Green cleanup of styrene-contaminated soil by carbon-based nanoscale zero-valent iron and phytoremediation: Sunn hemp (Crotalaria juncea), zinnia (Zinnia violacea Cav.), and marigold (Tagetes erecta L. )

Abstract

Accidental chemical spills can result in styrene-contaminated soil. Styrene negatively affects human health and the environment. The objective of this study was to remediate styrene-contaminated soil using a combination of activated carbon-based nanoscale zero-valent iron (nZVI-AC) and phytoremediation by sunn hemp (Crotalaria juncea), zinnia (Zinnia violacea Cav.) and marigolds (Tagetes erecta L.). The results showed that all three plant types could potentially increase the removal efficiency of styrene-contaminated soil. At 28 days, all three plants showed complete removal of styrene from the soil with 1 g/kg of nZVI-AC, activated carbon-based nZVI synthesized by tea leaves (Camellia sinensis) (T-nZVI-AC), or activated carbon-based nZVI synthesized by red Thai holy basil (Ocimum tenuiflorum L.) (B-nZVI-AC). However, styrene removal efficiencies of sunn hemp, zinnia, and marigold without carbon-based nZVI were 30%, 67%, and 56%, respectively. Statistical analysis (ANOVA) revealed that the removal efficiencies differed significantly from those of phytoremediation alone. With the same removal efficiency (100%), the biomass of sunn hemp in nano-phytoremediation treatments differed by approximately 55%, whereas the biomass of zinnia differed by >67%, compared with that of the control experiment. For marigold, the difference in biomass was only 30%. Styrene was adsorbed on surface of soil and AC and then further oxidized under air-water-nZVI environment, while phytovolatilization played an important role in transporting the remaining styrene from the contaminated soil to the air. Marigold was used as an alternative plant for the nano-phytoremediation of styrene-contaminated soil because of its sturdy nature, high biomass, tolerance to toxic effects, and ease of cultivation. Remediation of one cubic meter of styrene-contaminated soil by a combination of carbon-based nanoscale zero-valent iron and phytoremediation by marigolds emitted 0.0027 kgCO₂/m³.

Keywords: Carbon-based materials; Nano zerovalent iron; Phytoremediation; Remediation; Styrene.

Fig. 1

TEM scanning images of nZVI (A), T-nZVI (B), and B-nZVI (C) nanoparticles.

Fig. 2

XRD patterns of green synthesized nZVI, JCPDS stands for Joint Committee on Powder Diffraction Standards). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

SEM and EDX images of AC (A and B) and nZVI-AC (C and D).

Fig. 4

Removal of 2.5 (A) and 5 (B) mg/L styrene solution with 1 g/L nZVI, nZVI-AC, T-nZVI-AC, and B-nZVI-AC in 72 h.

Fig. 5

Mass spectra of styrene and intermediate of styrene from the treatment of styrene by nZVI-AC.

Fig. 6

Proposed mechanism of the treatment of styrene by nZVI-AC in aqueous solution, (1) reduction of Fe, (2) oxidation of oxygen, and (3) Fe²⁺ catalyzation.

Fig. 7

Removal of 5 mg/kg styrene-contaminated soil using nZVI, nZVI-AC, T-nZVI-AC, and B-nZVI-AC.

Fig. 8

Relative seed germination of five plants for phytoremediation of styrene at 96 h.

Fig. 9

Nano-phytoremediation of styrene in soil with marigolds (Tagetes erecta L.), sunn hemp (Crotalaria juncea), and zinnia (Zinnia violacea Cav.).