Sigauke et al. 2024 - Spent bleaching earth synthesis, properties, characterisation, and application

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.

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.

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 Pb, Cd, As, Hg, Ni, Al, Cr, Sn, Sb, or 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.

Wiki pages this source may touch

• copper
• iron
• manganese
• zinc
• vegetable-oil
• vegetable-oils
• cooking-oils-other

Verification notes

• DOI 10.46754/jssm.2024.03.014 prints on the first page; submission and acceptance dates are 31 August 2023 and 29 October 2023, publication 15 March 2024. eISSN 2672-7226. UMT Press is the publisher; the article header carries a ”© UMT Press” notice, so license is recorded as publisher copyright (reference-only).
• Table 3 prints “Cu 41.4 ± 0.6%” under a header that states “Results (wt.% unless specified)“. A copper concentration of 41.4 wt% in spent bleaching earth is physically implausible — Loh et al. 2015 (the cited primary source) reports trace metals in SBE in ppm. The review has carried over what is almost certainly a unit-label error from the source, where Cu should read as ppm consistent with Zn (30.1 ppm), Mn (359 ppm), and Fe (10026 ppm) in the same column. The page reproduces what the review prints and flags the apparent typo here; the primary Loh et al. 2015 paper would resolve it.
• The review uses the term “heavy metals” in a broad sense that includes Cu, Fe, Mn, Zn (essential trace elements) and references “leachable trace heavy metal” in landfill SBE without specifying which elements. None of HMI’s primary toxic-metal analytes (Pb, Cd, As, Hg, Ni, Al, Cr, Sn) are quantified in the review tables.
• Matrices use two terms not in the common-matrices list: spent-bleaching-earth and vegetable-oil-refining-residue. These describe the industrial byproduct that is the subject of the review and are descriptive analogues to existing slugs such as agro-industrial-waste; both should be confirmed against the matrices controlled vocabulary on the next Karen review.
• Jurisdictions are left empty: this is a global narrative review citing primary studies from Malaysia, Saudi Arabia, Algeria, Poland, Japan, Iran, Iraq, China, Brazil, Indonesia, Botswana, and elsewhere. The Botswana institutional affiliation of the authors does not bound the scope of the review.
• The review covers cottonseed, sunflower, palm, niger seed, soya bean, and colza oils within its bleaching-process tables, but does not break the SBE composition tables down by source oil. Frontmatter therefore uses the umbrella vegetable-oil / vegetable-oils ingredient slugs rather than per-oil slugs.
• No brand names are reported in the review; method/instrument vendor names are not referenced in detail in this review either, so no Part 12 firewall calls were required during ingest.
• Audit subagent (2026-06-02) flagged Table 4 citation-value pairings on PV, Fe, Cu rows where values had been re-sorted low-to-high but cited-source columns left in source-publication order. Independently verified against source p. 203; the audit was correct. Same issue extended to the FFA row (audit did not flag but it had the same defect). All four rows corrected so that the value order matches the citation order as printed in source Table 4 (Kheang 2013; Moshi 2017; Kheang 2006; Huang & Chang 2010 left-to-right). Phosphorus, beta-carotene, and total vitamin E rows were already correctly paired and were not changed.
• Audit subagent (2026-06-02) flagged the “(Loh et al. 2015)” attribution on the 1.5-2.0 Mt SBE / 128.2 Mt cooking-oil figure as over-specified. Independently verified against source p. 191; the global tonnage figure is printed in the review text without an immediate in-text citation, while the adjacent Loh et al. 2015 cite anchors specifically to the Malaysia 170,000-tonne figure. The Key numbers process-context bullet was corrected to attribute the global figure to the review text (unattributed in source) and to attach Loh et al. 2015 only to the Malaysia-specific number.

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.