Wollastonite Dissolution Research Articles

Long-term interaction of wollastonite with acid mine water and effects on arsenic and metal removal

Abstract This paper reports the results of a laboratory experiment conducted to investigate the effects of wollastonite dissolution on removal of potentially toxic trace elements from stream waters affected by acid mine drainage (AMD). Nearly pure wollastonite was treated with natural acid mine water (pH 2.1) for different lengths of time (15, 30, 50 and 80 days). The compositional and textural characterization of the solid reaction products suggests that wollastonite was incongruently dissolved leaving a residual amorphous silica-rich phase that preserved the prismatic morphology of the parent wollastonite. The release of Ca into solution resulted in a pH increase from 2.1 to 3.5, and subsequent precipitation of gypsum as well as poorly crystallized Fe–Al oxy-hydroxides and oxy-hydroxysulfates whose components derived from the AMD solution. A geochemical modeling approach of the wollastonite–AMD interaction using the PHREEQC code indicated supersaturation with respect to schwertmannite (saturation index = 10.7–15.7), jarosite (SI = 8.7–10.2), alunite (SI = 5.1), goethite (SI = 4.7) and jurbanite (SI = 2.2). These secondary phases developed a thin coating on the reacted wollastonite surface, readily cracked and flaked off upon drying, that acted as a sink for trace elements, especially As, Cu and Zn, as indicated by their enrichment relative to the starting wollastonite. At such low pH values, adsorption of As oxyanions on the positively charged solid particles and coprecipitation of metals (mainly Cu and Zn) with the newly formed Fe oxy-hydroxides and oxy-hydroxysulfates seem to be the dominant processes controlling the removal of trace elements.

Dissolution of Wollastonite in a Packed Bed Contactor

In a packed-bed reactor grains of the calcium silicate mineral wollastonite dissolved incongruently. A silicon-rich residue layer appeared to form on the grain surfaces and this quickly limited the overall rate of mineral dissolution. Approximately 54% of the silicon associated with the dissolved calcium was retained on the grains and about 46% went into solution with the calcium. As the amount of Ca dissolved per unit area of grain surface approached approximately 0.5μmol∕cm2 the effluent concentration of Ca decreased to a value that was significantly less than the equilibrium concentration (approximately 300μmol∕L) and the effluent pH decreased from approximately 10 to slightly greater than the influent value of 6.8. The effluent Ca and Si concentrations were effectively predicted by a finite difference model that used layer-by-layer calculation of a residue layer mass transfer resistance (for the Ca ion) acting in series with an extraparticle mass transfer resistance. The effluent pH was predicted using chemical equilibrium calculations and the assumption that the dissolved Si controls the proton balance across the column.

Incongruent dissolution of wollastonite measured with vertical scanning interferometry

In this study, we investigated the dissolution kinetics of wollastonite, a pyroxenoid with composition CaSiO3, at earth-surface temperatures. We used vertical scanning interferometry (VSI) to quantify the surface normal retreat (or advance) of a crystallographically defined cleavage face on a single wollastonite crystal as a function of time. Nanometer-scale measurements of (100) faces show that the wollastonite surface undergoes a net expansion normal to the surface and relative to the pristine mineral surface. These observations were made during the dissolution process at acidic conditions. The swelling is the result of the formation of a cation-depleted layer at the wollastonite-water interface. We combined VSI measurements with electron microprobe measurements to track the chemical evolution of the leached layer during dissolution, and then used previously reported Ca release rates to calculate the density of the cation-depleted layer. Using an average Ca release of 9.09 × 10-9 mol/m2·s at pH 3 and 25 °C, the density of the leached layer is 1.2 g/cm3, and grows to a thickness of ~50 nm after 20 hours of reaction. Although our observations do show direct evidence of the formation of a cation-depleted layer during dissolution, they do not favor a particular mechanism of formation of this layer. At this time, our results cannot rule out either cation migration through the depleted crystal structure or re-precipitation at the mineral surface.

