Sanky peel. Phosphate binding and suspension coagulation

Two aims and two types of material

The fruit peel of Corryocactus brevistylus, known as sanky or sancayo, was studied as a raw material for phosphate removal and reduction of water turbidity. The first aim concerns ions dissolved in solution. The second involves a suspension of fine kaolin particles [Contreras-López et al. 2021].

The phosphate study compared dried, ground peel with two heat-treated materials, one of which was additionally chemically activated. Ground peel released phosphates into solution. Both processed materials reduced their concentration. The thermal treatment was selected for further experiments, also accounting for preparation complexity and costs [Contreras-López et al. 2021].

Calcium participates in phosphate binding

Calcium ions were added to the solution. In infrared spectra of the material after the experiment, the authors identified bands consistent with brushite, hydrated calcium hydrogen phosphate. The proposed explanation combines ion retention at the surface with formation of a mineral phase. At pH 6,5 and 9,0, phosphate concentration decreased, while at pH 4,0 the material released phosphates. The pH change therefore reversed the direction of the observed process [Contreras-López et al. 2021].

Peel as a suspension coagulant

In 2026, Herrera-Linares and colleagues assessed peel powder in model water made turbid with kaolin. A coagulant facilitates destabilisation of a suspension, after which particles can be separated. The available abstract describes comparisons of powder dose, pH and initial turbidity. These conditions define the scope of the result obtained [Herrera-Linares et al. 2026].

The authors reported an 82% reduction in turbidity at a dose of 0,1 g/L, pH 3 and an initial turbidity of 200 NTU, units measuring light scattering by the suspension. At pH 6 and the same initial turbidity, the powder produced a 48% reduction and the comparator aluminium sulphate 99%. The result depended strongly on the conditions under which the two coagulants were compared [Herrera-Linares et al. 2026].

The pH dependence can be compared with experiments on dried Opuntia spp. material. Miller and colleagues found greatest activity in alkaline kaolin suspensions while also measuring zeta potential and imaging flocs. Particles remained negatively charged and clay was distributed along a thread-like matrix. Together with reduced performance after overdosing, these observations made adsorption and bridging the authors’ best-supported explanation [Miller et al. 2008].

Both studies measured suspension clarification, while favourable pH conditions differed. Raw material, plant part, preparation and particle environment also differed. The Opuntia mechanism study identifies a set of measurements useful for further characterisation of sanky peel. Direct charge and aggregate-shape measurements would establish how pH changes the interaction of this particular preparation with kaolin [Miller et al. 2008] [Herrera-Linares et al. 2026].

What is compared between studies

Phosphate removal and turbidity reduction describe separate properties. Combining the two approaches in one process remains a research question. Assessment of further applications includes real-water composition, contact time, material separation and the fate of retained components. The reference points in the current papers are defined laboratory solutions and suspensions [Herrera-Linares et al. 2026].

The comparison also involves the form of the material being removed. Phosphate remains dissolved until it binds to the material or enters a newly formed mineral phase. Kaolin begins as particles dispersed in water that can settle after aggregation. The response measure follows the process, phosphate concentration or turbidity. This distinction supports planning successive treatment stages around pH and the fate of the resulting solids [Contreras-López et al. 2021] [Herrera-Linares et al. 2026] [Miller et al. 2008].