Phenanthrene sorption studies on coffee waste- and diatomaceous earth-based adsorbents, and adsorbent regeneration with cold atmospheric plasma

Phenanthrene (PHE) is a polycyclic aromatic hydrocarbon categorized as a high priority organic pollutant being toxic for the ecosystem and human health, and its sorption on natural organic or inorganic substances seems a well-promising method for its removal from water streams. The goals of the present work are (i) to assess the capacity of low-cost adsorbents fabricated by treating coffee wastes and diatomaceous earth to remove PHE from water; (ii) to elucidate the role of the pore structure on PHE sorption dynamics; and (iii) to assess the potential to regenerate adsorbents loaded with PHE, by using the novel technology of cold atmospheric plasma (CAP). Diatomaceous earth (DE) and DE pre-treated with sodium hydroxide (NaOH) or phosphoric acid (H3PO4) were chosen as inorganic adsorbents. Coffee waste (CW) and activated carbons (AC) produced from its pyrolysis at 800 °C (CWAC), either untreated (CWAC-800) or pre-treated with NaOH (CWAC-NaOH-800) and H3PO4 (CWAC-H3PO4-800), were chosen as organic adsorbents. The adsorbents were characterized with nitrogen adsorption-desorption isotherms, attenuated total reflectance-Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and mercury intrusion porosimetry. Based on the PHE sorption capacity and pore structure/surface characteristics, the CWAC-NaOH-800 was chosen as the most efficient adsorbent for further equilibrium and kinetic sorption studies. The multi-compartment model was used to describe the PHE sorption dynamics in CWAC-NaOH-800 by accounting for the pore/surface diffusion and instantaneous sorption. The CWAC-NaOH-800 exhibited remarkable values for (i) the specific surface area (SBET = 676.5 m2/g) and meso- and micro-pore volume determined by nitrogen sorption (VLN2 = 0.415 cm3/g); (ii) the macro- and meso-pore volume determined by mercury intrusion porosimetry (VMIP = 3.134 cm3/g); and (iii) the maximum PHE sorption capacity (qmax = 142 mg/g). The percentage of adsorbent recovery after its regeneration with CAP was found to be ~ 35%. From the simulation of sorption dynamics, it was found that at early times, the sorption kinetics is governed by the film diffusion towards the external surface of grains, but at late times, most of the adsorbed mass is transferred primarily to meso-/macro-pores via diffusion, and secondarily to micro-porosity via surface diffusion. Based on the adsorbent characteristics, effect of pH on sorption efficiency, and numerical analysis of sorption dynamics, it was concluded that probably the dominant adsorption mechanism is the π-π interactions between hydrophobic PHE aromatic rings and CWAC-NaOH-800 graphene layers. The high PHE removal efficiency of CWAC-NaOH-800, the successful interpretation of sorption dynamics with the multi-compartment model, and the potential to regenerate PHE-loaded adsorbents with the green and economic technology of CAP motivate a strategy for testing CWACs towards the adsorption of other PAHs, application of adsorbents to real wastewaters, and scaling-up to pilot units.

Graphitic carbon nitride-adorned PDMS self-cleaning floating photocatalyst for simultaneous removal of Rhodamine B, Crystal Violet and Malachite Green from a ternary dye mixture

Graphitic carbon nitride-adorned polydimethylsiloxane (PDMS) floating catalyst was prepared by a simple procedure. The prepared catalyst was utilized for the simultaneous mitigation of recalcitrant organic pollutants such as Rhodamine B, Crystal Violet and Malachite Green from their ternary mixture for the first time. Derivative spectroscopic method was used to calculate the degradation efficiencies of individual dyes in the mixture. The prepared catalyst showed a consistent degradation performance up to 4 cycles inducing a degradation of 94.02%, 92.1% and 97.13% of Rhodamine B, Crystal Violet and Malachite Green, respectively, in a dye(s) solution with a catalytic loading of 0.5 g L-1. A kinetic analysis of the dye(s) degradation under visible light was carried out during the course of this work up to 120 min. A detailed characterization of the surface of this novel catalyst was carried out in this study by SEM, EDX, XRD, DRS, DTG, FTIR, Raman spectroscopy and UV-Vis spectroscopy and provided the experimental proof for the catalyst presenting high hydrophobicity, self-cleaning ability, good recyclability and high chemical stability.

Adsorption of Rhodamine B from an aqueous solution by acrylic-acid-modified walnut shells: characterization, kinetics, and thermodynamics

A batch experiment was used in studying the effect of acrylic-acid-modified walnut shell (MWNS) as a low-cost adsorbent for removing Rhodamine B (RB) cationic dye in aqueous solutions. The adsorbent dosage, initial dye concentration, contact time, temperature, pH, and supporting electrolyte concentration on the adsorption behaviour of the adsorbent were explored. The adsorbent was characterized using the point of zero charge (pH_PZC), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), automatic specific surface analysis (BET), and X-ray photoelectron spectroscopy (XPS). Results showed that MWNS had abundant active groups and rough surface, which is conducive to the adsorption process. The kinetics and equilibrium data of MWNS-to-RB adsorption were in accordance pseudo-second-order kinetic and Freundlich isotherm models, respectively. Under optimal adsorption conditions, the maximum adsorption capacity of RB was 48.87 mg·g^-1. Thermodynamic results showed spontaneously and exothermically the adsorption process. Moreover, the addition of electrolyte had a negative effect on equilibrium adsorption capacity and adsorption rate.HIGHLIGHTS Acrylic-acid-modified walnut shells was used as an adsorbent for the removal of Rhodamine B (RB).The adsorption of RB by modified walnut shells was greatly affected by pH.Pseudo-second-order kinetic and Freundlich model fit the experimental data.The modified walnut shell can remove RB through electrostatic attraction, hydrogen bonding, and electron donor-acceptor interaction.