Resumen de Avaluació i caracterització d'una apatita biogènica pel tractament in situ d'aigües subterrànies i sòls contaminats per activitats mineres
Given the problems of contamination with heavy metals and acidity that mining industry can generate in groundwater and soils. One of the passive remediation techniques is the permeable reactive barrier (PRB). This work has evaluated the application of biogenic apatite, as a material applied to a PRB, or as an amendment to contaminated soil. This biogenic apatite was obtained from fish bones and hard parts, US Patent of Apatite II TM. The evaluation was done with the aim of being applied as an in situ treatment to remediate groundwater and contaminated soils by mining activities.
The first part is the characterization with the determination of chemical properties for the purposes which were being sought for this application, kinetic and constant of dissolution. The solubility constant has been determined in batch experiments, and the result was log KSO = - 50.8±0.8 at 25°C, according to the dissolution reaction:
The kinetics of dissolution has been determined assuming that the reaction of dissolution is controlled by the fast adsorption of a proton in a specific area, followed by a slower phase of hydrolysis, and modelling it with the Langmuir isotherm. The rate in mol/m2 /s, can be written, depending on the activity of H+, with the following expression: The dissolution is independent of pH in acidic pHs, pH=5, and it decreases for pH>5. These values are comparable to other apatite in the literature. The Apatite II TM is dissolved stoichiometrically (Ca/P = 1.67) in steady state.
The second step was determining the capacities and mechanisms of retention by Apatite II TM with unimetall column systems, simulating the conditions of pH, a concentration and a flow similar to that of groundwater contaminated by mining activities. In all cases, the solution that passed through the column of Apatite II TM left more than 95% of the metal evaluated withheld until breaking point (97% for Zn, 99% lead, 96% manganese, 98% iron , 98.1% cadmium, 97.4% copper, cobalt 96.4%, 97.5% nickel and 99.6% mercury).
The main mechanism for metals retention in the columns was the dissolution of the Apatite II TM followed by the precipitation of metal phosphate. For the case of zinc, hopeite was detected (Zn3(PO4)2¿ 4H2O(s)); for lead, pyromorphite (Pb5(PO4)3OH(s)); for manganese, metaswitzerite (Mn3 (PO4)2¿4H2O(s)); for iron, vivianite (Fe3(PO4)2¿8H2O(s)); and for cobalt, Co3(PO4)2¿8H2O(s).
In other cases the speciation and the SEM analysis showed that the metal phosphate was also responsible of the retention, for the case of cadmium, Cd3(PO4)2(s); for copper, Cu2(PO4)OH(s); for nickel, Ni3(PO4)2(s); and for mercury, Hg3(PO4)2(s).
The retention capacity was also measured. In experiments at pH 5, the values were>59, >35, 18.5, 57.1, 108, 76, 12, 24 and 38 mg of metal/ g Apatite II TM, for zinc, lead, manganese, iron, cadmium, copper, nickel, cobalt and mercury, respectively.
The simulation of the application of Apatite II TM as a reactive material in PRB, indicated that with 1 m thick the durability of Apatite II TM would be between 5-10 years, if the pH is greater than 4.
For the treatment of water more acidic (pH<4), it should be considered the mixing with other materials (e.g. limestone, magnesium oxide), which could be used to regulate the acidity, and to increase durability of Apatite II TM.
The third step is the evaluation of Apatite II TM as an amendment material used in a contaminated soil to retain pollutants species. It was used a sludge collected in the spill of the pyritic mine of Aznalcóllar (Seville) in 1998. The sludge was mixed with two kinds of soils, a sandy and a gravel one of the same area, and also with Apatite II TM, 20/1 ratio. These columns were operative more than 200 days, with irrigation of 500 mL at increasing time intervals, equivalent to a rainfall of 85.3 mm.
