Heat-treated biochar impregnated with zero-valent iron nanoparticles for organic contaminants removal from aqueous phase: Material characterizations and kinetic studies

https://doi.org/10.1016/j.jiec.2019.03.041Get rights and content

Highlights

  • Heat-treated biochar (HBC) showed improved hydrophilicity and adsorption capacity.

  • Heat treatment duration affected nZVI particles’ morphology and oxidation state.

  • pNDA bleaching efficiency determined the optimum synthesis conditions for BC/nZVI.

  • HBC/nZVI removed 88% of trichloroethylene from aqueous phase after 20 min.

Abstract

Biochar (BC) is an inexpensive and widely available carbon-based material with a variety of applications. Zero valent iron nanoparticles (nZVI), on the other hand, are highly reactive species. However, agglomeration and difficulty of separation from the treated media are the major reported drawbacks associated with nZVI application for water treatment. In this study, BC was modified by a simple heat-treatment, producing hydrophilic heat-treated biochar (HBC) with enhanced absorptive features, and was impregnated with nZVI, producing BC/nZVI composite for efficient organic contaminant removal. Synthesis conditions of BC/nZVI composite were optimized by evaluating p-nitrosodimethylaniline (pNDA) bleaching efficiency of various BC/nZVI samples synthesized under different conditions of pH, ultrasonication amplitude, and iron concentration. Variously-synthesized HBCs were then used to synthesize HBC/nZVI composites, and were characterized for surface morphology, surface chemistry, and elemental composition. The best-performing HBC/nZVI for pNDA bleaching was then used for trichloroethylene (TCE) removal from water. Using HBC/nZVI or BC/nZVI composites, the pseudo-second order model fit indicated a chemisorption mechanism for organic contaminants removal. Using 250 mg L−1 of the best-performing HBC/nZVI, an 88% TCE reduction (initial concentration of 40 μg L−1) was achieved after 20 min at pH = 3.0, with a rate of 3.318 g mg−1 min−1.

Introduction

Contamination of groundwater and soil by chlorinated organic compounds (COC) is a current environmental issue [1]. This is due to the adverse health effects of COCs such as carcinogenicity and toxicity [2], as well as difficulties associated with the remediation of COC-contaminated sites in comparison with other hydrocarbons [1]. One COC of interest is trichloroethylene (TCE), used in various industrial applications such as fabricated metal parts degreasing, paint stripping, and pharmaceutical and plastic manufacturing [3], [4]. Due to serious human health hazards, including liver problems and increased risk of cancer [2], the United States environmental protection agency (USEPA) considered TCE as a priority pollutant, and defined its maximum contaminant level at 5 μg L−1 in drinking water [5]. Many different methods have been investigated for TCE removal from the aqueous phase, including adsorption [1], [6], Fenton reactions [7], [8], [9], [10], [11], [12], biological treatments [13], [14], and reduction [15], [16]. Removal of TCE from groundwaters is crucial because groundwater is the main source of drinking water in most areas all over the world. Groundwater remediation using traditional pump and treat methods has several disadvantages, including back diffusion, tailing, rebound, and long treatment periods, causing significant environmental footprints and high energy consumption [17], [18], [19]. To overcome these drawbacks, in-situ remediation techniques have recently been developed for a more sustainable treatment of contaminated aquifers [17], [20].

Biochar (BC) is an inexpensive carbon-based adsorbent produced from an oxygen-limited thermal or hydrothermal process of carbonaceous biomass [21]. BC, with a highly-ordered multiple interconnected networks of micropores, mesopores and macropores, has been recently considered as an economical substitute for conventional adsorbents, such as activated carbon, and used to remove a wide variety of contaminants, including VOCs, from aqueous, gaseous, and solid phases [21], [22], [23], [24], [25]. However, because the surface functional groups and the elemental composition of BCs depend on the biomass source as well as the carbonization process [21], BCs may have limited selectivity to adsorb some groups of contaminants [26]. Hence, various modification methods have been investigated for BCs to increase their removal capacities [27], [28], [29]. Ahmed et al. [26], classified BC modification methods aiming to enhance its aqueous contaminants removal performance into three main groups: (i) steam activation, in which the feed material is firstly pyrolyzed at 300–700 °C under N2, limited air, or sometimes ambient air, followed by steam activation at 800–900 °C for 0.5 to 3 h [30]; (ii) heat treatment, in which BCs are heated to elevated temperatures of 800–900 °C, and then hydrogen, air, or argon is introduced to BC to form new functional groups; and (iii) chemical modification, including alkaline and acidic modifications–both of which require subsequent pyrolysis–, as well as impregnation techniques in which the metal salts or oxides are mixed with BCs. Li et al. [31] reviewed synthesis methods and applications of metal-biochar composites, and proposed investigating the effects of biochars and biochar-composites synthesis factors on their sorptive properties for future studies. Tan et al. [32] also reviewed synthesis methods and applications of activated biochars for activated carbon production, and suggested designing biochars for specific target contaminants and characterizing them in the future studies.

Nanoscale zero valent iron (nZVI) particles are highly reactive species consisting of elemental iron/iron oxide [33], capable of remediating a wide variety of environmental contaminants from soil and water, including organic contaminants [33], [34], [35], [36] and heavy metals [37], [38]. These nano particles have also attracted special interest for in-situ remediation applications [17], [33], [39].

