Mesoporous Biopolymer Architecture Enhanced the Adsorption and Selectivity of Aqueous Heavy-Metal Ions

Abstract

Halloysite nanotubes (HNT) and ball-milled biochar (BC) incorporated biocompatible mesoporous adsorbents (HNT-BC@Alg) were synthesized for adsorption of aqueous heavy-metal ions. HNT-BC@Alg outperformed the BC, HNT, and BC@Alg in removing cadmium (Cd), copper (Cu), nickel (Ni), and lead (Pb). Mesoporous structure (∼7.19 to 7.56 nm) of HNT-BC@Alg was developed containing an abundance of functional groups induced from encapsulated BC and tubular HNT, which allowed heavy metals to infiltrate and interact with the adsorbents. Siloxane groups from HNT, oxygen-containing functional groups from BC, and hydroxyl and carboxyl groups from alginate polymer play a significant role in the adsorption of heavy-metal ions. The removal percentage of heavy metals was recorded as Pb (∼99.97 to 99.05%) > Cu (∼95.01 to 90.53%) > Cd (∼92.5 to 55.25%) > Ni (∼80.85 to 50.6%), even in the presence of 0.01/0.001 M of CaCl₂ and Na₂SO₄ as background electrolytes and charged organic molecule under an environmentally relevant concentration (200 μg/L). The maximum adsorption capacities of Ni, Cd, Cu, and Pb were calculated as 2.85 ± 0.08, 6.96 ± 0.31, 16.87 ± 1.50, and 26.49 ± 2.04 mg/g, respectively. HNT-BC@Alg has fast sorption kinetics and maximum adsorption capacity within a short contact time (∼2 h). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping exhibited that adsorbed heavy metals co-distributed with Ca, Si, and Al. The reduction of surface area, pore volume, and pore area of HNT-BC@Alg (after sorption of heavy metals) confirms that mesoporous surface (2-18 nm) supports diffusion, infiltration, and interaction. However, a lower range of mesoporous diameter of the adsorbent is more suitable for the adsorption of heavy-metal ions. The adsorption isotherm and kinetics fitted well with the Langmuir isotherm and the pseudo-second-order kinetic models, demonstrating the monolayer formation of heavy-metal ions through both the physical sorption and chemical sorption, including pore filling, ion exchange, and electrostatic interaction.

Figure 1

Properties of adsorbents (HNT-BC@Alg): (a) XRD pattern of (i) HNT-BC@Alg (pre-adsorption); (ii) HNT-BC@Alg (post-adsorption); surface properties based on nitrogen gas adsorption–desorption isotherm: (b) incremental and cumulative pore area of the HNT-BC@Alg; (c) sorption and desorption of N₂ gas by HNT-BC@Alg (pre-sorption and post-sorption of heavy metals); and (d) incremental and cumulative pore volume of the HNT-BC@Alg.

Figure 2

Thermal stability (a) and derivative weight loss (b) of HNT, BC, HNT-BC@Alg, and bare alginate beads (alg).

Figure 3

SEM image of the adsorbents (HNT-BC@Alg): (a–c) SEM image of HNT-BC@Alg beads under lower to higher magnification; (d–f) cross-sectional SEM image of HNT-BC@Alg under lower to higher magnification.

Figure 4

(a–d) Comparative sorption performances of adsorbents HNT, BC, BC@Alg, and HNT-BC@Alg (where m/v = 5 g/L, T = 12 h, pH = 5.80 ± 0.1, C₀ = 20 mg/L of Pb, Cu, Cd, Ni).

Figure 5

Isotherm study for heavy-metal removal onto HNT-BC@Alg: (a) Langmuir isotherm model fitting; (b) Freundlich isotherm model fitting (where m/v = 3 g/L, C₀ = 0.5 to 80 mg/L of Pb, Cu, Cd, Ni, T = 12 h, and pH = 6.25 ± 0.1).

Figure 6

(a) Pseudo-first-order (PFO) and (b) pseudo-second-order (PSO) kinetics study for heavy-metal sorption by HNT-BC@Alg (where m/v = 3 g/L, T = 0.08 to 24 h, pH = 6.25 ± 0.1, and C₀ = 20 mg/L of Pb, Cu, Cd, Ni).

Figure 7

(a) Effect of solution pH on heavy-metal removal from water (where m/v = 5 g/L, T = 12 h, C₀ = 20 mg/L of Pb, Cu, Cd, Ni). (b) Effect of background electrolytes on adsorption of heavy metals in environmentally relevant concentrations (C₀ = 200 μg/L of Pb, Cu, Cd, Ni; T = 12 h; m/v= 20 g/L, and background electrolytes: i = in the absence of electrolytes; ii = in the presence of 0.001 M Na₂SO₄; iii = in the presence of 0.001 M CaCl₂; iv = in the presence of 0.01 M Na₂SO₄; v = in the presence of 0.01 M CaCl₂; vi = in the presence of 1 mg/L methylene blue; vii = in the presence of 10 mg/L methylene blue).

Figure 8

Effect of adsorbent dose on the (a) adsorption capacity and (b) removal percentage of heavy metals (where m/v = 1.0 g/L (20 mg in 20 mL) to 20 g/L (400 mg in 20 mL), T = 12 h, pH = 5.80 ± 0.1, and C₀ = 20 mg/L of Pb, Cu, Cd, Ni).

Figure 9

SEM and EDS spectra of the adsorbents. SEM image of HNT-BC@Alg: (a) before adsorption and (c) after adsorption. Elemental spectra, mass percent, and atomic percentage of HNT-BC@Alg (b) before adsorption and (d) after adsorption.

Figure 10

(a) TEM images of HNT-BC@Alg (post-adsorption of heavy-metal ions); (b–i) elemental mapping of (a) for C, Ca, Al, O, Si, Pb, Cd, and Ni, respectively.

Figure 11

FTIR spectra of (a) BC, (b) HNT, (c) HNT-BC@Alg (post-sorption), and (d) HNT-BC@Alg (pre-sorption).

Figure 12

Schematic demonstration of the halloysite nanotubes (HNT) and ball-milled biochar (BC)-incorporated alginate biopolymer beads (HNT-BC@Alg).