Keywords

1 Introduction

Lignin constitutes one of the main component of lignocellulosic biomass (15–30% by weight), which is second only to cellulose in mass on the earth. Lignin acts as the essential glue that gives plants their structural integrity and resistance against microbial, chemical attack and prevents other outside stresses from destroying the structure of plant cell walls [1]. Lignin is a complex and recalcitrant phenolic macromolecule composed of three phenylpropane units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), cross-linked by β-o-4, α-o-4, β-β, and 5-5′ bonds etc. (as depicted in Fig. 1). Among them, the main linkage is the β-o-4 bond, about 40–60% of all inter-unit linkages in lignin belong to this bond [2]. The component and structural characterizations of lignin are quite different even it is separated from the same plant. It is to say, the lignin obtained from a plant is a mixture of different lignin polymers which is dependent on the source plant, species, and growing ages of the plant, such as softwood, hardwood, and grass [3].

Fig. 1
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a The phenylpropane type units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S); and b β-o-4, c α-o-4, d β-β, and e 5-5′ linkages in lignin

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A large quantity of technical lignin was produced as a main component in black liquor by the pulp and papering industry, which is a big threat to the environment [4]. It is estimated that the global annual production of technical lignin in pulp making is ca. 70 million tons, much of which is consumed as a low-value fuel. Although there are some other applications, such as a binder or dispersing agent, no large-scale application has so far been found [4]. Lignin has significant potential as a source for the production of the bio-renewable polymer [5, 6]. The main active sites of lignin are comprised of phenolic or alcoholic hydroxyl groups, which accounts for its reactivity, hydrophilicity as well as other chemical and physical attributes of lignin [2]. Recently, due to the unique polyphenol structure, chemical stability and wide availability, different kinds of adsorbents, particularly for dyes and heavy metals removal from wastewater are potentially obtainable from lignin [7].

Water pollution by different kinds of contaminants is currently serious concerned [8,9,10,11,12,13]. A large amount of organic, inorganic, and biological compounds have been reported as water contaminants. For example, the dyes, particularly azo dyes are recalcitrant molecules and resistant to aerobic digestion and are also stable to oxidizing agents that makes them being hard to treat [14]. The heavy metals, lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr) and arsenic (As), etc., are notorious with high toxicity and carcinogenicity. These toxic pollutants, even with a trace amount in water, can pollute water resources and transfer throughout the food chain to accumulate in animals and mankind, causing various diseases and disorders [15]. Therefore, the pollutants should be removed before its discharge into the environment.

Several physical and chemical treatment techniques have been reported for the removal of pollutants from wastewater, including precipitation, oxidation, biodegradation, ion exchange, adsorption [16], membrane separation [17,18,19], coagulation and flocculation, flotation, electrochemical methods. These methods still suffer from the limitations of low effectiveness and high cost.

Adsorption using cost-effective adsorbents is now considered as an efficient, convenient and economical method for wastewater treatment, due to the feasibility in design and operation, effectiveness in treating and the recyclability of the adsorbents [20, 21]. Activated carbon (AC) is being recognized as one of the most popular adsorbents used in the removal of contaminants from water. However, AC is of high capital cost due to the prolong production process [22]. In recent years, the development of various environmentally friendly adsorbents from biomass, biopolymers, especially the natural-occurring polymers, including cellulose, lignin, and chitosan, etc., has been intensified [23,24,25]. It could be seen from the literature that the adsorption of dyes and heavy metals using lignin is one of the most reported methods for the removal of contaminants from water [26,27,28].

Unfortunately, original lignin is less competitive in adsorption applications due to the low adsorption capability, and lignin does not own specific selectivity for a certain kind of pollutant from complex polluted water [29, 30]. The goal for the lignin modification for pollutants adsorption is to promote the adsorption capability, including adsorption capacity, stability, selectivity and recyclability [15]. The ease of separation from wastewater after the operation and the cost-effectiveness of the adsorbents should also be considered. The modification of lignin could be achieved by various methods including crosslinking, hybridization, hydrogen bonding formation, condensation, grafting and copolymerization [31]. Therefore, the main objective of this article is to collect and compile the most recent reported literature regarding the modified lignin for adsorption applications and to discuss the advantageous and disadvantageous issues of the resulting lignin as an adsorbent to remove o different environmental pollutants from the aqueous phase.

