Overview
Sigauke and colleagues review the literature on spent bleaching earth (SBE), the solid waste generated when bentonite-derived bleaching earth is used to decolourise and decontaminate crude edible vegetable oils. The review covers bentonite acid activation methods, the bleaching unit operation, SBE characterisation by XRD/FTIR/TGA, residual oil composition, and downstream applications (biodiesel, biogas, briquettes, bio-organic fertiliser, lubricating grease, foamed concrete, non-fired wall tiles). Heavy-metal-relevant content is limited: the authors state that bentonite-based bleaching earth removes heavy metals and phosphorus from crude oil during the bleaching unit operation, and compiled tables report trace-element composition of SBE (Cu, Zn, Mn, Fe) and of residual oil retained in SBE (Cu, Fe) drawn from prior primary studies.
Key numbers
SBE compositional ranges compiled from prior literature (Table 3, p. 202)
Values are reported in wt% unless otherwise specified and represent characterisation results compiled by the review authors from the cited primary studies.
| Parameter | Reported value | Cited source |
|---|---|---|
| Moisture content | 1.8 to 16 % across cited studies | Beshara & Cheeseman 2014b; Suhartini et al. 2011; Kheang et al. 2013; Yuan et al. 2020; Moshi 2017 |
| SiO2 | 37.45 to 70.87 % | Mana et al. 2011; Kheang et al. 2013; Yuan et al. 2020; Moshi 2017 |
| Al2O3 | 8.01 to 11.83 % | Mana et al. 2011; Kheang et al. 2013; Yuan et al. 2020; Moshi 2017 |
| Carbon | 15.5 to 28.52 % | Loh et al. 2015; Mana et al. 2011; Yuan et al. 2020; Moshi 2017 |
| Nitrogen | 0.06 to 0.71 % | Loh et al. 2015; Kheang et al. 2013; Yuan et al. 2020; Moshi 2017 |
| C:N ratio | 256.5 to 293:1 | Loh et al. 2015; Kheang et al. 2013; Moshi 2017 |
| pH | 4.5 to 5.33 | Loh et al. 2015; Kheang et al. 2013 |
| K2O | 0.27 ± 0.02 % | Loh et al. 2015 |
| CaO | 3.58 ± 0.36 % | Loh et al. 2015 |
| MgO | 1.55 ± 0.06 % | Loh et al. 2015 |
| Cu | 41.4 ± 0.6 % (as printed; see Verification notes) | Loh et al. 2015 |
| Zn | 30.1 ± 1.7 ppm | Loh et al. 2015 |
| Mn | 359 ± 4 ppm | Loh et al. 2015 |
| Fe | 10026 ± 663 ppm | Loh et al. 2015 |
Residual oil composition in SBE (Table 4, p. 203)
Compiled by the review authors from Kheang et al. 2013, Kheang et al. 2006, Moshi 2017, and Huang & Chang 2010.
| Parameter | Reported value(s) | Cited source |
|---|---|---|
| Free fatty acids, FFA (%) | 12.6; 13.01; 11.5; 24.1 | Kheang et al. 2013; Moshi 2017; Kheang et al. 2006; Huang & Chang 2010 |
| Peroxide value, PV (meq/kg) | 3.4; 2.98; 3.1 | Kheang et al. 2013; Moshi 2017; Kheang et al. 2006 |
| Phosphorus, P (mg/kg) | 18.7; 19.3 | Kheang et al. 2013; Kheang et al. 2006 |
| Iron, Fe (mg/kg) | 1.24; 0.22 | Kheang et al. 2013; Kheang et al. 2006 |
| Copper, Cu (mg/kg) | 0.38; 0.32 | Kheang et al. 2013; Kheang et al. 2006 |
| beta-carotene (mg/kg) | 0.401; 3; 6 | Moshi 2017; Kheang et al. 2006; Kheang et al. 2013 |
| Total vitamin E (mg/kg) | 0; 38.8; 39.03 | Kheang et al. 2006; Kheang et al. 2013; Moshi 2017 |
Process-context statements
- “Bentonite activated with acid are the common bleaching agents… During the bleaching process, activated bentonite adsorbs oxidation products, peroxides… pigments and fatty acids, salts residues, phosphatides, gums, trace metals, phospholipids, soap, carotenoids” (p. 191).
