Materials preparation
The Cd-defiant microbial strains Cd-1, Cd-2, Cd-3, Cd-5, Cd-6, and Cd-7 identified in the previous study were used to carry out this research. The Cd-1, Cd-2, Cd-5, and Cd-6 were strains of Stenotrophomonas sp., while Cd-3 and Cd-7 were strains of Achromobacter sp. and Staphylococcus sp., respectively (Fan et al. 2023). All strains were stored as glycerol stocks in a deep freezer.
Preparation of Cd2+ mother liquor: CdCl2·5H2O (purity > 99%) was utilized to formulate 500 mg·L− 1 Cd2+ stock solution with sterile water.
Beef extract peptone liquid medium: The medium was prepared using beef extract powder (3 g), NaCl (5 g), peptone (10 g), and the solution was made up to 1 L using double distilled water. After tuning the pH to 7, medium was subjected to sterilization at 121°C for 20 min in autoclave. LB solid medium was sterilized after adding 1.5% agar.
Preparation of bacterial adsorbent: Firstly, the stored strains were activated. A single colony was picked and used as the inoculum for beef extract peptone liquid medium. The inoculated medium was incubated at 30°C and 140 r·min− 1 for 36 h. The bacteria were separated from the medium by centrifugation (9000 r·min− 1, 4°C, 15 min). The precipitate containing bacterial cells was washed thrice using sterilized water. After each washing, the bacterial cells were centrifuged for 15 min at 9000 r·min− 1 and then recollected in the form of precipitate. After final washing, the bacterial cells were heated at 70°C to remove moisture and then ground into powder.
Biosorption experiments
Impact of initial conditions: 0.02 g bacterial biomass was supplemented to the shake flask containing 10 mL Cd2+ solution (pH 7). Thus, the adsorbent (bacterial) dose was 2 g·L− 1. The Cd biosorption experiment was carried at 140 r·min− 1 and 30°C for 24 h. After 24 h, the supernatant was subjected to centrifugation for 15 min (9000 r·min− 1, 4°C). The Cd2+ content in the supernatant was measured through inductively coupled plasma emission spectrometer (ICP-OES, 5110, Agilent Technologies, USA). Each treatment was conducted in triplicates. The biosorption rate (R) and biosorption capacity (Q) of Cd2+ were determined by using Eq. (1) and Eq. (2) (Masoudzadeh et al. 2011):
$$\text{Q}\text{=}\frac{\text{(}\text{C}\text{0}\text{-}\text{C}\text{t}\text{)}\text{V}}{\text{m}}$$
2
Here, R is the rate of Cd biosorption (%); C0 and Ct are the initial Cd2+ content and the content after biosorption (mg·L− 1), respectively; m is bacterial dry weight (g); Q is the biosorption capacity (mg·g− 1); and V is the volume of solution (L).
Impact of pH variations: Influence of pH was assessed by using 100 mg·L− 1 Cd2+ solutions with pH set at different levels: 5, 6, 7, 8 and 9. 0.02 g microbial adsorbent was supplied to 10 mL of these Cd2+ solutions of different pH. After the biosorption, Cd2+ content in the supernatant was measured.
Impact of temperature variations: 0.02 g adsorbent was supplemented to 10 mL of Cd2+ solution (Cd2+ content: 100 mg·L− 1, pH 7). The biosorption experiment was conduction at varying temperatures of 10, 20, 30, 35 and 40°C with the flasks shaking at 140 r·min− 1 for 24 h. After biosorption, the supernatant was collected and the Cd2+ content in supernatant was determined.
Impact of biosorption time: 0.02 g adsorbent was supplemented to 10 mL of Cd2+ solution (Cd2+ content: 100 mg·L− 1, pH 7). The biosorption of Cd2+ by microbial adsorbent was allowed to occur at 140 r·min− 1 and 30°C, for different durations of 5, 10, 20, 30, 60, 120, 240 and 360 min. After biosorption for the chosen durations, the solutions were centrifuged (15 min, 9000 r·min− 1, 4°C) to obtain the supernatant and determine the Cd2+ content in it.
Impact of adsorbent dose: 100 mg·L− 1 Cd2+ solution was prepared (pH 7). Afterwards, the bacterial adsorbents were added into 10 mL Cd2+ solution at varying quantity of 0.004, 0.01, 0.02, 0.04, and 0.08 g. Therefore, the initial adsorbent concentration in these solutions were 0.4, 1, 2, 4 and 8 g·L− 1, correspondingly. After the completion of biosorption experiment, supernatant was obtained by centrifugation of solutions for 15 min (9000 r·min− 1, 4°C) and Cd2+ concentration in the supernatant was quantified.
