Rhamnolipids as Green Stabilizers of nZVI and Application in the Removal of Nitrate From Simulated Groundwater

Abstract

Environmental contamination caused by inorganic compounds is a major problem affecting soils and surface water. Most remediation techniques are costly and generally lead to incomplete removal and production of secondary waste. Nanotechnology, in this scenario with the zero-valent iron nanoparticle, represents a new generation of environmental remediation technologies. It is non-toxic, abundant, cheap, easy to produce, and its production process is simple. However, in order to decrease the aggregation tendency, the zero-iron nanoparticle is frequently coated with chemical surfactants synthesized from petrochemical sources, which are persistent or partially biodegradable. Biosurfactants (rhamnolipids), extracellular compounds produced by microorganisms from hydrophilic and hydrophobic substrates can replace synthetic surfactants. This study investigated the efficiency of a rhamnolipid biosurfactant on the aggregation of nanoscale zer-valent iron (nZVI) and its efficiency in reducing nitrate in simulated groundwater at pH 4.0. Two methods were tested: 1) adding the rhamnolipid during chemical synthesis and 2) adding the rhamnolipid after chemical synthesis of nZVI. Scanning electron microscopy field emission, X-ray diffractometry, Fourier transform infrared spectroscopy, thermogravimetric analysis, Dynamic Light Scattering, and zeta potential measurements were used to characterize bare nZVI and rhamnolipid-coated nZVI. The effects of the type of nZVI and initial NO₃ concentration were examined. Nanoscale zer-valent iron with the addition of the rhamnolipid after synthesis achieved the best removal rate of nitrate (about 78%), with an initial nitrate concentration of 25 mg L^-1. The results suggest that nZVI functionalized with rhamnolipids is a promising strategy for the in situ remediations of groundwater contaminated by NO₃, heavy metal, and inorganic carbon.

Keywords: NZVI; groundwater; nitrate removal; rhamnolipids; stabilizer.

FIGURE 1

FTIR spectrum of rhamnolipids.

FIGURE 2

X-ray diffraction peaks associated with nZVI particles were recorded: freshly synthesized samples (lighter colour) and 30-day old samples (darker colour). The Pattern Diffraction File (PDF) of metallic iron (PDF 87-7194), lepidocrocite (PDF 8-98) and, magnetite (PDF 74-419) were displayed as well.

FIGURE 3

Zeta potential and pzc of nZVI.

FIGURE 4

Average particle diameter size distribution of nZVI particles.

FIGURE 5

SEM images of (A) bare-nZVI (B) nZVI-A (C) nZVI-S.

FIGURE 6

TGA and DTA curves for (A) bare-nZVI, (B) nZVI-A and (C) nZVI- S.

FIGURE 7

Effect of time and initial nitrates concentration on nitrates reduction using nZVI at pH 4 (A) 25 mg/L NO₃ (B) 50 mg/L NO₃ (C) 100 mg/L NO₃ and effect of time and initial nitrates concentration on ammonia concentration using nZVI at pH 4 (D) 25 mg/L NO₃ (E) 50 mg/L NO₃ (F) 100 mg/L NO₃.