The Development of Fe3O4-Monolithic Resorcinol-Formaldehyde Carbon Xerogels Using Ultrasonic-Assisted Synthesis for Arsenic Removal of Drinking Water

Abstract

Inorganic arsenic in drinking water from groundwater sources is one of the potential causes of arsenic-contaminated environments, and it is highly toxic to human health even at low concentrations. The purpose of this study was to develop a magnetic adsorbent capable of removing arsenic from water. Fe₃O₄-monolithic resorcinol-formaldehyde carbon xerogels are a type of porous material that forms when resorcinol and formaldehyde (RF) react to form a polymer network, which is then cross-linked with magnetite. Sonication-assisted direct and indirect methods were investigated for loading Fe₃O₄ and achieving optimal mixing and dispersion of Fe₃O₄ in the RF solution. Variations of the molar ratios of the catalyst (R/C = 50, 100, 150, and 200), water (R/W = 0.04 and 0.05), and Fe₃O₄ (M/R = 0.01, 0.03, 0.05, 0.1, 0.15, and 0.2), and thermal treatment were applied to evaluate their textural properties and adsorption capacities. Magnetic carbon xerogel monoliths (MXRF600) using indirect sonication were pyrolyzed at 600 °C for 6 h with a nitrogen gas flow in the tube furnace. Nanoporous carbon xerogels with a high surface area (292 m²/g) and magnetic properties were obtained. The maximum monolayer adsorption capacity of As(III) and As(V) was 694.3 µg/g and 1720.3 µg/g, respectively. The incorporation of magnetite in the xerogel structure was physical, without participation in the polycondensation reaction, as confirmed by XRD, FTIR, and SEM analysis. Therefore, Fe₃O₄-monolithic resorcinol-formaldehyde carbon xerogels were developed as a potential adsorbent for the effective removal of arsenic with low and high ranges of As(III) and As(V) concentrations from groundwater.

Keywords: adsorption; arsenate and arsenite; carbon xerogels; resorcinol-formaldehyde; sonication.

Figure 1

Powder X-ray diffraction patterns of the xerogels (RFX), Fe₃O₄-monolithic resorcinol-formaldehyde xerogels synthesized through direct sonication (MX1 and MX2), and indirect sonication with different R/C (MC1-MC4), along with their corresponding JCPDS card assignments.

Figure 2

SEM images of RF gels between (a) inside and (b) outside.

Figure 3

SEM mages of Fe₃O₄-monolithic resorcinol-formaldehyde xerogels prepared by (a,c) direct (MX1) and (b,d) indirect (MC4) ultrasonication with magnifications.

Figure 4

SEM and EDX images of Fe₃O₄-monolithic resorcinol-formaldehyde xerogels prepared by (a,c) direct (MX1) and (b,d) indirect (MC4) ultrasonication.

Figure 5

SEM Images of Fe₃O₄-monolithic resorcinol-formaldehyde xerogels with R/C ratios of (a) 50 (MC1) and (b) 200 (MC4).

Figure 6

Particle size distribution of RF xerogel and Fe₃O₄-monolithic resorcinol-formaldehyde xerogels prepared by the sol-gel method under ultrasonic irradiation and presented in (a) grouping and (b) separation graphs.

Figure 7

FTIR spectra of RF xerogel and Fe₃O₄-monolithic resorcinol-formaldehyde xerogels prepared by (a) direct and indirect sonication ultrasonication-assisted and (b) varying of R/C molar ratios.

Figure 8

Effect of pH on the adsorption of As(V) using Fe₃O₄-monolithic resorcinol-formaldehyde xerogels (Condition: initial concentration 100 µg/L, dose 1 g/L, 6 h, and temperature 25 °C).

Figure 9

SEM images and EDX mapping analysis of magnetic xerogels prepared with R/C = 200, R/W = 0.04, and varying M/R of MX3 = 0.03 (a,f), MX4 = 0.05 (b,g), MX5 = 0.1 (c,h), MX6 = 0.15 (d,i), and MX7 = 0.2 (e,j), respectively.

Figure 10

XRD patterns of magnetic xerogels prepared by (a) direct (MX3-MX7) and (b) indirect (MX8-MX11) ultrasonic with varying M/R ratios of 0.03–0.2.

Figure 11

FTIR Spectra of magnetic xerogels prepared by (a) direct (MX4-MX7) and (b) indirect sonication (MX8-MX11).

Figure 12

As(V) removal using Fe₃O₄-monolithic resorcinol-formaldehyde xerogels prepared by (a) direct sonication with low power output and (b) indirect sonication with high power output. (Conditions: initial concentration 200 µg/L, pH of 3, dose 2 g/L, 6 h, and temperature 25 °C).

Figure 13

SEM and EDAX images of (a,b) Fe₃O₄-Monolithic resorcinol-formaldehyde xerogels (MXRF), and (c,d) Fe₃O₄-Monolithic resorcinol-formaldehyde carbon xerogels (MXRF600).

Figure 14

(a) N₂ adsorption isotherms and (b) pore size distributions of xerogel (RFX) and Fe₃O₄-monolithic resorcinol-formaldehyde xerogels (MXRF).

Figure 15

FTIR analysis before and after adsorption of As(III) of (a) magnetic xerogels of resorcinol formaldehyde (MXRF) and (b) magnetic carbon xerogels of resorcinol formaldehyde (MXRF600).

Figure 16

XRD diffractogram of Fe₃O₄-monolithic resorcinol-formaldehyde xerogels (MXRF) and carbon xerogel (MXRF600) before and after adsorption of As(III).

Figure 17

Adsorption kinetics using MXRF with 0.025, 0.05, and 0.075 mg/L of As(III) concentrations, pH of 3, and dosage of 2 g/L.