Sustainable Low-Concentration Arsenite [As(III)] Removal in Single and Multicomponent Systems Using Hybrid Iron Oxide-Biochar Nanocomposite Adsorbents-A Mechanistic Study

Abstract

Rice and wheat husks were converted to biochars by slow pyrolysis (1 h) at 600 °C. Iron oxide rice husk hybrid biochar (RHIOB) and wheat husk hybrid biochar (WHIOB) were synthesized by copyrolysis of FeCl₃-impregnated rice or wheat husks at 600 °C. These hybrid sorbents were characterized using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, physical parameter measurement system, and Brunauer-Emmett-Teller (BET) surface area techniques. Fe₃O₄ was the predominant iron oxide present with some Fe₂O₃. RHIOB and WHIOB rapidly chemisorbed As(III) from water (∼24% removal in first half an hour reaching up to ∼100% removal in 24 h) at surface Fe-OH functions forming monodentate ≡Fe-OAs(OH)₂ and bidentate (≡Fe-O)₂AsOH complexes. Optimum removal occurred in the pH 7.5-8.5 range for both RHIOB and WHIOB, but excellent removal occurred from pH 3 to 10. Batch kinetic studies at various initial adsorbate-adsorbent concentrations, temperatures, and contact times gave excellent pseudo-second-order model fits. Equilibrium data were fitted to different sorption isotherm models. Fits to isotherm models (based on R ² and χ²) on RHIOB and WHIOB followed the order: Redlich-Peterson > Toth > Sips = Koble-Corrigan > Langmuir > Freundlich = Radke-Prausnitz > Temkin and Sips = Koble-Corrigan > Toth > Redlich-Peterson > Langmuir > Temkin > Freundlich = Radke-Prausnitz, respectively. Maximum adsorption capacities, Q _RHIOB ⁰ = 96 μg/g and Q _WHIOB ⁰ = 111 μg/g, were obtained. No As(III) oxidation to As(V) was detected. Arsenic adsorption was endothermic. Particle diffusion was a rate-determining step at low (≤50 μg/L) concentrations, but film diffusion controls the rate at ≥100-200 μg/L. Binding interactions with RHIOB and WHIOB were established, and the mechanism was carefully discussed. RHIOB and WHIOB can successfully be used for As(III) removal in single and multicomponent systems with no significant decrease in adsorption capacity in the presence of interfering ions mainly Cl^-, HCO₃ ^-, NO₃ ^-, SO₄ ^2-, PO₄ ^3-, K⁺, Na⁺, Ca²⁺. Simultaneous As(III) desorption and regeneration of RHIOB and WHIOB was successfully achieved. A very nominal decrease in As(III) removal capacity in four consecutive cycles demonstrates the reusability of RHIOB and WHIOB. Furthermore, these sustainable composites had good sorption efficiencies and may be removed magnetically to avoid slow filtration.

Figure 1

SEM–EDX spectra of the hybrid biochars before and after As(III) adsorption: (a) unloaded RHIOB, (b) unloaded WHIOB, (c) As(III)-loaded RHIOB, and (d) As(III)-loaded WHIOB biochar composites.

Figure 2

SEM micrographs of RHIOB at different magnifications: (a) 200× and (b) 2k× before As(III) adsorption and (c) 500× and (d) 2k× after As(III) adsorption.

Figure 3

SEM micrographs of WHIOB at different magnifications: (a) 500× and (b) 2k× before As(III) adsorption and (c) 1k× and (d) 3k× after As(III) adsorption.

Figure 4

Full range XPS spectra of RHIOB after As(III) adsorption. High-resolution deconvoluted XPS spectra of RHIOB: (a) Fe 2p, (b) O 1s, (c) C 1s, and (d) As 3d.

Figure 5

Full range XPS spectra of WHIOB after As(III) adsorption. High-resolution deconvoluted XPS spectra of WHIOB: (a) Fe 2p, (b) O 1s, (c) C 1s, and (d) As 3d.

Figure 6

TEM micrograph of RHIOB after As(III) adsorption (a) at 100k× magnification, (b) lattice fringes of Fe₃O₄, and (c) SAED pattern of the 220 plane of Fe₃O₄. TEM micrograph of WHIOB after As(III) adsorption (d) at 100k× magnification, (e) lattice fringes of Fe₃O₄, and (f) SAED pattern of the 220 plane of Fe₃O₄.

Scheme 1. Dissociative and Associative Paths for Monodentate As(III) Complex to a Monodentate Corner-Sharing ²C or ¹V Complex Formation

Scheme 2. Dissociative and Associative Paths for a Monodentate As(III) Complex Conversion to a Bidentate Edge-Sharing ²C or ¹V Complex

Figure 7

XRD spectra of (a) RHB, (b) WHB, (c) RHIOB after As(III) adsorption, (d) WHIOB after As(III) adsorption, (e) RHIOB before As(III) adsorption, and (f) WHIOB before As(III) adsorption and pure magnetite (i) akagenite (β-FeOOH), (ii) hematite (α-Fe₂O₃), (iii) pure magnetite (Fe₃O₄), and (iv) different mineral oxides (Table 2).

Figure 8

FTIR spectra of (a) RHIOB after As(III) adsorption, (b) RHIOB before As(III) adsorption, (c) WHIOB after As(III) adsorption, and (d) WHIOB before As(III) adsorption.

Figure 9

pH dependence of As(III) on RHIOB and WHIOB (adsorbent dose = 2.0 g/L; initial As(III) concentration = 100 μg/L; agitation speed = 100 rpm; temperature = 25 °C; contact time = 24 h) and fractional composition curves for As(III) speciation.

Figure 10

Langmuir nonlinear adsorption isotherms of As(III) adsorption by (a) RHIOB and (b) WHIOB at different temperatures (pH = 7.5; concentration = 100 μg/L; temp = 25 °C; agitation = 100 rpm; particle size = 0.3–0.5 mm (30–50 B.S.S. mesh size).

Figure 11

As(III) adsorption on (A) RHIOB and (B) WHIOB in the absence and presence of individual interfering ions at 100 and 200 mg/L [adsorbent dose: 2.0 g/L for RHIOB and 1.0 g/L for WHIOB; temp.: 25 °C, initial pH: 7.5, agitation speed: 100 rpm, contact time: 24 h].

Figure 12

As(III) adsorption on (A) RHIOB and (B) WHIOB in the absence and presence of combined interfering ions at Cl^–/NO₃^–/SO₄^2–/HCO₃^–/PO₄^3–/Na⁺/K⁺/Ca²⁺: 100:100:100:100:100:100:100:100 mg/L and Cl^–/NO₃^–/SO₄^2–/HCO₃^–/Na⁺/K⁺/Ca²⁺: 100:100:100:100:100:100:100:100 mg/L [adsorbent dose: 2.0 g/L for RHIOB and 1.0 g/L for WHIOB; temp. = 25 °C; initial pH = 7.5; agitation speed = 100 rpm; contact time = 24 h].

Figure 13

Percent As(III) adsorption/desorption in four consecutive cycles for RHIOB and WHIOB [adsorption studies were conducted at adsorbent dose = 2.0 g/L; initial As(III) concentration = 100 μg/L; initial pH = 7.5; agitation speed = 100 rpm; temperature = 25 °C; and contact time = 24 h; desorption studies were carried out using 50 mL of wash solution prepared by mixing 0.1 N NaOH + 0.1 N NaCl].