin was modified by grafting polyethylenimine via Mannich reaction. The chemical modification on lignin favored the PFAS chemical interaction with the composite, and the 3D design enabled the spatial accommodation of multiple contaminants, which contributed to the high adsorption capacity. Because RAPIMER was derived from plant biomass, and resembled the cell wall, the material itself thus served as the fungal feedstock and enhanced the fungal bioremediation capacity. This example exemplifies a sustainable perspective on developing new adsorption materials that would be biodegradable by nature. One great advantage is the RAPIMER system can potentially contain the contaminant adsorption, concentration, and degradation in the same space, which could overcome the treatment train limitations. Furthermore, using renewable agricultural waste residues for manufacturing adsorption materials, material degradability, contaminant degradation, and lack of secondary pollutants provide additional environmental benefits compared with the traditional adsorbent treatments.

Figure 2.

The concept and mechanism of the RAPIMER system for contaminant treatment. RAPIMER is a plant-derived biomimetic nano-framework which achieves a high adsorption efficiency for PFAS and other co-existing contaminants to enable synergistic fungal degradation The components of the RAPIMER system are cellulose and lignin, which are reverse engineered to develop the composite. The RAPIMER composite works as the sole carbon source to sustain fungus growth and the adsorbed PFAS is synergistically biodegraded. (Reproduced with permission from Li et al. [173]).

(From top middle) Corn stover residual lignin is crafted with polyethylenimine to form modified lignin particles. (Bottom right) Cellulose nanofibrils and modified lignin form the RAPIMER composite. (Bottom left) Contaminated water is treated with RAPIMER to remove PFAS. RAPIMER absorbs PFAS and feeds it to the fungus Irpex lacteus.

Concluding remarks

The traditional environmental remediation materials derived from biomass carbonization heavily rely on adsorption to remove organic or inorganic contaminants. Thus, the recovery and regeneration capacity are desired adsorbent properties to minimize the treatment cost. Furthermore, incineration and landfill have been the primary disposal approach to process the spent adsorbents [174], which is not environmentally friendly. Nevertheless, future material designs should consider the whole life cycle of the remediation materials, especially the spent adsorbents (see Outstanding Questions).

Outstanding questions.

1. How can we address the whole life cycle of environmental remediation materials, which are hazardous materials after enriching contaminants at very high concentrations?

2. Can we utilize sustainable bio-based materials to remediate environmental contamination and eliminate secondary pollution, while not compromising remediation efficiency?

3. How to design a cost-effective degradable media that can accomplish contaminant removal and degradation synergistically?

4. Can we simplify the treatment train approach and achieve contaminant removal and degradation by designing multifunction biomaterials?

Recent biotechnology developments have catalyzed the biomaterial design for environmental remediation. Compared with the thermal biomass process, functional modification of biopolymers has enabled more diversified design to improve adsorption capacity during environmental remediation. Biotechnology has profoundly transformed traditional biosorbent development to allow treatment train integration. Enzymes and microbes with degradation capacities can be immobilized into lignocellulosic biomass-based, cellulose- or lignin-based materials to achieve synergistic and enhanced contaminant enrichment and degradation. Furthermore, biotechnology and biomaterial design have advanced new types of biomimetic and biomass-based materials to provide multitudes of benefits in that the biomaterial can concentrate pollutants from the environmental matrices, present them to the bioremediation microbes, feed the organism as the nutrient, and enhance the contaminant degradation. Using abundant lignocellulosic biomass presents a potential low-cost route to manufacture economic degradable biomimetic materials with high adsorption efficiency. The multifunctional materials thus can integrate pollutant removal and degradation in one system, which has the potential to remove the treatment train approach. The future biomaterial design will extend the consideration to the whole life cycle of such media, maximizing the positive environmental outcome and promoting sustainability.

Highlights.

Traditional biobased adsorbents heavily rely on biomass carbonization and aim at high adsorption capacity and recyclability.

Chemical or biological modifications of lignocellulosic biomass enable diversified remediation material designs. Integrating biomolecules such as enzymes or microbes with lignocellulosic biomass facilitates contaminant removal and degradation in the same space.

While most of the available biomass-derived remediation materials can efficiently remove pollutants from the target environmental matrices, they do not address the whole life cycle of the spent adsorbents. Degradable biomimetic adsorbents, which also serve as the feedstock for bioremediation organisms, can potentially eliminate the treatment train approach and sustainably remediate contaminants.

Coupling bioremediation-capable bioagents and adsorptive media enable a partial treatment train approach for environmental remediation where contaminant adsorption, enrichment, and degradation happens in the same space.

The whole life cycle for environmental remediation media is critical for sustainability. Biodegradable adsorptive materials present a new perspective to eliminate multiple steps of hazardous material processing and maximize the environmental benefits.

Glossary

Biochar:

carbon enriched material resulting from biomass carbonization.

Lignocellulosic biomass:

plant-based material that is primarily made of cellulose, hemicellulose, and lignin, and typically not used for food or feed.

PFAS:

per- and polyfluoroalkyl substances are synthetic organic compounds with multiple fluorine atoms attached to the carbon alkyl chain.

Pyrolysis:

thermal degradation of an organic material in the absence of oxygen.

Technology readiness level (TRL):

a metric for measuring technology maturity, used by many US government agencies to assess technologies for adoption, on a scale from one to nine with nine being the most mature.

Treatment train:

multiple technologies are used in sequential order to process environmental contaminants.