2 Modified Lignin for Dyes Adsorption

Dyes contaminated wastewater is hard to treat because of their inert properties and the residual trace amount of dyes in water. Recently, adsorption techniques using modified lignin have been widely reported in removing dyes as a promising wastewater treatment process. The dyes adsorption capacities of the modified lignin-based adsorbents are tabulated in Table 1. And the modified lignin-based materials for removing dyes will be fully discussed in the following section.

Table 1 Reported modified lignin for adsorption of dyes in water
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Nair et al. [32] reported a novel chitosan-alkali lignin composite prepared from chitosan and alkali lignin (as shown in Fig. 2). The weak interactions between β-1,4-glycosidic linkage, amide and hydroxyl groups of chitosan, and ether, an aromatic ring and hydroxyl groups of alkali lignin, impart enhanced surface and chemical properties to the composite than chitosan and alkali lignin. The composite with 50:50 chitosan: alkali lignin exhibited maximum adsorption (111.11 mg/g) of Remazol Brilliant Blue R (RBBR) compared to chitosan (76.92 mg/g), which showed that the composite exhibits 33% improvement in the maximum adsorption amount. Batch adsorption of RBBR on the composite followed the Langmuir equation, while the dynamic adsorption followed the pseudo-second-order equation. A lignin-chitosan pellet (ALiCE) was prepared for adsorption of methylene blue (MB) [33]. The results indicated the ALiCE had a maximum adsorption capacity of 36.25 mg/g for MB according to the fitting of the Langmuir equation (R2 = 0.997), yielding. The adsorption kinetic data could be fitted well by the pseudo-second-order-model [33]. A chitin/lignin hybrid biosorbent was prepared and used to adsorb C.I. Direct Blue 71 (DB71) from water [28]. The adsorption capacities of DB71 by the chitin and chitin/lignin hybrid biosorbent were 30.7 and 40 mg/g, respectively, which indicated that the hybrid chitin/lignin material has a stronger adsorptive affinity for DB 71 than the chitin. The adsorption of DB 71 onto the hybrid could be described by the Freundlich equation and the pseudo-second-order equation, respectively. The optimum solution pH for dye removal was in the range 2.4–8.4. The adsorption of DB 71 by the chitin/lignin hybrid was spontaneous and endothermic in nature [28].

Fig. 2
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An illustrative diagram of chitosan-alkali lignin composite. Reprinted with permission [32]. Copyright 2014 Elsevier

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Lou et al. [34] developed a ternary graft copolymer based on chitosan, acrylamide, and lignin (CAML) by a microwave-assisted method (as shown in Fig. 3). The copolymer was loosely aggregated powder with a particle size range of 1–3 μm. Adsorption experiment showed that the CAML (chitosan:acrylamide:lignin = 1:1:1) exhibited maximum removal efficiency of 99.3 and 67.0% for reactive orange C-3R and methyl orange, respectively. The CAML had a wide suitable pH range, although the removal efficiency was slightly higher under lower pH values. The removal mechanism was combined charge neutralization and bridging effects [34]. Guo et al. [48] prepared a lignin-based flocculant (LBF) via grafting dimethyldiallylammonium chloride and acrylamide onto lignin. Adsorption results demonstrated that a high removal efficiency of 93% was achieved for the reactive dye by LBF with the addition of PAC. The high removal efficiency of dye by LBF was because of the electrostatic attraction effect and bridging action [48].

Fig. 3
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Synthetic process of CAML. Reprinted with permission [34]. Copyright 2018 Elsevier

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Li et al. [35] prepared magnetic lignin spheres (MLS) from different organosolv lignins with maleic anhydride and Fe3O4 nanoparticles. The adsorption amounts of methylene blue and Rhodamine B by the MLS from larch lignin (31.23, and 17.62 mg/g, respectively) were higher than that from poplar lignin (25.95 and 15.79 mg/g, respectively). The adsorption kinetics and isotherm were could be described by the pseudo-second-order equation and Langmuir equation, respectively. Moreover, the MLS from larch and poplar lignin had a good recyclability, after three cycles of adsorption-desorption, the removal efficiencies for the dyes still remained at 98 and 96%, respectively [35].

