Overview
Four-page Springer book chapter from a Tunisian–Belgian group testing whether three local clay deposits in the Gabes region (Haidoudi, Romana, Hmaymet) can adsorb heavy metals (the authors emphasise cadmium (Cd) and Cr) from phosphogypsum — the by-product of phosphoric-acid manufacture at the Groupe Chimique Tunisien plant. The paper is qualitative in published form: it asserts that phosphogypsum from Gabes carries “high” concentrations of Cd, chromium (Cr), and uranium (U) “compared to other wastes of phosphogypsum in the world and to the soil remediation limits” without printing the mg/kg values, and reports column-leaching results as “the concentration of metals decreased … all the way down to 0 mg/kg” for the Haidoudi and Romana clays without printing breakthrough curves or numeric removal efficiencies. The Hmaymet clay is reported as “less substantial” in adsorption performance, again without numbers.
Phosphogypsum is upstream of the agricultural-soil cadmium pathway (phosphate-fertiliser raw-material chain), and the paper documents a remediation technology rather than a food-occurrence finding. It is retained in the Heavy Metal Index as a C-tier remediation-leads source for the Tunisian phosphogypsum-storage context, not as occurrence evidence for any food matrix.
Key numbers
The paper does not print the underlying ICP-OES values for Cd, Cr, U, or Th in either the phosphogypsum or the leachate. The numerical statements that survive in the published chapter are:
- Phosphogypsum sampling year: April 2018 (Groupe Chimique Tunisien, Gabes).
- Three clay deposits tested: Haidoudi, Romana, Hmaymet (all from the Gabes area, southern Tunisia).
- Qualitative XRD mineralogy: all three clays contain kaolinite, gypsum, and calcite; quartz and mica appear as accessory minerals. Silicon oxide is the major constituent and is highest in the Hmaymet clay.
- FT-IR signal: the band at 1430 cm⁻¹ is reported as indicating the disappearance of carbonate after purification.
- Qualitative column-experiment outcome: post-leaching metal concentration “decreased … all the way down to 0 mg/kg” for Haidoudi and Romana columns; Hmaymet column gave a positive but “less substantial” reduction.
- The authors flag Cd, Cr, and U as the elements present at the highest concentration in Gabes phosphogypsum relative to international comparators and to soil-remediation limits (Rutherford et al. 1994), but the comparator values and the Gabes values are not printed.
- Radioactivity statement: Ur and Th activity (Bq/kg) in the Gabes phosphogypsum samples was reported as “acceptable and not dangerous to human health” and “lower” than other phosphogypsum samples cited from other countries. Numeric activity values are not printed.
Methods (brief)
Three Tunisian clay deposits (Haidoudi, Romana, Hmaymet) and phosphogypsum waste from the Groupe Chimique Tunisien (Gabes) were sampled, air-dried, and characterised. Mineralogy: X-ray diffraction (Bruker D8 ECO Advance, Cu tube anode, Lynxe Eye XE energy-dispersive position-sensitive detector). Functional-group characterisation: FT-IR (EQUINOX 55 spectrometer, KBr pellet technique). Both XRD and XRF measurements were carried out at the Department of Geology, Research Group Mineralogy and Petrology, Faculty of Sciences, Ghent University, Belgium. Column experiments used three laboratory columns, each with a clay layer, a transition layer, and a phosphogypsum waste layer above the clay; leachate composition was measured by inductively coupled plasma–optical emission spectrometry (ICP-OES, Varian Vista MPX, Varian, Palo Alto, California, USA). The paper does not report column dimensions, packing density, hydraulic loading, leaching solution chemistry, contact time, pH, replicate structure, detection limits, or quantitative removal-efficiency calculations.
Limitations
C-tier conference / book-chapter contribution; four printed pages; no quantitative data tables for either the input phosphogypsum metal content or the post-leaching effluent metal content. Removal claims are stated only as “decreased … down to 0 mg/kg” without breakthrough curves, isotherm fits, kinetic models, or replicate measurements. No detection limits or method-blank data are reported, so the “0 mg/kg” claim cannot be distinguished from “below detection by ICP-OES under the unreported analytical conditions”. Sample size (single composite per matrix?) is not specified. The chapter’s framing emphasises Cd and Cr as “the most hazardous metals” but the underlying ICP-OES dataset for those elements is not printed; the reader is told that Gabes phosphogypsum exceeds soil-remediation limits but neither the Gabes concentration nor the soil-remediation limit is printed. No statistical analysis. No comparison against a control column without clay. No food matrix is measured. Sampling year is 2018; chapter published 2022. Two of five authors are co-surnamed Moussa; the Belgian co-author (J. De Grave) is associated with the analytical-instrumentation contribution at Ghent University rather than the field sampling. Reference list is short (nine references — Beveridge & Pickering 1983; Briffa et al. 2020; Dammak et al. 2014; Eloussaief et al. 2011; Futalan et al. 2011; Masindi & Muedi 2018; Rutherford et al. 1994; Rutherford et al. 1995; Tran et al. 1999) and does not include the Tunisian-clay sorption literature that would normally be expected for a paper of this kind.
Implications
Minimal direct value for the Heavy Metal Index. The wiki’s scope is heavy-metal occurrence in food and personal-care supply chains; this paper is a remediation-engineering study on an industrial waste stream. It is retained as a leads document for two narrow purposes:
Phosphate-rock and phosphogypsum supply-chain context: Phosphogypsum is the principal solid by-product of phosphoric-acid manufacture, and phosphoric acid feeds the phosphate-fertiliser supply chain that is upstream of cadmium accumulation in agricultural soils and cereal grains worldwide. This paper documents that the Gabes (Tunisia) phosphogypsum stream is enriched in Cd and Cr relative to international comparators (per the authors’ qualitative claim; the underlying numbers would need to be retrieved from prior Rutherford et al. or Tunisian-Chemical-Group characterisation papers cited in this chapter). The Tunisian Chemical Group is among the major phosphate-rock processors in the Mediterranean basin, and Tunisian phosphate rock is exported into European and African fertiliser supply chains — this is supply-chain provenance context, not occurrence data.
Adsorbent-material leads: Kaolinite-bearing natural clays from arid regions are documented here as candidate low-cost sorbents for Cd and Cr from phosphogypsum leachates. This is methodological-leads context for the broader low-cost-adsorbent literature already represented in the corpus (e.g., Heavy Metals (Pb+2, Ni+2, Zn+2) Removal from Wastewater Using Low Cost Adsorbents: A Review) and for environmental-remediation context on Cadmium and Chromium, not direct occurrence evidence.
Related evidence
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