Determination of 5 Morphologies of Silicon in Produced Water of Heavy Oil by Spectrophotometry after High Temperature Digestion

Based on Si-Mo yellow method, the method for determination of 5 morphologies of silicon (calculated as SiO₂), including active silicon, colloidal silicon, adsorbed silicon, dissolved organic silicon and granular silicon in the produced water of heavy oil was proposed by spectrophotometry after high temperature digestion. The water sample (10.00 mL) was taken and tested by the following steps, respectively. ① Water sample was passed through PTFE filter, and mixed with hydrochloric acid. In the above solution, 400 g·L^-1 sodium potassium tartrate tetrahydrate solution of 1 mL and 250 g·L^-1 potassium oxalate monohydrate solution of 1 mL were added to mask the interference of Fe³⁺ and PO₄^3- (suitable when the mass of total Fe³⁺ and P were less than 3 000, 250 μg, respectively). In the experimental group, ammonium paramolybdate was used for color reaction (ammonium chloride was added in the chroma compensation group). With water as reference, the absorbance of the color system was measured at 400 nm. The target absorbance was calculated by the absorbance difference between the experimental group and the chroma compensation group, and the mass concentration of active silicon (ρ₁) was calculated based on the target absorbance. ② Water sample was passed through PTFE filter, hydrofluoric acid was added to decompose colloidal silicon in water into active silicon, and the above solution was digested in a hydrothermal kettle at 100 ℃ for 20 min. After aluminum chloride was added to eliminate excess F^-, the mixed solution was washed, diluted, and done as follow-up steps with ①. The mass concentration of colloidal silicon was obtained by difference between the result (ρ₂) and ρ₁. ③ Water sample was not filtered, and directly pretreated and detected in accordance with the method provided by ②. The mass concentration of adsorbed silicon was obtained by the difference between the result (ρ₃) and ρ₂. ④ Hydrofluoric acid and sodium persulfate for digesting dissovled organic silicon to active silicon were added to the water sample, and the mixed solution was digested at 120 ℃ for 30 min in a hydrothermal kettle. The solution obtained was mixed with aluminum chloride, washed, diluted, and done as follow-up steps with ①. The mass concentration of dissolved organic silicon was obtained by the difference between the result (ρ₄) and ρ₃. ⑤ Sodium hydroxide (for digestion of granular silicon and colloidal silicon) and sodium persulfate were added to the water sample, and the mixed solution was digested at 200 ℃ for 120 min in a hydrothermal kettle. The solution obtained was mixed with sulfuric acid washed, diluted, and done as follow-up steps with ④. The mass concentration of granular silicon was obtained by the difference between the result (ρ₅) and ρ₄. The series of standard solutions prepared by SiO₂ datum chemical reagent were detected after treatments with interference masking, digestion and color reaction, and linear relationships obtained between mass of silicon (calculated as SiO₂) and its corresponding absorbance were kept in the range of 500-5 000 μg, with detection limits of silicon (calculated as SiO₂) in the range of 4.0-7.1 mg·L^-1. Tests for accuracy and precision were made on standard solution of silicon at intra-and inter-laboratory. RSDs (n=5) of the determined values of the total silicon from intra-laboratory and inter-laboratory were found in the range of 0.85%-4.5% and 2.1%-5.0%, relative errors were in the range of -7.4%-0.47% and -4.5%--1.7%. Values of test for spiked recovery ranged from 95.8% to 96.8%. The proposed method was applied to the analysis of actual samples, with the contents of active silicon accounted for 94%-97% of that of the total silicon, and it was suggested to remove silicon impurities in the produced water of heavy oil by chemical precipitation.