Comparative study of the removal of phenolic compounds by biological and non-biological adsorbents

Abstract

The ability of biological and non-biological adsorbents to remove 2-nitrophenol (2-NP) and 2-chlorophenol (2-CP) from aqueous solutions in batch experiments at room temperature was compared. The marine seaweeds Macrocystis integrifolia Bory (S1) and Lessonia nigrescens Bory (S2) were cross-linked with CaCl₂ to enhance their mechanical properties. Natural bentonite was chemically exchanged with hexadecyltrimethylammonium bromide (B1) and bencyltriethylammonium chloride (B2) to increase their affinity towards organic compounds as well. The adsorption capacity of all of the adsorbents strongly depends on solution pH, whereas equilibrium assays showed a mixed mechanism according to the Langmuir and Freundlich isotherms. The maximum adsorption capacity of 2-NP follows the trend: S1 > S2 > B2 > B1 within the range of 97.37 and 18.64 mg g⁻¹ whereas for 2-CP, it ranged between 24.18 and 9.95 mg g⁻¹ with the trend: S1 > S2 > B2 ≈ B1. The importance of the octanol–water partition coefficient as the main factor on the adsorption of these compounds on two different kinds of adsorbents is discussed.

Introduction

At present, one of the most serious concerns faced in the natural environment is chemical contamination with organic and inorganic substances catalyzed by the presence of heavy metals and phenolic compounds [1], [2]. The presence of such compounds in an aquatic medium originates changes in chemical (pH, chemical oxygen demand, alkalinity, acidity, dissolved oxygen, etc.), physical (color, temperature, odor, viscosity, turbidity, etc.) and biological properties, damaging the water quality for human usage and upsetting the environmental equilibrium [2], [3]. The uptake of toxic and heavy metals ions from waste waters and industrial wastes by certain types of microbial biomass especially algae [4], [5], fungi [6], [7] and other biological adsorbents [8], [9] has been the subject of many recent studies. Natural clays have also shown an increasing number of applications on the removal of heavy metals [10], [11], [12].

Among the different pollutants of aquatic systems, phenols are considered as priority contaminants since they are harmful to plants, animals and humans, even at low concentrations [3], [13]. Due to their extensive use and slow degradation, nitrated and chlorinated phenols readily enter the environment by agricultural runoff of pesticides, effluents from oil refineries and plastic industries and by leachates emerging from waste deposits produced by microbial hydrolysis and photodegradation of several organophosphorous pesticides, such as parathion [14], [15].

Activated carbon is one of the most efficient adsorbents for these organic molecules, as it possesses a high surface area per unit mass and exhibits a high affinity for phenolic compounds [16]. Unfortunately, due to the high cost of activated carbon, it is not an economical adsorbent compared with low cost, naturally occurring alternatives. These non-expensive adsorbents remove trace amounts of organic contaminants from wastewaters where other sophisticated techniques such as ionic exchange, osmosis, and solvent extraction are not applicable. Consequently, the environmental biotechnology is in a constant search of alternative, less expensive and viable techniques for the removal of these phenolic compounds at very low concentrations. Adsorption using naturally occurring adsorbents has become a cheap and efficient tool, easily applicable in the detoxification of residual waters from industries and mines showing positive results. High efficiency on the elimination of artificial colorants and phenols from aqueous solutions by marine seaweeds and natural clays has been observed in previous studies [17], [18], [19], [20], [21], [22].

Peru is widely known for its diversity of natural resources. Marine seaweeds are in such abundance on the shores of beaches as to cause unsightly accumulation. Also, natural clays of the type montmorillonite, saponite and kaolinite are extracted from natural mines in the northern part of the country at very low price. For reasons of accessibility and abundance, Peruvian algae and natural clay of the type bentonite were chosen and compared for the removal of 2-nitrophenol and 2-chlorophenol from aqueous solutions.

Several research groups have investigated the adsorptive capacity of other adsorbents such as lignins, coconut shells and papermill sludges [15], [23], [24] in the removal of nitrated and chlorinated phenols. Previous work has already been carried out utilizing marine algae and clays for the elimination of phenolic compounds [17], [19] but none have compared the adsorption capacity of these two adsorbents or explained a relationship between their chemical and mechanical properties and adsorption.