Removal and Recovery of AgNPs from Water by Sustainable Magnetic Nanoflocculants

Abstract

The presence of silver nanoparticles (AgNPs) in water bodies has emerged as a new environmental concern and the efficient separation of these nanoparticles remains a critical challenge. Here, we developed novel magnetic nanoflocculants for the recovery of AgNPs from water. Alternating layers of biopolymers, in particular, chitosan, alginate, and polymeric bio-based soluble substances (BBS) derived from urban waste, were coated on magnetic nanoparticles via the layer-by-layer technique to prepare reusable magnetic nanoflocculants (MNFs). The MNFs obtained were characterized with diverse physicochemical techniques. Surface response methodology, based on the Doehlert matrix, has shown to be a useful tool to determine the effect of pH (in the range 5-9), concentration of AgNPs (7-20 mg L^-1), and MNFs (50-1000 mg L^-1) on the performance of AgNPs removal. The model predicts a high AgNPs removal percentage at low pH values and high MNF concentration. In particular, for the most efficient MNFs, 90% of AgNPs removal was obtained at pH 5 and 600 mg L^-1 MNF concentration. Additionally, the effects of AgNPs size, ionic strength, the presence of humic acids, and two types of surfactants (LAS anionic and TWEEN 20 nonionic) on the AgNPs removal were evaluated. Finally, recovery and reuse experiments showed that MNF made of Chitosan-BBS can be reused in ten cycles, losing only 30% of the initial removal capacity. Therefore, magnetic flocculation could represent a sustainable alternative for AgNPs separation with potential applications in water treatment and remediation of nanoparticle contamination.

Keywords: biopolymers; flocculants; magnetic nanomaterials; silver nanoparticles; water treatment.

Figure 1

(A) XRD diffraction patterns of the silica-coated magnetic iron oxide nanoparticles. (B) ATR-FTIR spectra of MNFs.

Figure 2

TEM images of MNFs: (A) MMCA5 and (B) MMCB5.

Figure 3

Magnetization curves of MNFs.

Figure 4

Zeta potential as a function of the biopolymer layer number for each MNFs. (A) MMCB5: the positive potentials represent chitosan coating, while the negative potentials represent BBS coating. (B) MMCA5: the positive potentials represent chitosan coating, while the negative potentials represent alginate coating. Nanoflocculants dispersed in 0.01 M NaCl solution at pH 5.

Figure 5

(A) Effect of MNF dosage on AgNPs removal. [AgNP1] = 13.5 mg L⁻¹, [chitosan] = 50 mg L⁻¹, pH = 5. (B) Effect of pH on AgNPs removal. [AgNP1] = 13.5 mg L⁻¹, [MNF] = 1000 mg L⁻¹.

Figure 6

Response surface plot and the isoresponse curve (at [AgNP1] = 13.5 mg L⁻¹) for MMCA5.

Figure 7

Response surface plot and the isoresponse curve (at [AgNP1] = 13.5 mg L⁻¹) for MMCB5.

Figure 8

TEM images of AgNPs after the magnetic flocculation process using MMCB5.

Figure 9

MNF performance in removing AgNP1 and AgNP2. [MNF] = 366 mg L⁻¹, [AgNPs] = 13.5 mg L⁻¹, pH = 5.

Figure 10

Effect of recycling of MNFs on the AgNP1 removal efficiency after various cycles of use. [MNF] = 1000 mg L⁻¹, [AgNP1] = 13.5 mg L⁻¹, pH = 5.