In the case of TCE contact with ZVI, two pathways have been considered for dechlorination [33], [40]: (i) beta-elimination, which results in TCE transformation to ethane and short-lived intermediates, while partially-dechlorinated by-products such as dichloroethene (DCE) and vinyl chloride (VC) are prevented; and (ii) sequential degradation, which is a step-wise removal of chlorine atoms, resulting in TCE degradation to cis-1,2 DCE, then to VC, and lastly to ethene and ethane; the reactions in this pathway are Fenton reagent oxidation.

Beta-elimination is the favorable process for TCE removal, because it produces non-persistent intermediates. On the other hand, an anaerobic condition is favorable for ZVI reductive reactions [33], as it decelerates the oxidative passivation of ZVI. Due to the dissolved oxygen demand from organic biodegradation activities, most of the contaminated groundwater aquifers have nearly anaerobic conditions [41]. Consequently, reduction of TCE by nZVI is potentially an effective approach for in-situ groundwater remediation.

Agglomeration of nZVI particles due to their magnetic properties, as well as their interaction with other materials in the surrounding environment results in reduced surface area and reactivity and thus, decreased efficiency [16], [37], [42]. Immobilization of nZVI particles onto a porous media, such as activated carbon [37], [43], biochar [9], [44], carbon nanotubes [45], SBA-15 [46], and bentonite [47] has been reported to significantly improve nZVI performance for aqueous contaminants removal.

In the present study, a two-step modification process was employed to enhance biochar’s performance for organic contaminants removal, consisting of: (1) heat treatment of BC, to make its surface hydrophilic, expecting to enhance its adsorption capacity, and (2) immobilization of nZVI particles onto the BC, to enhance its contaminant removal efficiency through Fenton-oxidation/reduction by Fe0.

In the heat-treatment step, BC particles were subjected to a simple heat-treatment process, under ambient air at 300 °C for various time periods of between 2–24 h, to produce a variety of heat-treated BCs (HBCs). The surface functional groups of the resultant HBCs were analyzed, and their hydro-phobicity/philicity was compared.

In the impregnation step, various composites of BC and nZVI–designated as BC/nZVI–, were synthesized using the raw BC, under different conditions of pH, ultrasonication amplitude, and iron concentration. In order to determine the synthesis conditions under which the most-efficient BC/nZVI could be produced, p-nitrosodimethylaniline (pNDA) bleaching efficiency of different BC/nZVI samples were compared.

Next, the two modification processes were combined: HBCs were used to synthesize various HBC/nZVI composites under the identified optimized conditions. The pNDA bleaching efficiency of various HBC/nZVIs was then compared, and the best-performing HBC/nZVI was selected for the trichloroethylene (TCE) removal experiments. Production of the chlorinated compound intermediates was traced, and kinetic studies were performed for both the pNDA and TCE removal. To the best knowledge of the authors, this is the first study which optimizes a BC/nZVI composite for aqueous organic contaminant removal by modifying the surface functional groups of the BC using a simple heat-treatment and optimizing the BC/nZVI composite’s synthesis conditions through varying the three most-affecting synthesis factors.

Section snippets

Chemicals

The biochar used in the present study was purchased from Biochar Now company (Berthoud, CO). The particle size of the biochar was in the range of 26–50 mesh. It was produced from beetle killed pine trees from National Forests in the United States, at the pyrolysis temperature between 550 °C and 600 °C under limited oxygen for 8 h. Iron (III) chloride hexahydrate (FeCl3 6H2O, 97% assay), sodium hydroxide (NaOH, 97.0% assay), p-nitrosodimethylaniline (pNDA), and trichloroethylene (TCE- ACS

Batch experiments for pNDA bleaching

In order to identify the optimum conditions for BC/nZVI synthesis yielding the greatest organic contaminant removal, the efficiencies of various samples were investigated using pNDA bleaching as the probe.

Bleaching of the spin-trap pNDA has been extensively investigated in the literature as a hydroxyl radical (radical dotOH) probe compound [54], [55], [56], [57], assuming that the bleaching of the yellow chromophoric groups in pNDA is solely due to the attack of hydroxyl radicals (radical dotOH) [57]. The pNDA is

Conclusions

In this study, a commercial biochar (BC) was modified by heat treatment, and impregnated with nZVI particles to produce an efficient carbon-based composite material for aqueous organic contaminant removal, and potentially for remediating TCE-contaminated groundwater aquifers. The FTIR analysis along with the water-droplet contact angle measurement showed that the hydrophilicity of biochar was enhanced by heating it at 300 °C, and that the heating time significantly affected the degree of its

Authors contribution

S. Mortazavian synthesized the materials, carried out pNDA and TCE experiments and analyzed experimental data, conducted kinetic studies, conducted FTIR, SEM, and sessile drop tests, analyzed XPS data, contributed in GCMS measurements, wrote the manuscript and discussed all the results. T. Jones-Lepp helped in GCMS measurement and the associated data analysis, and contributed in writing the GCMS measurement method under the materials and methods section and reviewed the manuscript. J.H. Bae and

Declaration of interest

None.

Acknowledgements

This work was supported by the U.S. Department of Energy Minority-Serving Institution Partnership Program (MSIPP), managed by the Savannah River National Laboratory under SRNS Contract No. 000033-29-78. The authors would like to thank Dr. Erica Marti, Assistant Professor at UNLV, for helping with GCMS analysis, and also Autumn Pietras for helping in conducting TCE and pNDA experiments. The authors greatly appreciate the anonymous reviewers of this manuscript for their constructive suggestions

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