Mass-Balance Validation in Bench-Scale Gravimetric Sorption Testing: A Case Study on Zeolite and Clay for Used Motor Oil and Diesel Fuel

Gravimetric sorption testing is widely used to screen candidate sorbents for petroleum hydrocarbon remediation. Using a five-material Romanian sorbent screening as a case study, this study demonstrates why explicit mass-balance validation should be mandatory in such protocols. Natural zeolite, clay, peat, wood sawdust, and hops were tested against used motor oil and diesel fuel under a standardized protocol. A mass-balance check showed that results for peat, wood sawdust, and hops implied hydrocarbon uptakes exceeding the physically available mass; because the underlying raw records could not be located, these three datasets were excluded, illustrating the kind of error such validation is designed to catch. The validated result is a two-material comparison: for used motor oil and diesel fuel, respectively, clay and zeolite showed the higher sorption capacity (1.210 ± 0.016 g/g vs. 1.170 ± 0.010 g/g, Welch’s t-test, t(6.7) = −4.78, p = 0.0022; and 1.444 ± 0.005 g/g vs. 1.352 ± 0.031 g/g, t(4.2) = 6.51, p = 0.0023), a reversal consistent with, but not confirmed by, a pore-size- and viscosity-selective sorption hypothesis. Both materials showed low inter-replicate variability (CV < 2.3%), confirmed by Mann–Whitney U tests (p = 0.0079 for both hydrocarbons). Because measured capacities are three- to thirteen-fold higher than literature values for raw clinoptilolite and bentonite, these results are presented as protocol-specific bench-scale rankings rather than generalizable material properties. This study is offered as a worked example of validation-first data screening in gravimetric sorption testing; deeper characterization and re-measurement of the excluded materials are identified as next steps.