The use of silica-immobilized brown alga (Pilayella littoralis) for metal preconcentration and determination by inductively coupled plasma optical emission spectrometry

Abstract

The brown alga Pilayella littoralis was used as a new biosorbent in an on-line metal preconcentration procedure in a flow-injection system. Al, Co, Cu and Fe were determined in lake water samples by inductively coupled plasma optical emission spectrometry (ICP-OES) after preconcentration in a silica-immobilized alga column. Like other algae, P. littoralis exhibited strong affinity for these metals proving to be an effective accumulation medium. Metals were bound at pH 5.5 and were displaced at pH<2 with diluted HCl. The enrichment factors for Cu^II, Fe^III, Al^III and Co^II were 13, 7, 16 and 11, respectively. Metal sorption efficiency ranged from 86 to 90%. The method accuracy was assessed by using drinking water certified reference material and graphite furnace atomic absorption spectrometry (GFAAS) as a comparison technique. The column procedure allowed a less time consuming, easy regeneration of the biomaterial and rigidity of the alga provided by its immobilization on silica gel.

Introduction

Several investigators have used algal materials as preconcentration media for trace metal analysis. Following a sorption–desorption elution cycle, the metals are usually determined by optical absorption or plasma emission spectrometry. Gupta et al. [1] studied the proprieties of filamentous algae Spirogyra species on the sorption of Cr^VI. The results indicated that this biomass is suitable for the development of efficient biosorbent for the removal and recovery of Cr^VI from wastewater. Hamdy [2] investigated the ability of four different algae to adsorb Cr^III, Co^II, Ni^II, Cu^II, and Cd^II ions. Metal uptake was dependent on the type of biosorbent, with different accumulation affinities towards the tested elements. The extent of uptake of the metals with the algae was assessed under different conditions such as pH, contact time of the algae with the metal, and concentration of the biomass. The amount of the metal uptake increased steeply by increasing the weight of the biosorbent. Figueira et al. [3] studied the biosorption of Cd by biomass of the brown seaweeds Durvillaea, Laminaria ecklonia, and Homosira presaturated with Ca, Mg, or K and coupled with the release of these light ions. The feasibility of biomass pretreatment to develop a better biosorbent was evaluated by its biosorption performance, the degree of its component leaching as well as by the number of ion-exchange sites remaining in the biomass after the pre-treatment. Gonzalez et al. [4] proposed on-line preconcentration of Cd^II, Cr^III, Cu^II, and Pb^II and chemical speciation of Cr^III and CrO₄²⁻ using a microcolumn packed with dealginated seaweed biomass. Batch experiments showed that about 90% of the metals in solution is taken up in less than 5 min, and that maximum binding was obtained between pH 6 and 7. The effective binding capacities for Cd, Cr, Cu, and Pb at pH 6 were determined from column breakthrough measurements. Suarez et al. [5] investigated the ability of Chlorella vulgaris to accumulate heavy metals in solutions. This alga was able to bind Cr, Cu, Mn, Ni, and Zn at pH 8 in 15 min of contact time and metals were simultaneously determined by capillary electrophoreses with a UV-Vis detector. Biosorption of rare earths has also been investigated. Neodymium [6] was sorbed from acidic solutions in batch experiments using the microalgae Monoraphidium sp. The equilibrium biosorbent-metal was achieved in approximately 2 h of exposure at pH 1.5.

A variety of inert supports [7], [8], [9], [10], [11], [12], [13] has been used to immobilize biomaterials either by adsorption or physical entrapment. Silica gel, an inert and efficient support for microorganisms has been used to immobilize Stichococcus bacillaris for Pb preconcentration [12] and determination by flame atomic absorption spectrometry. This algae–silica material was also used to simultaneous preconcentrate Cu, Cd, Pb, and Zn in simulated riverine water, brine and seawater solutions [13].

Pilayella littoralis, a filamentous free-living brown alga has been previously investigated by Carrilho and Gilbert [14]. The authors describe a series of experiments designated to determine the potential of dead biomass from the marine alga P. littoralis for biosorption of metal from solution in batch systems. The effect of pH on metal uptake and the kinetic of metal sorption were assessed. Metals were bound to the algae within the first 5 min of exposure at pH 5.5 and were efficiently desorbed with 0.12 mol l⁻¹ HCl. In a recent work, Carrilho et al. [15] proposed some procedures to characterize metal binding sites on P. littoralis using nuclear magnetic resonance (NMR) spectroscopy and Fourier transformed infrared spectrometry (FTIR). The results provided information on the type of functional groups responsible for metal uptake such as carboxylates, ethers, amines and hydroxyls. Metal interaction with this alga and sorption sites competition among metals were assessed by ²⁷Al and ¹¹³Cd NMR.

The present work proposes the use of this new P. littoralis-based material for preconcentration in trace metal analysis. Unlike our previous work, biosorption is assessed in lake water samples using an on-line metal preconcentration procedure applying a flow-injection system and metal determination by ICP-OES. This new approach proposes a faster and more efficient method, in which several parameters were investigated.