Couch et al. obtained [36] oxidized lignin products from softwood lignin by using HNO3 (as shown in Fig. 4). The products were used to remove ethyl violet (EV) and basic blue (BB) dyes from simulated water. The results showed that the dyes removal efficiency was 70–80 wt% for EV and 80–95 wt% for BB within the dyes concentrations of 50 and 400 mg/L. The dye removal was pH and ionic strength dependent. Feng et al. [41] prepared a methylene blue (MB) adsorbent from acetic acid lignin (AAL) via deacetylation in NaOH aqueous solution followed by fractionation in methanol. The maximum adsorption capacity of MB reached to 63.3 mg/g by the AAL. In addition, the adsorption of MB was pH and dosage dependent.

Fig. 4
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Scheme of oxidized softwood lignin products. Reprinted with permission [36]. Copyright 2016 American Chemical Society

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Xu et al. [37] developed an acrylic-lignosulfonate resin (ALR) from calcium lignosulfonate and acrylic acid. The resin had a high surface area of 190.55 m2/g with a porous structure in an average pore diameter of 11.34 nm. The maximum adsorption capacity of crystal violet (CV) by the ALR was 150.40 ± 4.80 mg/g at 25 °C. The kinetic and equilibrium data could be fitted by the pseudo-second-order equation and Freundlich equation, respectively. In addition, the calculation of thermodynamic parameters demonstrated the adsorption of CV on ALR was exothermic and spontaneous in nature.

Wang et al. [38] prepared a hybrid hydrogel from lignin grafted with acrylamide and N-isopropyl acrylamide as well as montmorillonite (lignin-g-p(AM-co-NIPAM)/MMT, as shown in Fig. 5). The prepared hydrogel presented thinner pore walls, good thermal stability and strong mechanical strength due to the existence of montmorillonite. The hydrogel showed an excellent removal efficiency for methylene blue in water, as indicated by the maximum adsorption capacity of 9646.92 mg/g. The adsorption was dependent on the pH and temperature. Dynamic adsorption could be described by the pseudo-second-order equation, while both of the Langmuir and Freundlich equations could describe the equilibrium adsorption. Furthermore, the hydrogel showed an excellent reusability within five adsorption-desorption cycles.

Fig. 5
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Diagram of synthesis of lignin-g-p(AM-co-NIPAM)/MMT. Reprinted with permission [38]. Copyright 2017 Elsevier

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Yu et al. [39] also prepared an LS-g-AA hydrogel by grafting of acrylic acid on lignosulfonate backbone using N,N′-methylene-bis-acrylamide and laccase/t-BHP (tert-butyl hydroperoxide) as cross linker and initiator respectively (Fig. 6). The successful grafting of the monomer was confirmed using FTIR. The prepared hydrogel showed a high adsorption capacity of 2013 mg/g for methylene blue (MB) dye in water. Besides, excellent reusability was shown by the LS-g-AA hydrogel with adsorption capacity of 1757 and 1681 mg/g for 3 and 4 cycles, respectively.

Kumari et al. [40] reported a pine needle lignin-based polyurethane foam (LPUF) as an adsorbent of dyes removal from water. The experiment results showed that the LPUF was effective in removing a cationic dye, malachite green (MG) with a maximum adsorption capacity of 80 mg/g, other than an anionic dye, methyl orange (MO), from the water. The adsorption kinetics and isotherms could be described well by the pseudo-second-order equation and Langmuir equation, respectively. Moreover, the LPUF could be used for 20 regeneration cycles with a cumulative adsorption capacity of 1.33 g/g to MG (Fig. 7).

Fig. 6
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Synthesis of LS-g-AA hydrogel from lignosulfonate and acrylamide catalyzed by laccase. Reprinted with permission [39]. Copyright 2016 Elsevier

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Fig. 7
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Schematics of LPUF synthesis and application. Reprinted with permission [40]. Copyright 2016 Royal Society of Chemistry

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Tang et al. [42] have reported their studies on the preparation of a lignin sulfonate polymer by a simple emulsion polymerization method and the adsorption properties of the lignin sulfonate polymer towards malachite green (MG) dyes (as shown in Fig. 8). The obtained lignin sulfonate polymer showed an effective adsorption of MG with a maximum adsorption capacity of 60.2 mg/g for MG according to the Langmuir equation. A Lignosulfonate-based mesoporous material was further prepared by grafting of acrylic acid and acrylamide onto the backbone of lignosulfonate for adsorbing MG from the water. This synthesized material was mesoporous confirmed by using N2 adsorption/desorption curve. The BET surface area was 118 m2/g and the mesopores are in an average diameter of 3.8 nm. Due to the presence of pores in the mesoporous material, it showed an enhanced adsorption capacity, 150.376 mg/g for MG.

