Bio-adsorbent preparation based on Chinese Radix isatidis residue for Pb(II) removal

In recent decades, water pollution caused by heavy metals – for example, lead, cadmium, chromium and mercury – has become a serious problem because of their adverse effects on ecological systems and human health (Badruddoza et al. 2013a; Afkhami et al. 2010). The major concern with heavy metals is that most are highly toxic, carcinogenic and mutagenic at even relatively low concentrations (Ahmed et al. 2013; Badruddoza et al. 2013b). They are also non-biodegradable and tend to accumulate in organisms (Squadrone et al. 2016; Liu et al. 2018), making them indirectly harmful to human beings in some degree.

The treatment methods for heavy metal pollutants include adsorption, precipitation, nano-filtration, ion exchange, osmosis, etc (Jurado-Sánchez et al. 2015). Adsorption with various synthesized materials, such as activated carbon (Nowicki et al. 2015), carbon nanotubes (Ge et al. 2014; Yang et al. 2015), graphene (Nagarjuna et al. 2015) and metal-organic frameworks (MOFs) (Abney et al. 2014) has emerged as a promising wastewater treatment process. However, high cost and low reusability limit the widespread use of adsorbents in commercial applications (Yin et al. 2016). Conversely, natural polymer materials, including cellulose (Alatalo et al. 2015; Ganesan et al. 2016), rice husks (Vyas et al. 2013), wheat straw (Mahmood-Ul-Hassan et al. 2015), coconut shell (Huang et al. 2013), chitosan (Jiang et al. 2019), etc (Singh et al. 2018), have attracted increasing attention in heavy metal removal due to their extensive availability, eco-friendly nature and low cost (Thakur & Thakur 2015).

Cellulose is arguably the most abundant natural polymer and is the main component of plant fibres, giving plant their rigidity (Wei et al. 2014). Herb residue – for example, from Radix isatidis – is rich in cellulose and seen as a representative kind of concentrated natural resource, but also has pollution potential because it is highly susceptible to rot (Guo et al. 2013). In order to improve the potential utility of herb residues and protect the environment, various reutilisation studies have been done. Zhao & Zhou (2016) prepared a variety of bio-adsorbents to remove synthetic dyes from wastewater using an extraction residue of Salvia miltiorrhiza. Feng & Zhang (2013) used Chinese ephedra residue as a bio-adsorbent for Pb(II) removal from aqueous solutions. Li et al. (2010) extracted pine needle residues to adsorb methylene blue from aqueous solution. Although the researchers had taken advantage of the cellulose in herb residues to remove contaminants from wastewater, it is wrapped in lignin and is compact, resulting in low adsorption capacities and rates (Xue et al. 2018). Currently, physical, chemical and biological methods are commonly used to treat cellulose in natural polymer materials (Hokkanen et al. 2016; Anastopoulos et al. 2017; Yin et al. 2017).

In this study, chemical modification and fermentation treatment were used to prepare the bio-adsorbents to remove Pb(II) from wastewater. White-rot fungi, a common fermentation agent, can penetrate through lignin into the cell cavity and degrade the cellulose, hemicellulose and lignin by laccase, peroxidase and manganese peroxidise (Hildén et al. 2013; Kunjadia et al. 2016; Pamidipati & Ahmed 2017). Compared with physical and chemical methods, biological fermentation is ideal for wastewater treatment, because it causes no secondary pollution and saves energy (Yang et al. 2014; Yu & Choi 2018). In order to increase degradation efficiency, the cellulose structure was loosened with NaOH before fermentation. The Pb(II) adsorption and desorption properties of four bio-adsorbents were studied, providing a theoretical basis for the preparation of water bio-adsorbents on the basis of herb residue use.

Fresh Radix isatidis residue (RIR) was obtained from a hospital affiliated to Gansu University of Chinese medicine (Gansu, China). White-rot fungi were obtained from China General Microbiological Culture Collection Center. Hydrochloric acid, sodium hydroxide, sodium carbonate, sulphuric acid and citric acid (all A.R. grade) came from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Distilled water was produced by the laboratory's water purification system (Elix Reference, Merck Millipore, USA). All other reagents were of analytical grade, and all aqueous solutions were prepared with distilled water.

