Environmental context. Large quantities of chromium are discharged into the environment as a result of its widespread use in modern industries, and consequently, chromium could constitute a serious pollution problem. Adsorption onto natural biomass offers real potential as a way of removing chromium from the environment, because such adsorbents contain biopolymers with particular chemical stability and selectivity towards metals. In addition, natural biomass constitutes an eco-friendly and cost-effective alternative to the existing methods. Here, specific interactions between chromium and the biomass are investigated.

Abstract. The chromium(III)-binding capacity of several biomaterials has been described under fixed conditions of pH (4.5) and initial metal concentration (100 mg L^–1). Three of these materials (Sargassum muticum, orange peel and bracken fern) have been selected and subjected to different studies. Fourier transform infrared and scanning electron microscopy techniques were used to describe the structure of the biomaterials, supporting the hypothesis of a mechanism of metal complexation via carboxylic groups. Potentiometric titrations revealed the quantity of carboxyl groups present in S. muticum, orange peel and bracken fern: 1.78, 0.49 and 0.67 mmol g^–1, respectively. Moreover, a model considering different types of binding sites was used to simulate the process and determine the apparent pK values of the main functionalities. The number of carboxylic groups was clearly correlated with the maximum amount of Cr^III binding by the materials. A Langmuir competitive model was used to determine the complexation constants for chromium, log K_Cr, which are very close (~3), supporting the idea of the implication of essentially one acid functionality. Desorption studies were conducted for different times employing H₂SO₄ and sodium citrate.