- “Disposing of SBE in landfills causes environmental pollution problems emanating from residual oil content, water content, leachable trace heavy metal, degradation of organic compounds and is a source of odours” (p. 190).
- “The acidic, catalytic properties and trace heavy metals in the clay speed up the decomposition of hydroperoxides in residual oil” (p. 200).
- The authors cite Tang et al. 2015, “One-step calcination of the spent bleaching earth for the efficient removal of heavy metal ions” (ACS Sustainable Chemistry and Engineering 3(6):1125-1131), as a reference treatment-of-SBE study but do not report quantitative heavy-metal removal performance values for that work in the review text.
- World SBE generation is estimated as 1.5-2.0 million tonnes/year against global cooking-oil production of 128.2 million metric tonnes in 2007 (review text, p. 191, unattributed in source); the same paragraph cites Loh et al. 2015 for the Malaysia-specific figure of up to 170,000 tonnes of SBE per annum from crude palm oil refining.
The review does not report measurements of lead, cadmium, arsenic (total or inorganic), mercury (total or methyl), nickel, aluminum, chromium (total or hexavalent), tin, antimony, or uranium in SBE, in residual oil, or in finished edible oils.
Evidence Fitness
This is a narrative review, not a primary measurement study. The quantitative compositional tables (Tables 3 and 4) are compiled values drawn from prior peer-reviewed studies and are presented here as a literature summary, not as new data. Public evidence label “Context only” is appropriate: the source supports process-context claims about the bleaching unit operation in edible-oil refining and points to specific primary studies (Loh et al. 2015 for SBE composition; Kheang et al. 2013, 2006 for residual oil composition; Tang et al. 2015 for SBE-as-sorbent for heavy metal ions) where the underlying observations live.
The source does not support occurrence claims for lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), nickel (Ni), aluminum (Al), chromium (Cr), tin (Sn), Sb, or uranium (U) in any food matrix and should not be used as the basis for HMTc threshold work or ingredient contamination_profile values. Its role in the corpus is to document that edible oil refining includes a bleaching step that adsorbs trace metals into a clay matrix, providing process context for vegetable-oil heavy-metal interpretation.
Methods (brief)
Narrative literature review without explicit search-and-selection protocol. The review compiles published characterisation data on SBE and residual oil in SBE from the cited primary studies; analytical methods for the underlying primary measurements are not re-stated in detail in the review tables and would need to be retrieved from each cited study (e.g., Loh et al. 2015 for Cu/Zn/Mn/Fe in SBE; Kheang et al. 2013, 2006 for residual oil characterisation). The authors describe characterisation techniques for SBE in general terms (XRD for crystalline phase, FTIR for functional groups, TGA for thermal decomposition) but do not present new spectra or measurements.
Implications
Certification: Process context only. The review supports the general claim that bentonite-based bleaching is a unit operation in edible-oil refining that adsorbs trace metals, but provides no occurrence values that should inform HMTc thresholds for finished edible oils. Threshold work on vegetable oils should rely on primary occurrence studies of finished oils, not on SBE compositional summaries that document what was removed.
Courses: Useful as background reading for explaining the edible-oil refining sequence (degumming, neutralisation, bleaching, deodorisation) and where trace metals exit the process stream. The Tang et al. 2015 reference cited here is worth pursuing as primary evidence on SBE as a heavy-metal sorbent.
App: No ingredient contamination_profile impact.
Microbiome: Not addressed beyond the noted role of trace elements (Mn, Fe, Ca, Mg, Ti) as anaerobic-digestion co-factors when SBE is used as a biogas substrate.
Related evidence
- Copper
- Iron
- Manganese
- Zinc
- Vegetable Oils
- Vegetable Oils
- Other cooking oils (canola, sunflower, coconut, avocado, sesame)
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