Chromium biosorption using the residue of alginate extraction from Sargassum filipendula

Highlights

• This study assesses the feasibility of chromium uptake onto the residue of alginate extraction.

• Superior maximum biosorption capacity for Cr total in comparison to Cr(III).

• The biosorption of Cr(VI) is followed by the reduction of hexavalent to trivalent chromium.

• Extraction residue is a suitable, economic and easily available biosorbent for chromium uptake.

Abstract

The Brazilian brown seaweed Sargassum filipendula was treated for alginate extraction and the residue was used for removing Cr(VI) and Cr(III) from aqueous solutions. The seaweed was characterized in terms of alginate and residue yields. Alginate and residue contents were 17% and 39%, respectively. Kinetic experiments were carried out and different models were applied in order to elucidate the rate-controlling mechanism: pseudo-first order, pseudo-second order and intra-particle diffusion. The biosorption of Cr(VI) in residue is followed by the reduction of hexavalent to trivalent chromium. The application of Langmuir model to equilibrium data showed a superior maximum biosorption capacity (q_max) for total chromium (0.819 mmol g⁻¹) in comparison to trivalent chromium (0.635 mmol g⁻¹). The biosorption capacities obtained were close to the values found for the removal of chromium by different species of brown seaweeds.

Introduction

Chromium can exist in oxidation numbers ranging from −2 to +6. However, chromium is stable only under the +3 and +6 oxidation states [1]. The differences in the oxidation state of chromium influence differences in the toxicity and the chemical properties of the metal in water. Hence, trivalent chromium is approximately 300-fold less toxic than the hexavalent form, but according to Suwalsky et al. [2], Cr(III) ions can cause more structural disturbances in erythrocyte membranes than Cr(VI). Hexavalent chromium is part of the United States Environmental Protection Agency (EPA) list, being one of the 129 most critical pollutants [3]. Cr(VI) is both carcinogenic and mutagenic [4], and it may cause damages to the kidney, lungs and ulcerations to the skin [5]. According to the World Health Organization (WHO) drinking water guidelines, the maximum allowable limit for total chromium is 0.05 mg L⁻¹ [6].

The major sources of chromium released into the environment are water streams from electroplating, welding, production of chromium–iron alloys, tannery, metal plating, production of chromates, dichromate, dyes and varnishes, as well as the use in electronic and metallurgy industries [7]. Several technologies are available to remove chromium species from wastewater before it can be launched into water bodies: coagulation–flocculation [8], flotation [9], membrane separation [10], ionic exchange resins [11] and biological treatments. The first processes can present a high cost due to the equipment and monitoring systems used, while the biological treatment is much more sensitive to the characteristics of the wastewater, such as pH and concentration [12].

Different seaweed species, especially brown seaweed, have been studied as alternative adsorbents for the removal of heavy metals from contaminated water by adsorption, including chromium, both in its trivalent and hexavalent forms [13], [14], [15], [16]. Brown seaweed contains alginate biopolymer, the main component of its cellular wall, responsible for its mechanical resistance and significant sorption capacity [17], [18]. Alginate is used in food, textile, cosmetic and pharmaceutical industries due to its jellification, viscosity and stability properties. The use of alginate as biosorbent has also been investigated by several researchers [19], [20], [21].

The residue originated from alginate extraction is discarded or used for the production of animal feed, and it has not yet been exploited as a biosorbent. Even after the extraction of alginate, it contains many of the constituents of the raw seaweed, presenting, therefore, potential for the sorption of heavy metals. The functional groups present in Sargassum biomass are alginate carboxyls, phosphate, sulfate, amino and hydroxyl groups: they are responsible for physical and chemical interactions of heavy metals with the biosorbent [22]. They are supposed to contribute to metal sorption on alginate–extraction residue. In addition, the use of alginate–extraction residue as a biosorbent is economically interesting, since it valorizes the waste from biopolymer production.

The main objective of this study was to characterize the residue from the extraction of alginate from a Brazilian seaweed, a natural renewable resource, and evaluate its capacity for separation and recovery of trivalent and hexavalent chromium ions from aqueous solutions. The study investigates the sorption isotherms at different temperatures, the uptake kinetics for both Cr(VI) and Cr(III); in addition, the materials are characterized using FT-IR spectrometry and SEM-EDX analysis for identifying metal distribution in the biosorbent and its interactions with the biomass.