The herb's clean root slices were boiled three times with distilled water to remove all extracts. Subsequently, the extracted herb residue was treated with methanol (1:5, w/v) for removal of bioactive ingredients and pigments, this prevents them affecting the adsorption experiment. The solid residue, dried at 60 °C in an oven to constant weight, was ground after cooling to 0.5 to 1.0 mm size (referred to as D-RIR). The powders were stored in a desiccator until assay.

The D-RIR was put into a beaker with 1.0 mol L⁻¹ NaOH (10:1, w/v), which was oscillated at constant temperature for 4 hours at 120 rpm. The solid residue was washed repeatedly with deionized water until neutral, and the product was dried to constant weight at 60 °C (referred to as D-RIR-NaOH).

After sterilisation and cooling, white rot fungus was dispersed in liquid containing a known amount of D-RIR, sealed, placed in an incubator at 32 °C and rotated at 120 rpm for 9 days. The solid residue was dried at 60 °C (referred to as D-RIR-F).

The combined modification of the herb residue was conducted both chemically – NaOH – and by fermentation. The resulting adsorbent is referred to as D-RIR-NaOH-F.

The morphology of the materials' surfaces was observed using a scanning electron microscope (JSM, 5600LV, Japan), and the active functional groups were determined using a Perkin Elmer FTIR spectrometer (Thermo Fisher, USA).

The SEM images of RIR treated in different ways are shown in Figure 1. RIR modification produced significant changes. The original RIR surface was smooth, but after fermentation or alkali treatment, it was wrinkled. The surface was roughest when RIR was treated with alkali before fermentation, and there were many pores, due to destruction of the lignocellulose structure by the NaOH. Chemical/fermentation modification (Figure 1(d)) yielded particularly large numbers of micropores with structures favourable for heavy metal ion entry, which would enhance adsorption capacity.

The FTIR spectroscopy provided information on the functional groups on the RIR surface before and after modification (Figure 2). D-RIR displayed several adsorption peaks at 3,390 to 3,396 cm⁻¹ (OH stretching vibration in cellulose, hemicellulose and polysaccharide), 2,925 to 2,932 cm⁻¹ (-CH₃ and -CH₂ vibration in cellulose), 1,649 cm⁻¹ (water adsorbed in hemicellulose and cellulose), 1,031 cm⁻¹ (Si-O stretching), 1,510 cm⁻¹ (characteristic absorption peak of lignin).

After fermentation treatment, new peaks were observed at 1,239 cm⁻¹. The peaks of D-RIR-NaOH also changed significantly, indicating that alkali treatment removes lignin to a large extent. New peaks were also observed at 616 cm⁻¹ in the FTIR spectra of D-RIR-NaOH-F, suggesting that these groups are the primary active adsorption sites.

The pH of heavy metal ion solutions can affect the main metal ion species and the charge distribution on the bio-adsorbent surface (Al-Ghouti & Salih 2018; Dai et al. 2019), so adsorption capacity depends on solution pH to some extent. The adsorption capacities at different pH are shown in Figure 3. At low pH (<4), the adsorption capacity of the D-RIR-Ts increased rapidly with increasing pH. At pH 5 the adsorption capacity of D-RIR-NaOH-F reached 31.0 mg g⁻¹, about 1.5 times that of D-RIR-NaOH (23.21 mg g⁻¹), and 3 times that of D-RIR (11.9 mg g⁻¹). When pH is between 5 and 7, the adsorption capacities decreased slightly. This could be explained by the change of the surface-active sites – at low pH, the surface of bio-adsorbent was protonated, which reduced the number of binding sites that were available for uptake of metal ions in absorption. H⁺ also competes with metal ions for adsorption sites, with increasing pH, the H⁺ concentration gradually decreases, and the competition with metal ions weakens while adsorption capacity increases. At pH 5, the adsorption capacity of D-RIR-NaOH-F was highest, which might arise because pre-treatment with NaOH and fermentation led to greater adsorption capacity for Pb(II). With the addition of OH⁻ ions, the negative charges on the treated D-RIR surface increased correspondingly, producing more negative functional groups, and increasing the number of active groups that could participate in adsorption and thus improving adsorption capacity.

