Copper, zinc, cadmium and lead biosorption by Gymnogongrus torulosus. Thermodynamics and kinetics studies

Abstract

Gymnogongrus torulosus adsorption efficiency for cadmium(II), copper(II), lead(II) and zinc(II) were studied in batch mode in different acidic conditions. The adsorbent removal efficiency was determined as a function of contact time, initial metal ions concentration, pH and temperature. G. torulosus was characterized by SEM, water adsorption surface area and EDS. The Langmuir, Freundlich, Dubinin–Radushkevich and Temkin models have been applied and results showed that the biosorption process was better described by the Langmuir model. Kinetic experiments demonstrated that fast metal uptakes follow a pseudo-second-order kinetic model and that intra-particle diffusion and/or chemisorption were the rate-limiting steps. Experimental results show that G. torulosus isotherm followed the biosorption series, Cu > Cd ∼ Zn ∼ Pb.

Biosorption capacities were affected by solution parameters. The maximum metal uptake (q_max) increased with increasing pH. The affinity constant, q_max, and the pseudo-second-order kinetic constants were calculated for the adsorption of all studied metals onto G. torulosus. The Gibbs free energy of the adsorption process as well as the process enthalpy and entropy were calculated from experimental results.

Introduction

The increasing concentration of heavy metals in waters is mainly due to effluent discharges from industries. Pollution of natural waters by metal ions has become a major issue all over the world because metal concentrations in waters often exceed the admissible values. Consequently, industries are required to diminish the contents of heavy metals in their effluents to acceptable levels.

The increasing waste products are forcing scientist to study new and alternative technologies to remove trace metals from polluted waters [1] and one of the main goals regarding heavy metals removal from waste waters consists in the reduction of these pollutants at very low levels [2]

Biosorption, known as the sorption of heavy metals onto biological materials, is becoming a potential alternative for toxic metals removal from waters [3], [4] and is a cost effective technology that uses readily available biomass from nature [5]. Among many biosorbents, marine seaweeds are excellent biosorbents for metals [6]. In recent years, the metal biosorption potential of various red, green and brown seaweeds were investigated by many researchers [7], [8], [9], [10], [11], [12], [13], and references therein.

Seaweeds have a high bonding affinity with heavy metals [1], [14], [15], [16], [17], [18], [19], [20]. Since their cell walls have different functional groups (such as carboxyl, hydroxyl, phosphate or amine) that can bind to metal ions [21], they are much more efficient than active carbon and natural zeolites and, depending on the pH, these groups are either protonated or deprotonated [22], [23].

Many works describing metals biosorption have been published [8], [24], [25], [26], [27], [28], [29], [30]. The advantages of using dried aquatic algae for metal removal derive from its high efficiency as biosorbents, easy handling, no nutrient requirements, low costs, and their availability.

Gymnogongrus torulosus (Rhodophyta) is a seaweed with great commercial interest in Argentina because it produces carrageenan, a polysaccharide commonly used in hydrocolloid industries. This seaweed predominates along the coastal and continental shelf areas of temperate and cold-water regions. Thus, the present study was focused on heavy metal removal from aqueous solution by G. torulosus under different experimental conditions in order to optimize the efficiency of the adsorption process. Equilibrium isotherm and kinetic models were carried out for a better understanding of the adsorption process.