Fig. 8
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Schematic diagram of the lignin sulfonate polymer. Reprinted with permission [43]. Copyright 2016 Royal Society of Chemistry

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Song et al. [44] prepared a hydrogel consisted of acylated hemicelluloses, acrylic acid, and sodium lignosulfonate by using initiator ammonium persulfate and N,N,N′,N′-tetramethylethane-1,2-diamine. The honeycomb-like morphology was observed in the prepared hydrogel. The adsorption kinetics of methylene blue (MB) by the hydrogel was fitted well with pseudo-second-order kinetics and the isotherm was fitted well with the Langmuir isotherm model, respectively. The adsorption capacity of MG by the hydrogel could reach to 2691 mg/g. Even after a further cycle, the hydrogel exhibited an approximately 80% adsorption efficiency for MG, and accordingly, it was proposed to be a promising material for dye removal from wastewater [44].

Adebayo et al. prepared carboxyl-methyl lignin CML from the acid hydrolysis lignin from sugarcane bagasse (as shown in Fig. 9) [46]. The CMLs was further bound with Al(III) (CML-Al) and Mn(II) (CML-Mn) for the removal of Procion Blue MX-R (PB) in aqueous solutions. The experiment optimum pH and contact time were 2.0 and 5 h, respectively. The CML-Al and CML-Mn showed a maximum adsorption capacity of 73.52 and 55.16 mg/g for PB at 25 °C, respectively. 98.33% of CML-Al and 98.08% of CML-Mn could be regenerated from dye-loaded adsorbents by using 50% acetone + 50% of 0.05 mol L−1 NaOH. After four adsorption-desorption cycles, the removal efficiency of the dyes still remained ca. 93.97% and ca. 75.91% by the CML-Al and CML-Mn, respectively. Silva et al. [47] also reported a carboxy-methylated lignin complexed with Fe3+ (CML-Fe) for the adsorption of Brilliant Red 2BE (BR-2) textile dye from aqueous solutions. The maximum adsorption capacity of BR-2 was 73.6 mg/g by the CML-Fe adsorbent. Besides, the dye-loaded adsorbent could be recycled with 0.050 mol/L NaOH.

Fig. 9
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Schematic of Carboxyl-methyl lignin (CML) from lignin and monochloroacetic acid. Reprinted with permission [46]. Copyright 2014 Elsevier

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3 Modified Lignin for Heavy Metals Adsorption

Heavy metal ions are more toxic than dyes and cover a wider region of pollutants during the past decades. Modification of lignin by introducing the desired properties in physical, chemical and mechanic properties is a key issue to achieve proper characters of lignin including the hydrophilicity, hydrophobicity and adsorption ability. The most common strategies in the modification of lignin for heavy metal ion adsorption include amination, methylolation, alkylation, carboxylation, acylation, sulfonation/sulfomethylation, phosphorylation, and copolymerization etc.

Ge et al. [49] prepared an aminated lignin-based adsorbent from alkaline lignin grafted by methylamine and formaldehyde, as shown in Fig. 10. Kinetic adsorption suggested the aminated lignin could adsorb Pb(II) in water quickly and the adsorption process could be described well by a second-order model. The obtained adsorbent presented a maximum adsorption amount of 60.5 mg/g for Pb(II) that was 4.2 fold of the original alkaline lignin. Ge et al. [50] further investigated the influence of numbers of the carbon in an alkyl, from C2 (ethyl) to C18 (octadecyl), on the adsorption capacity of lignin for the lead ion. The results indicated that the carbon number had a strong influence on the adsorption of Pb(II). A suitable carbon number of alkyl (C4) helped the adsorption of Pb(II), due to the electron donating ability of the alkyl groups. Recently, Huang et al. [51] reported a modified enzymatic hydrolysis lignin-containing nitrogen and sulfur moieties, which showed a high adsorption capacity of 180 mg/g for Hg(II) at 25 °C. The adsorption kinetics could be fitted well with the pseudo-second-order equation, while the adsorption isotherms could be fitted well with the Freundlich equation. Besides, the increasing value of the constant (n) with temperature indicated the adsorption became more favourable at a higher temperature.

