Microwave-assisted digestion of organic samples: how simple can it become?

Advancements in sample preparation for performing trace analysis of inorganic analytes are coming from the dissemination of microwave-assisted procedures, but there is still room for improvements by looking for simple and easily applied procedures. Recently it was proposed a new approach called single reaction chamber with capability for digestions at high pressures and temperatures using simple vials and racks. This was a limitation of former cavity microwave ovens with closed vessels. It was demonstrated here that the use of single reaction chamber approach allows the implementation of efficient digestions using diluted solutions of nitric acid and also allows addressing a critical need of sample preparation for inorganic analysis by running mixed batches of samples. The feasibility of this procedure was demonstrated for organic samples and accuracy was proved by using certified reference materials of apple leaves, bovine liver and whole milk powder. Digestions performed of whole milk powder and bovine liver using 2.0 mol L(-1) nitric acid solution plus concentrated hydrogen peroxide at 240 °C led to residual carbon contents of 0.825 and 1.50% and residual acidities of 1.04 and 0.618 mol L(-1), respectively. These parameters are fully compatible with further measurements using ICP OES or ICP-MS. Al, Cu, Fe, Mn, Mo, Rb, Se, Sr, and Zn were accurately determined by ICP OES or ICP-MS depending on their concentrations in digests.

Reduction of heavy metal contents in liquid effluents by vermicomposts and the use of the metal-enriched vermicomposts in lettuce cultivation.

The removal of Cu, Ni and Zn from electroplating effluents by adsorption in cattle manure vermicompost has been discussed. A glass column 38 cm long and 7 cm i.d. was loaded with cattle manure vermicompost and effluents were passed through it. The metal concentrations were measured in the elutant. The experiments on adding effluent aliquots into the columns were continued until the metal concentrations in the elutant reached the maximum values established for effluent discharges in water courses by the Brazilian quality criteria, i.e., Cu=1.0 mg L(-1), Ni=2.0 mg L(-1), and Zn=5.0 mg L(-1). The amount of Cu retention by the vermicompost was determined at the natural effluent pH (2.0). The Zn and Ni retentions were evaluated at the natural effluent pH (6.9 and 7.4, respectively) as well pH 2.0. Vermicompost residues obtained from this process were used for lettuce cultivation. The vermicompost was found to be efficient in removing metals from the electroplating wastes, as well as in the increase of its pH values. Metal retention values were close to 100%. The Cu concentrations in lettuce leaves from the treatment with vermicompost enriched with this metal were below the range of critical toxicity level to plants, i.e., from 20 to 100 mg L(-1). However, the estimated Cu concentrations in the roots from the treatment with vermicompost enriched with Cu were much larger than that of the treatment with the natural vermicompost, reaching 246.3 mg L(-1). The Ni and Zn concentrations in lettuce leaves from the treatments, with vermicomposts enriched with the respective metals, were above the range of critical toxicity levels to plants, i.e., from 10 to 50 mg kg(-1) and from 15 to 30 mg kg(-1), respectively. However, no symptom of toxicity was found visually. Larger accumulations of Cu, Ni and Zn were found in the lettuce leaves than in the roots after the treatments with the uncontaminated vermicompost. A greater absorption of Cu and Ni by roots was found in treatments with vermicompost enriched with these elements, whereas Zn was found preferentially in the leaves. The statistical analysis was done by analyses of variance and regression.