Experimental determination of the effect of dissolved CO 2 on the dissolution kinetics of Mg and Ca silicates at 25 °C

Dissolution rates of diopside, forsterite (Fo 100 and Fo 92), wollastonite, and hornblende were measured at 25 °C in NaCl solutions as a function of pH (1≤pH≤12), pCO 2 (0 and 1 atm) and activity of HCO 3 − and CO 3 2− (10 −5≤∑CO 2≤0.1 M). Dissolution rates of diopside, forsterite, and horneblende are not affected, within the experimental uncertainty, by 1 atm pCO 2 at pH∼4 and by the presence of bicarbonate (up to 0.1 M) and carbonate (up to 0.01 M) ions at pH of 8 and 11–12, respectively. Although wollastonite dissolution rates are unaffected by pCO 2 at pH∼4, its rates are increased by the presence of HCO 3 − at 0.01≤[HCO 3 −]≤0.1 M and pH=7–8 and decreased slightly by CO 3 2− at [CO 3 2−]>0.0001 M and pH∼12. These results should be useful for modeling the effect of silicate rock weathering on the global biogeochemical cycle of carbon. For the first time, we were able to distinguish the net effect of pCO 2 versus that of pH. It is observed, in general accord with previous results, that the direct effect of CO 2 on basic silicates dissolution rates is very weak. As a result, the major parameter governing basic silicates dissolution is pH of soil solutions in contact with minerals.

Dissolution of wollastonite and its flotation and surface interactions with tallow-1,3-diaminopropane (duomeen T)

Wollastonite finds an important place among industrial minerals due to its high chemical and thermal resistance and also as a substitute for asbestos. With the depletion of high-grade ores and increased demand, it becomes inevitable to process low-grade ores. Flotation can be employed effectively for processing these ores. Flotation behavior of wollastonite was studied using tallow-1,3-diaminopropane as collector in the Hallimond micro-flotation cell. The flotation recoveries obtained were correlated with contact angle measurements. Dissolution studies clearly bring out the incongruent dissolution of wollastonite, resulting in a silica rich layer on the surface of the mineral. The release rates of calcium and silicon decrease with the particle size. Maximum flotation obtained at 1 × 10 −5 M diamine concentration. DRIFT studies indicate that the intensity of alkyl chain bands increases with increasing diamine concentration, thus corroborating increased presence of diamine on wollastonite resulting in maximum contact angle and flotation recovery. Surface free energy values of wollastonite were determined by different approaches using contact angle data of well-defined components of polar and apolar liquids. An attempt has been made to correlate between the flotation response and the surface free energy values of wollastonite in the presence diamine.

Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1

Powdered and pelletized wollastonite (CaSiO3) was applied to an 11.8 ha forested watershed at the Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire, U.S.A. during October of 1999. The dissolution of wollastonite was studied using watershed solute mass balances, and a 87Sr/86Sr isotopic tracer. The wollastonite (87Sr/86Sr = 0.70554) that was deposited directly into the stream channel began to dissolve immediately, resulting in marked increases in stream water Ca concentrations and decreases in the 87Sr/86Sr ratios from pre-application values of 0.872 mg/L and 0.72032 to values of ∼2.6 mg/L and 0.71818 respectively. After one calendar year, 401 kg of the initial 631 kg of wollastonite applied to the stream channel was exported as stream dissolved load, and 230 kg remained within the stream channel as residual CaSiO3 and/or adsorbed on streambed exchange sites. Using previously established values for streambed Ca exchange capacity at the HBEF, the dissolution rate for wollastonite was found to be consistent with dissolution rates measured in laboratory experiments. Initially, Ca was released from the mineral lattice faster than Si, resulting in the development of a Ca-depleted leached layer on mineral grains. The degree of preferential Ca release decreased with time and reached stoichiometric proportions after ∼6 months. Using Sr as a proxy for Ca, the Ca from wollastonite dissolution can be accurately tracked as it is transported through the aquatic and terrestrial ecosystems of this watershed.

Apatite formation on the surface of wollastonite/tricalcium phosphate composite immersed in simulated body fluid.

Wollastonite/tricalcium phosphate composites were prepared and immersed in SBF for various periods to investigate the apatite-formation mechanism on their surfaces. Surface morphologies and composition before and after immersion were analyzed by SEM and EDS technologies. The concentration changes of calcium, silicon, and phosphorus in SBF due to the immersion of the samples were measured with inductively coupled plasma atomic emission spectroscopy, and the corresponding pH values in SBF were recorded. XRD and IR were used to examine the changes in crystalline phases and functional groups. It was found that, after immersion for 1 day, the samples were covered by pom-pom-like apatite granules, resulting from the dissolution of wollastonite and the simultaneous transformation of TCP. This quickly led to the subsequent precipitation of another apatite layer, which covered the earlier-formed surface completely. These results indicate that the wollastonite/tricalcium phosphate composite is a potential biomaterial candidate because of its high bioactivity and in situ porous structure.

