A two-step pH control method to remove divalent metals from near-neutral mining and metallurgical waste drainages by inducing the formation of layered double hydroxide

A neutral M2+-rich and M3+-poor (M = metal) metallurgical waste drainage was used to test a metal removal method based on the precipitation of layered double hydroxide (LDH). The LDH precipitation was induced by adding a salt of Al3+ (trivalent metal missing in the drainage) and maintaining or restoring the pH to a circum-neutral value. The precipitates were characterized by chemical analysis, XRD, ESEM, HRTEM and XAS. The main parameter controlling the removal of metals and the type of precipitate appeared to be the pH. As a function of pH variation during the experiments, analyses of precipitates and solutions showed either the formation of poor crystalline LDH combined with very high removal of Zn, Ni and Pb (92–100%), more variable removal of Mn (46–98%) and less Cd (33–40%), or the formation of more crystalline LDH combined with lower removal of Zn (62%), Mn (43%), Ni (88%), Pb (64%) and especially Cd (1%). The different metal removal efficiency in the two cases is only indirectly due to the different LDH crystallinity, and it is clearly affected by the following factors: 1) the two pH steps of the method; 2) the direction of pH variation within each step. In particular, the highest removal of metals is obtained when the first pH step goes towards acidic conditions, as a consequence of Al salt addition, and precipitation of a quasi-amorphous hydrated hydroxysulfate of Al (probably a precursor of felsӧbányaite Al4(SO4)(OH)10 · 4H2O) occurs. This first acidic pH step removes little or no metals (just 0–3%) but it is essential so that the second pH step towards slightly alkaline conditions, as a consequence of NaOH addition, can be highly efficient in removing divalent metals as the quasi-amorphous hydrated hydroxysulfate of Al gradually turns into an LDH incorporating Zn, Mg and other metals. On the contrary, when both pH steps remain in the neutral-alkaline range, only LDH precipitation occurs and a lower metal removal is observed. These results encourage further investigations on the removal of metals by inducing LDH precipitation as a simple and effective method for the treatment of circum-neutral polluted drainages

Crystal chemical characterisation of red Beryl by ‘standardless’ laser-induced breakdown spectroscopy and single-crystal refinement by X-Ray diffraction. An example of validation of an innovative method for the chemical analysis of minerals

Laser-induced breakdown spectroscopy (LIBS) is a valuable technique for performing qualitative and quantitative chemical determinations of all elements in one shot, including low atomic number elements such as Li and Be. This technique does not require any sample preparation to reveal the atomic species, even when present in trace amounts (< 0.01% m/m). In this study, for the first time, we provide an accurate mineral formula for a Cs-rich red beryl by combining crystallographic data obtained using the traditional single-crystal X-ray diffraction technique and quantitative chemical data obtained with an innovative ‘standardless’ method: Calibration-free-LIBS (CF-LIBS). In particular, a new LIBS prototype coupled with a petrographic microscope (CF-μLIBS) was used to analyse chemically homogeneous areas of about 10 μm spot size, causing minimal damage to the mineral. The results showed that calibration-free quantitative analysis is suitable for the quantification of major and minor low and high atomic number elements in beryl. The accuracy of quantification of low atomic number elements by CF-μLIBS led to the empirical formula: [12](Cs0.006Na0.019K0.017Ca0.019)Σ0.061[4](Be2.989Li0.011)Σ3.000[6](Ti0.053Mn0.051Mg0.007Al1.890)Σ2.000[4](Be0.116Fe0.024Si5.860)Σ6.000 O18. This formula is consistent with the crystal-structure refinement data and demonstrates the validity of CF-μLIBS for chemical analyses of minerals containing low atomic number elements