Adsorption of Cu(II) on Araucaria angustifolia wastes: determination of the optimal conditions by statistic design of experiments.

Wastes of Araucaria angustifolia (named pinhão) natural (PW) and also loaded with Congo red (CRP) were tested as low-cost adsorbents for Cu(II) removal from aqueous solutions. In order to reduce the total number of experiments to achieve the best conditions of the batch adsorption procedure, three sets of statistical designs of experiments were carried-out for each adsorbent. Initially, a full 2(4) factorial design for each adsorbent with two central points (18 experiments) were performed, to optimize the following factors: mass of adsorbent (m), pH, time of contact (t) and initial metallic ion concentration (Co). These results indicated that almost all the main factors and its interactions were significant. It was verified for both adsorbents, that a mass of 30.0mg leaded to higher Cu(II) uptake and that the best pH for Cu(II) adsorption was 5.6. In order to continue the batch adsorption optimization of the systems, a central composite surface analysis design with two factors (Co, t) containing 13 experiments, divided in to four cube points, four axial points and five centre points was carried-out for each adsorbent. By performing these two sets of statistical design of experiments, the best conditions for Cu(II) uptake using pinhão wastes (PW) and pinhão wastes loaded with Congo red (CRP) using batch adsorption system, where: m=30.0mg of adsorbent; pH 5.6; t=2.5h. After optimizing the batch adsorption system by statistical design of experiments, isotherms for Cu(II) uptake using PW and CRP were performed. These isotherms fitted to the linear Langmuir and Freundlich models.