Assessing and correcting the effects of the chemical weathering of potsherds: A case study using soft‐paste porcelain wasters from the Longton Hall (Staffordshire) factory site

Unglazed soft-paste porcelain wasters from the Longton Hall factory site are variably depleted (75–80 rel %) in CaO relative to comparatively insoluble components (e.g., Al2O3, TiO2) due to the dissolution of wollastonite (CaSiO3, a pyroxenoid) by subsurface water. The degree of desilicification is variable (0–45 rel % SiO2). Petrographic data and element-abundance plots suggest that these were the principal effects of the chemical weathering process in most samples. The preferential dissolution of a single phase in the unglazed Longton Hall sherds permits the semiquantitative “reversal” of weathering phenomena. Alteration effects can be corrected using porosity–volume data to constrain the amount of wollastonite originally present in the weathered sherds. The original compositions of the unglazed wasters are bracketed by arithmetically “adding back” the missing pyroxenoid components according to two endmember assumptions concerning element mobility: (1) the total leaching of wollastonite components and (2) the preferential leaching of wollastonite-derived CaO. These calculations—particularly the latter—yield results that compare favourably with the compositions of relatively unaltered (wollastonite-bearing), glazed samples from the Longton Hall site. Given the potential susceptibility of archaeological ceramics to chemical weathering, it would seem prudent that these phenomena be carefully assessed, and corrected where possible, so that analytical data for these artifacts can be judiciously interpreted. © 1998 John Wiley & Sons, Inc.

Dissolution stoichiometry and adsorption of alkali and alkaline earth elements to the acid-reacted wollastonite surface at 25°C

The Ca 2+ H + exchange reaction on the wollastonite surface was investigated at 25°C with both short-term (<2.5 h) and long-term (>48 h) dissolution studies. In acidic solutions, the dissolution of wollastonite is nonstoichiometric with a greater release of Ca than Si relative to the wollastonite stoichiometry. Both short-term and long-term Ca 2+ H + exchange reaction stoichiometries are 0.5. Rapid desorption of Ca 2+ from the surface of untreated wollastonite caused a rise of the suspension pH to about 10 in a couple of minutes. Therefore, potentiometric titrations were performed with an acidreacted wollastonite where most surface detachable Ca 2+ had been removed. Addition of alkali and alkaline earth metal chloride solutions to the acid-reacted wollastonite suspension results in a pH decrease with K +> Na + > Ba 2+ > Mg 2+ > Ca 2+ in equal molal solutions. This suggests that the cations in these solutions are adsorbed to the wollastonite surface. Surface protonation properties of the acid-reacted wollastonite are found to be similar to those of microporous silica but with the point of zero salt effect (pzse) of 4.5–5.5 rather than the 3.0 of microporous silica. The surface protonation-deprotonation as a function of pH is modeled with a one-site double layer model which includes Na adsorption from the background electrolyte to reasonable accuracy. The adsorption of CrO 4 2−, MoO 4 2−, Ca 2+, Mg 2+, Ba 2+, and Na + from aqueous solutions to the acidreacted wollastonite/water interface was determined as a function of the pH and ionic strength of the solution. CrO 4 2− and MoO 4 2− were not adsorbed to the wollastonite surface at pH above 3. The extent of cation adsorption increases with increasing pH and decreases with increasing ionic strength. Ca 2+ adsorption depends on both the surface area of wollastonite and total amount of Ca 2+ in the suspension. For alkaline earth metals at the same concentration, the adsorption sequence is Ba 2+> Ca 2+> Mg 2+. At pH 8.5, the maximum Ca 2+ adsorption density on the acid-reacted wollastonite is about 0.83 μmol m 2−.

A survey study of the kinetics of wollastonite dissolution in H 2 O-CO 2 and buffered systems at 25 degrees C

A survey study of the kinetics of wollastonite dissolution in H 2 O-CO 2 and buffered systems at 25 degrees C