Impact of Cd2+ concentration: The influence of initial Cd2+ content was assessed by using the Cd2+ solutions of 1, 5, 10, 20, 50, 100 and 200 mg·L− 1 concentrations (pH 7). After preparing the solutions, 0.02 g microbial adsorbent was added to 10 mL Cd2+ solutions (microbial dose of 2 g·L− 1). After 26 h of biosorption experiment, supernatant was collected from the solutions through centrifugation and the final Cd2+ content was determined.
Isothermal biosorption
Langmuir (3) and Freundlich (4) isothermal models were employed to explain the Cd2+ biosorption by the bacterial strains (Freundlich 1906; Langmuir 1918):
$$\text{q}\text{e}\text{=}\frac{\text{K}\text{L}\text{q}\text{m}\text{C}\text{e}}{\text{1+}\text{K}\text{L}\text{C}\text{e}}$$
3
Here, qe and Ce are biosorption capacity (mg·g− 1) and Cd2+ concentration (mg·L− 1) at equilibrium, correspondingly; qm is the highest biosorption capacity (mg·g− 1); and KL is the biosorption equilibrium constant (L·mg− 1).
$$\text{q}\text{e}\text{=}\text{K}\text{F}\text{C}\text{e}\text{1/}\text{n}$$
4
Here, qe and Ce are same as in Eq. (3); while KF and n are biosorption constants.
Biosorption kinetics
Cd2+ biosorption by bacterial adsorbents was fitted by employing quasi first- and second-order kinetic models. Moreover, the steps of Cd2+ biosorption responsible for restricting the biosorption rate were identified (Ho and McKay 1998; Lagergren 1898). The linear fitting equations have been shown in (5) and (6):
$$\text{ln(}{\text{q}}_{\text{e}}\text{-}{\text{q}}_{\text{t}}\text{)=ln}{\text{q}}_{\text{e}}\text{-}{\text{K}}_{\text{1}}\text{t}$$
5
Here, qe and qt represent the amount of Cd2+ (mg·g− 1) adsorbed by the bacteria at the biosorption equilibrium and at biosorption time t, correspondingly; while K1 is first-order rate constant (min− 1).
$$\frac{\text{t}}{\text{q}\text{t}}\text{=}\frac{\text{1}}{\text{q}\text{e}\text{K}\text{2}}\text{+}\frac{\text{t}}{\text{q}\text{e}}$$
6
Here, qe and qt are same as in (5), while K2 is second-order rate constant, expressed as g·mg− 1· min− 1.
Furthermore, intraparticle diffusion was determined as per Eq. (7) to understand the process of Cd2+ diffusion in the microbial adsorbents (Lim and Lee 2015):
$$\text{q}\text{t}\text{=}\text{K}\text{d}\text{t}\text{1/2}\text{+}\text{C}$$
7
Here, t represents contact time (t); qt represents biosorption capacity at time t (mg·g− 1); Kd represents the intraparticle diffusion constant (mg·g− 1·min1/2); and C is correlation coefficient of boundary layer thickness.
Biosorption thermodynamics
Variations in Gibbs free energy (ΔGθ), entropy (ΔSθ), and enthalpy (ΔHθ) are used to determine the possibility as well as spontaneity of the biosorption process (Batool et al. 2018). Therefore, the changes in these thermodynamic parameters were calculated based on the biosorption results at different temperatures. ΔGθ was determined by using Eqs. (8) and (9) (Segneanu et al. 2022).
$$\text{Δ}\text{G}\text{θ}\text{=-}\text{R}\text{T}\text{ln}\text{K}\text{c}$$
8
$$\text{K}\text{c}\text{=}\frac{\text{q}\text{e}}{\text{C}\text{e}}$$
9
$$\text{Δ}\text{G}\text{θ}\text{=}\text{Δ}\text{H}\text{θ}\text{-}\text{T}\text{Δ}\text{S}\text{θ}$$
10
The Van't Hoff equation was derived as follows:
$$\text{ln}\text{K}\text{c}\text{=-}\frac{\text{Δ}\text{G}\text{θ}}{\text{R}\text{T}}\text{+}\frac{\text{Δ}\text{S}\text{θ}}{\text{R}}$$
11
Here, C0 and Ce are the Cd2+ contents in the beginning and at biosorption equilibrium, correspondingly (mg·L− 1); R is the gas constant, Kc is the biosorption equilibrium constant (L·mol− 1), and T is temperature (K). The lnKc-1/T straight line graphs were drawn as per Eqs. (10) and (11). ΔHθ and ΔSθ were determined as per the slope and intercept of the straight line, respectively.
Statistical analysis
SPSS 26.0 was employed to conduct one-way analysis of variance (ANOVA) and Duncan test. Origin 2021 software was used to draw the graphs. Variations with p ≤ 0.05 were regarded as substantial. Data were expressed as mean ± SD of three quantifications.