Fig. 10
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Synthetic diagram of a Mannich base from lignin. Reprinted with permission [49]. Copyright 2015 Elsevier

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Carboxyl is a common component of lignin. The frequency of the group can be adjusted by chemical modifications as well as the hydrophilicity and polyelectrolyte characters. Quintana et al. [52] investigated the oxidized lignins from sulfuric acid pretreated cane bagasse, soda pulping bagasse, eucalypt Kraft lignin and commercial Kraft lignin. The results indicated the oxidized lignins showed higher adsorption capacities than the original lignins, due to the higher contents of carboxyl groups in the oxidized lignins. Peternele et al. [53] reported a functionalized formic lignin from sugarcane bagasse for the adsorption of Pb(II) and Cd(II). Batch adsorption equilibrium could be described well by the Langmuir equation. The oxidative modification of a wheat straw organosolv lignin has been investigated by Dizhbite et al. [54] to introducing –COOH and –OH groups. The oxidation was conducted under a polyoxometalate H3[PMo12O40] and O2 or H2O2. The oxidative modification did not damage the lignin skeleton, while the carboxyl and hydroxyl group’s contents in lignin increased distinctly. As expected, the oxidized lignin showed high adsorption amounts of 35.9 and 155.4 mg/g toward Cd(II) and Pb(II), respectively, at pH 5, 20 °C.

Sulfonate functional groups can be introduced into lignin via sulfomethylation and sulfonation, which has been reported as an effective way to improve the hydrophilicity of lignin [55, 56]. Li et al. [29] prepared a modified lignin-containing both amino and sulfonic groups (as shown in Fig. 11). It was found that the modified lignin could adsorb heavy metals effectively even at low pH values. The dynamic adsorption and equilibrium adsorption could be fitted well by the pseudo-second-order equation and D–R equation, respectively. Xu et al. [57] developed a mesoporous lignin-based biosorbent (MLBB) from rice straw. The MLBB had an excellent adsorption performance for Pb(II) with a maximum capacity of 952 ± 31 mg/g at 20 °C, which was due to the large surface area (186 m2/g), plenty of mesopores (dp = 5.5 nm) and high content of sulfonic groups (S: 2.51 ± 0.01%)].

Fig. 11
figure 11

Synthetic diagram of a modified lignin-containing amino and sulfonic groups. Reprinted with permission [29]. Copyright 2014 Elsevier

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Dithiocarbamated lignin is the most reported modified lignin due to its good adsorption ability toward heavy metal ions. Ge et al. [58] developed a lignin-based dithiocarbamate (LDTC), as shown in Fig. 12. The developed LDTC showed a high adsorption amount of 175.9 and 103.4 mg/g toward copper and lead ions, respectively. Li et al. [15] developed a porous lignin-containing a large number of mesopores and functional groups. The surface area of the modified lignin increase 11 fold of lignin to 22.3 m2/g. Accordingly, the modified lignin had a high adsorption capacity of 188 mg/g to Pb(II), 13 fold of the lignin and 7 fold of activated carbon. Ge et al. [27] further prepared a dithiocarbamate modified lignin from organosolv lignin. The modified lignin showed a high adsorption amount of 210 mg/g to Hg(II). The adsorption kinetics could be fitted well by a pseudo-second-order equation, and the adsorption equilibrium could be fitted well by the Freundlich equation. Li et al. [59] prepared a lignin xanthate resin (LXR) with xanthate functional groups (–CSS−) (as shown in Fig. 13). The adsorption capacity of Pb(II) was 64.9 mg/g at pH 5.0, 30 °C.