Figure 4 shows that there are two stages in the adsorption process: in the first 30 minutes adsorption is rapid, thereafter it is slower until equilibrium is reached. This could arise from the limited numbers of active binding sites on the bio-adsorbent surfaces (Abdel-Halim & Al-Deyab 2013). The adsorption capacity of D-RIR-NaOH-F was always the highest.

The pseudo-first-order model's calculated value for D-RIR (12.4 mg g⁻¹) was closer to the experimental value (13.7 mg g⁻¹) than that from the pseudo-second-order model. Moreover, the correlation coefficient (R₁²) was higher than R₂², indicating that adsorption was described better by the pseudo-first-order model. This, in turn, indicated that Pb(II) adsorption of D-RIR is a physical process. However, the correlation coefficients (R₂²) of D-RIR-F, D-RIR-NaOH, D-RIR-NaOH-F of the pseudo-second-order kinetic model all exceeded 0.99, and the calculated adsorption capacities were closer to the experimental data, indicating that their Pb(II) adsorption was chemical.

The initial concentration of metal ions has a significant influence on bio-adsorbent performance. To investigate this in relation to Pb(II) with respect to D-RIR-Ts, initial Pb(II) concentrations were adjusted in the range 10–100 mg L⁻¹. As can be seen in Figure 5, adsorption capacity increased gradually with increasing initial Pb(II) concentration. The increase in metal ion concentration provides an adsorption driving force for bio-adsorbents (Liu et al. 2010). When the initial concentration was increased further, the active adsorption sites tended to become saturated. The removal capacity of Pb(II) decreased in the order D-RIR-NaOH-F >D-RIR-F >D-RIR-NaOH > D-RIR, possibly due to the formation of an extracellular mucus sheath connecting mycelia and biomass, which enables the fungi to degrade lignin and hemicellulose, to expose the active groups of cellulose and increase the adsorption capacity of cellulose to metal ions (Mäkelä et al. 2013; Waliszewska et al. 2019).

Two isothermal models were used to fit the experimental data – see Table 2. The Langmuir correlation coefficient, R_L² (>0.99), for D-RIR-NaOH-F, D-RIR-F and D-RIR-NaOH were higher than the Freundlich correlation coefficients, R_F², and the calculated values using the Langmuir equation were closer to the experimental values. On this basis it could be concluded that the adsorption process was fitted better by the Langmuir isothermal model, indicating that Pb(II) adsorption in this case was a single-layer chemisorption process. However, the Freundlich correlation coefficient, R_F², was close to 1 for D-RIR, and there was little difference between the calculated and experimental values, suggesting that the Freundlich isothermal model provides a better description of D-RIR adsorption.

Table 3 is a comparison of q_max for Pb(II) obtained in this study with bio-adsorbents used in other work. The adsorption capacity of D-RIR-NaOH-F exceeds that of some other bio-adsorbents.

D-RIR-NaOH-F, the best bio-adsorbent for Pb(II) in this work, was used to study adsorption and desorption conditions. Its reusability for Pb(II) adsorption is shown in Figure 6. After five consecutive adsorption/desorption cycles, the adsorption rate was still 75% of the original level and only about 10% lower than after its first regeneration. In addition, the desorption rate of HCl was as high as 50% (Sajab et al. 2013). In other words, the recycling performance of D-RIR-NaOH-F is promising.

The bio-adsorbents based on Radix Isatidis residues to remove Pb(II) from wastewater were prepared using two modification methods. The adsorption capacities of the different adsorbents followed the order: D-RIR-NaOH-F >D-RIR-F >D-RIR-NaOH >D-RIR. The highest capacity was achieved by D-RIR-NaOH-F, at about 31.1 mg g⁻¹ in 50 minutes. The adsorption capacity of modified D-RIR remained above 75% of its initial level after five consecutive adsorption/desorption cycles. Not only can D-RIR-NaOH-F be used to remove Pb(II) from wastewater, it is also a means of reusing a common natural waste material.

This work was supported by Natural Science Foundation of Gansu Science and Technology Department (17JR5RA170) and the Foundation of Gansu Educational Committee of China (2018A-047).

All relevant data are included in the paper or its Supplementary Information.