Comparison of quantitative X-ray diffraction methods for characterizing composition and yield between distilled water and dispersant-aided extractions
This research attempted to quantify the relative advantages and disadvantages of competing methods for extraction and characterization of mineral colloid material (effective spherical diameter 100 - 1000 nm) in soils. Quantification of colloid constituents in soils allows for greater precision in nutrient and contaminant transport modeling in the shallow subsurface. Mineral colloids increase transport rates for certain immobile solids by serving as a semi-mobile phase to which those solids can readily sorb and desorb. This study provides a comparison of mineral colloid yield and composition from synthetic rainwater and sodium diphosphate (TSPP) extraction. Compositional differences are expected to arise from pH increase, solution strength increase, and interlayer sodium infiltration in silicate clays from the addition of TSPP. The compositional effects of TSPP were quantified using two methods of quantitative X-ray diffraction (QXRD) to compare the identities and weight fractions of extracted mineral species. Colloid characterization with QXRD is complicated by errors associated with peak broadening, micro-absorption effects, and non-uniform background noise arising from small crystallite size. Results from internal-standard quantification were compared to results from direct-estimation quantification to determine the accuracy and precision of direct estimation. Direct estimation was proposed in the 1960s but did not displace the internal standard method in common practice. The method directly calculates weight percentages using a linear system of explicit equations for each mineral species. By quantifying the increase in yield and change in composition from using TSPP as a peptizing agent and by evaluating the validity of direct estimation in QXRD, mineral colloid characterization may be made cheaper and quicker, increasing its utility for stakeholders in agriculture and environmental remediation.