Fig. 12
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Synthetic diagram of dithiocarbamated lignin. Reprinted with permission [58]. Copyright 2016 Royal Society of Chemistry

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Fig. 13
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Synthetic diagram of the lignin xanthate resin (LXR). Reprinted with permission [59]. Copyright 2015 Elsevier

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Ge et al. [26] prepared a new kind of lignin microspheres (LMS) through an inverse suspension copolymerization method. The LMS was in diameter of 348 μm with plenty of amine groups (total N: 7.5 mmol/g). LMS showed an adsorption capacity of 33.9 mg/g for lead ions at pH 6.0, 25 °C, and the adsorption kinetics could be fitted well by the pseudo-second-order equation. Liang et al. [60] prepared a lignin-based resin (LBR) from sodium lignosulfonate and glucose. The maximum adsorption capacity of Cr(VI) by the LBR was 57.68 mg/g. The adsorption equilibrium could be fitted well by the Freundlich equation. The calculated thermodynamic parameters of the adsorption (ΔG, ΔH and ΔS) indicated that the adsorption of Cr(VI) by the LBR is a spontaneous and endothermic process. Parajuli et al. [61] developed a crosslinked lignocatechol gel from catechol and wood lignin. The gel showed a saturated adsorption amount of 37.05 mg/g toward Pb(II) at pH 5.2. The adsorption process was mainly due to the cation exchange mechanism.

Lignin composites are now considered as a talented candidate to traditional adsorbents for cleanup of heavy metal ions in water. Qin et al. [62] synthesized a composite from poly (ethylene imine) and lignin. The composite presented a high adsorption amount of 98.0 mg/g to Cu(II), 78.0 mg/g to Zn(II) and 67.0 mg/g to Ni(II) at pH 6.0, 25 °C. In addition, the composite had good recyclability stability within 5 adsorption-desorption cycles. Klapiszewskia et al. [63] prepared new TiO2/lignin and TiO2-SiO2/lignin hybrids. The adsorption capacity of Pb(II) by the TiO2/lignin and TiO2-SiO2/lignin was 35.7 and 59.9 mg/g, respectively, at pH 5.0, 20 °C. Kinetic analysis revealed that the adsorption followed by the pseudo-second-order equation that meant a chemical interaction occurred during the adsorption process. Equilibrium adsorption was well described well by the Langmuir equation that meant a monolayer coverage of the adsorbates on the homogeneous surface of the adsorbents. Li et al. [64] prepared a composite lignin sphere from sodium alginate and epichlorohydrin. The obtained sphere showed an excellent removal efficiency (95.6 ± 3.5%, C0 = 25.0 mg/L) to Pb(II). Li et al. [65] reported a new nano-composite composed of lignin and carbon nanotubes (L-CNTs). The as-prepared nano-composite not only showed a good water-dispersibility and environmentally friendliness but also presented an excellent adsorption ability to Pb(II) with a maximum adsorption amount of 235 mg/g. The nano-composite with a lignin layer has advanced adsorption ability, low in cost and environmentally friendliness, and therefore is a talented alternative for wastewater treatment. Klapiszewski et al. [66] synthesized a ‘green’ adsorbent with a high surface area of 223 m2/g from the commercial silica Syloid®244 and Kraft lignin for the removal of nickel(II) and cadmium(II). The results indicated the adsorbents had maximum sorption capacities of 77.11 mg/g for Ni(II) and 84.66 mg/g for Cd(II), respectively. The kinetics adsorption data could be described well by a pseudo-second-order equation while the adsorption isotherms fitted well with Langmuir equation. Yao et al. [67] synthesized a composite composed of bentonite/sodium lignosulfonate with acrylamide and maleic anhydride (BLPAMA). Results showed that the adsorption of Pb(II) by the BLPAMA was correlated with pH values but not with the temperatures. It showed a maximum adsorption capacity of 314.8 mg/g for Pb(II) at pH 5.0, 25 °C.

The above mentioned modified lignin adsorbents for adsorption of heavy metals are tabulated in Table 2, from which it can be seen that the adsorption capacity was much dependent on the chemical methods for lignin modification and heavy metals species as well as the temperature and pH values. According to the analysis of the published literature, the research on the development of advanced lignin-based adsorbents, especially lignin-based nano-composites, is greatly needed in the coming decades.

Table 2 Comparison of adsorption capacity of heavy metals with modified lignins
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4 Modified Lignin for Other Pollutants Adsorption

Except for the above discussed main pollutants (dyes and heavy metals), some other pollutants by lignin-based adsorbents are also reported in literature during the past decades. Although there are few papers on the topic, the removal of some other toxic and hazardous pollutants is also very important. Saad et al. [70] investigated two lignins, including alkaline lignin and organosolv lignin, for the adsorption of 2,4-dinitroanisole (DNAN) from the water. The adsorption of DNAN on both lignins could be described well with pseudo-second-order equation. The organosolv lignin showed a maximum adsorption amount of 7.5 mg/g to DNAN while the alkaline lignin showed a maximum adsorption amount of 8.5 mg/g to DNAN. The adsorption equilibrium for either alkali or organosolv lignin could be described well by the Freundlich model.

