The surface oxidation of synthetic and natural samples of enargite and tennantite has been monitored by X-ray photoelectron spectroscopy, XPS. The minerals were conditioned at pH 11.0 in an aqueous solution purged with nitrogen gas for 20 min or with oxygen gas for 60 min. The XPS results show that the oxidation layer on the mineral surface is thin. The surface oxidation products comprise copper and arsenic oxide/ hydroxide, sulfite, and a sulfur-rich layer made of metal-deficient sulfide and/or polysulfide. The proportion of all of these oxidation products at the mineral surface is more important when the minerals are treated in more oxidizing conditions (i.e., with oxygen gas and for a longer time) for tennantite than for enargite and for the natural samples than for the synthetic samples. Different arsenic sulfide species have been found at the surfaces of enargite and tennantite: As4S4 or As2S3 constitutes the major arsenic sulfide species at the surface of enargite, but these are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. This difference is not related to a surface impurity in the natural enargite sample as it is also observed in the synthetic enargite sample.
The surface of commercial 30-nm colloidal silica particles were modified by reaction with functional silanes. The high specific surface area and reactivity of the particles, due to their small size, makes the process susceptible to irreversible aggregation not found previously with larger particles. The present study compares surface charge results from amino silanes with one or three alkoxy groups. Measurements of the zeta potential as a function of pH, and gelation kinetics shed light on the mechanism of surface charge changes from the modification. Instability in suspensions before and after the surface modification is also studied using a new data analysis technique from simple light-scattering equipment. Experimental results show very stable particles are obtained by amino silane surface modification. Factors affecting susceptibility of small particles to irreversible aggregation caused by a non-aqueous solvent or high concentration of a trialkoxy silane, including the large number of reactive silanol groups in the surface gel layer of the particles, are discussed.
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