**1. Introduction**

Despite its toxic character, chromium(VI) is widely used in various industries, being its recovery from the corresponding liquid effluents a primary target before their discharge to natural waters. Several technologies have found application to remove and/or recover chromium(VI) from these process wastes: pseudo-emulsion strip dispersión pertraction [1, 2], adsorption onto activated carbons [3], liquidliquid extraction [4], biomass adsorption [5, 6], adsorption onto natural zeolites [7], adsorption onto phosphates [8], ion exchange [9] and electro-assisted and photo-assisted technologies [10]. Among them, adsorption onto carbon nanotube (CNT) technology could be competitive when the metal is present at low concentrations in the aqueous solution. Various carbon nanotubes configurations can be found, being the most commonly used the single-walled carbon nanotubes (SCNTs) and multi-walled carbon nanotubes (MWCNTs) configurations, together with functionalized multi-walled carbon nanotubes. In either configuration, and

after the metal adsorption, a subsequent operation or elution is needed in order to recover the metal to a solution where it is concentrated and purified, and thus it can be conveniently recovered or even recycled to the original industrial process.

A number of examples using these CNTs for chromium(VI) recovery from aqueous solutions can be found in the literature. Naghizadeh [11] investigated the adsorption efficiency of activated carbon and multi-walled carbon nanotubes respect to cadmium(II) and chromium(VI) in the 3–12 pH range. Whereas both adsorbents presented high metal adsorption capacities over the whole pH range investigated, the experimental results indicated that MWCNTs had a greater potential for the removal of chromium(VI) and cadmium(II) from aqueous solutions than activated carbon. In an investigation by [12], oxidized (–COOH) multiwalled carbon nanotubes were used to remove Cr(VI) (hazardous element) and Au(III) (valuable element), from aqueous solutions. Experiments were performed in order to investigate the influence of different variables on the adsorption kinetics, i.e., the stirring speed (250–2000 min<sup>−</sup><sup>1</sup> ) and adsorbent dosage (0.25–1.5 g/L) in the case of chromium(VI) as well as temperature (20–60°C) and HCl (0.1–10 M) concentration in the case of gold(III). The performance of these carbon nanotubes was excellent in the removal of both elements, presented as the anions HCrO4 <sup>−</sup> and AuCl4 <sup>−</sup>, from the aqueous solutions.

Anastopoulos et al. [13] reviewed the removal of chromium(III) and (VI) from aqueous solutions by carbon nanotubes. In both cases, the pH of the solution seemed to control the adsorption process, with a maximum adsorption of Cr(VI) occurring at pH 1–4 (in the case of Cr(III), the above occurs at pH values of 5–8). Furthermore, it is stated that most of the investigations are reported using non-real wastewater, conditions that very often are not repeated in real wastewaters. Xing et al. [14] presented a novel remediation protocol for Cr(VI) featured with high-capacity adsorption and electrochemical regeneration of the adsorbent. In their study, MWCNTs modified carbon cloth (CC) is used as a useful carrier for electrodepositing polypyrrole (PPy) film and the resultant nanocomposite CC-MWCNTs-PPy is used as an adsorbent with high adsorption capacity and stability. CC-MWCNTs-PPy is electrically regenerated to reduce secondary wastes.

In the present work, results obtained for the adsorption of chromium(VI) using multi-walled carbon nanotubes are presented. Several variables that could affect the adsorption process, such as the stirring speed of the aqueous solution, metal concentration and adsorbent dosage, temperature, etc., are investigated. Several equilibrium, kinetics and thermodynamic parameters are also reported. The desorption of the Cr(VI)-loaded MWCNTs is accomplished using aqueous solutions of hydrazine sulfate.