Chen et al. [71] reported spherical lignin beads as an adsorbent to adsorb L-lysine in water. The results indicated the beads showed a maximum adsorption capacity of 67.11 mg/g to L-lysine. Dynamic adsorption was fitted well by the pseudo-first-order equation, and the adsorption of L-lysine on the beads was initially determined by film diffusion, and then by intra-particle diffusion. Adsorption equilibrium was described well with the Langmuir equation.

Żółtowska-Aksamitowska et al. [72] firstly investigated the use of chitin modified Kraft lignin as an effective sorbent of ibuprofen and acetaminophen. Batch adsorption results indicated the modified lignin showed an adsorption capacity of 400.39 μg/g to ibuprofen and 267.07 μg/g to acetaminophen, respectively. Adsorption isotherms data could be described with the Langmuir equation for ibuprofen, and with the Freundlich equation for acetaminophen, while the adsorption kinetics followed well to the pseudo-second-order equation for both pollutants (R2 = 0.999). These results indicated that the adsorption belonged to a chemisorption. Furthermore, the used adsorbents could be easily regenerated with ethanol (yield 82.2%) in the case of ibuprofen and methanol (yield 80.8%) in the case of acetaminophen.

Application of the oxidative modification of sulfate kraft lignin with sodium periodate under mild conditions is suggested in order to obtain a sorbent for detoxication of spillage places of rocket fuels based on 1,1-dimethylhydrazine and to purify wastewaters containing this compound [73]. It was found that processing with sodium periodate at 55 °C for 20 h resulted in a more than two-fold increase in the content of carbonyl and quinone groups in lignin and a three-fold increase in the adsorption capacity for 1,1-dimethylhydrazine. The adsorbent can bind 6.7% of 1,1-dimethylhydrazine and substantially surpasses in this parameter other lignin-based adsorbents.

Table 3 listed all these reported modified lignin-based adsorbents for adsorption of some other pollutants in water. As could be seen, the adsorption capability of lignin-based adsorbents for other species is much lower than those for dyes and heavy metals. Therefore, it is in a great need of developing new lignin-based adsorbents with enhanced adsorption capability toward the emerging pollutants in the future.

Table 3 Adsorption capacity of modified lignin for other pollutants in water
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5 Outlook and Conclusions

With the depletion of fossil-based resources and increasing environmental concerns, the research and development of new low-cost adsorbents derived from renewable resources have received more and more attention. The utilization of aromatic lignin in replace of the fossil-based carbon will help to the establishment of the sustainable society. In this content, the adsorption of dyes, heavy metals and some other pollutants by various modified lignin materials have been collected and discussed in this study. Based on the above discussions, adsorption performance of these modified lignin materials for dyes, heavy metals, and other pollutants is expected to be amplified in the near future. It should be noted that the cost-effectiveness is firstly important because low manufacturing cost and high adsorption capability of an adsorbent is desired in practical large-scale applications. Lignin is an abundant polymer derived from plant kingdom that can provide CO2 neutral, cost-effective, environmentally friendly and therefore can be used as building blocks to create “green” adsorbents. Secondly, although modified lignin is efficient to capture different kinds of pollutants through physical or chemical interactions, regeneration by feasible methods should be studied carefully as it is very important for the improvement of economics. The regeneration of lignin-based materials could be carried out by the most common solvent extraction method with EDTA, HCl, HNO–, NaCl, and NaOH solutions et al. Thirdly, experiments should not only stay in the lab. As all known, the industrial wastewater always contains many kinds of contaminants. Although different kinds of modified lignin adsorbents can be obtained with good adsorption capacity and selectivity for the target contaminants, such as methylene blue (MB), toxic metals (Hg), and (Pb, Cu) et al., it is desirable in developing a multipurpose adsorbent from lignin that can be used to adsorb different kinds of contaminants simultaneously. Therefore, specific attention should be focused on the modification of lignin matrix, via hybriding, cross-linking and grafting to develop lignin-based advanced composite, and accordingly broadening the kinds of pollutants for efficient removal and improving the reusability of the modified lignin-based composites.