By Vuyisile L. Dube

EVERY year, millions of tonnes of cashew nut shells are discarded as agricultural waste across Africa and other cashew-producing regions. 

Now, research published in the Zimbabwe Journal of Science and Technology (ZJST), housed at the National University of Science and Technology (NUST) has revealed a way to put that waste to work, turning it into tiny particles capable of pulling toxic metals out of contaminated water.

The research, centres on cashew nut shell liquid (CNSL), a natural oil found within the spongy layer between a cashew’s outer shell and its kernel. 

The CNSL used in the study had a viscosity of 820 centipoise, a measure of how thick and flow-resistant the liquid is, which made it suitable for the processing technique the team employed. 

By mixing it with formaldehyde through emulsion polycondensation, the research led by Sempheho Siafu and Wilfred Johnson produced what they call micron-sized polymeric particles, or MSPPs, ranging from 0.1 to 0.4 micrometres in size, roughly 200 times thinner than a human hair.

The particles on their own already showed some ability to capture heavy metals from water. But the team went further, chemically modifying the surface of the MSPPs to introduce functional groups designed specifically to attract and hold metal ions. 

Using three different reagents, they created three distinct variants: carboxylated, sulfonated, and aminopropylsilylated particles, each with a different surface chemistry and a different affinity for lead and cadmium.

The results showed clear improvement across the board. 

“The amount of Pb2+ and Cd2+ exchanged by the unmodified MSPP was 0.022 mg/g and 0.034 mg/g, respectively. The amounts of Pb2+ exchanged by the modified resins were 0.056 mg/g, 0.023 mg/g, and 0.103 mg/g for the carboxylated, sulfonated and aminopropylsilylated MSPPs, respectively,” noted the paper.

The aminopropyl silylated, a material whose surface has been chemically modified with aminopropyl silane molecules, variant stood out for lead removal, achieving nearly five times the uptake of the unmodified particles. 

For cadmium, the sulfonated particles performed best. 

“The amount of Cd2+ exchanged by the carboxylated, sulfonated and aminopropylsilylated MSPPs was 0.062 mg/g, 0.093 mg/g, and 0.040 mg/g,” noted the paper.

The team used Atomic Absorption Spectroscopy to verify the measurements, confirming that column-packed versions of the particles successfully exchanged lead and cadmium ions out of contaminated water samples. 

They also found a strong correlation between how well the particles performed and the pH of the water being treated, meaning the acidity or alkalinity of the water plays a significant role in the process.

Reporting on the findings, Siafu and Johnson were candid about the limitations. The current uptake values are low, and the paper calls for further study to optimise the particles' capacity. 

But the direction of travel is clear: each chemical modification improved performance, and the team believes further refinement of the surface chemistry could yield much stronger results.

Below is the abstract and link to the paper.

Abstract

Cashew nut shell liquid (CNSL)-based micron-sized polymeric particles (MSPPs) of average particle sizes of approximately 0.1 µm to 0.4 µm, were prepared by formaldehyde emulsion polycondensation of technical CNSL. The technical CNSL of 820 centipoise viscosity was used. Surface chemical modifications of MSPPs were carried out to introduce appropriate functional groups for cation exchange. Through suitable reactions with monochloroacetic acid, concentrated sulfuric acid, and aminopropyltrimethoxysilane, respectively, carboxylic, sulphonic, and aminopropylsilyl functional groups were introduced onto MSPPs. Column-packed MSPPs and chemically modified MSPP were successfully used to exchange lead and cadmium ions in contaminated water. The extent of Pb2+ and Cd2+ exchanged was analysed by AAS. The amount of Pb2+ and Cd2+ exchanged by the unmodified MSPP was 0.022 mg/g and 0.034 mg/g, respectively. The amounts of Pb2+ exchanged by the modified resins were 0.056 mg/g, 0.023 mg/g, and 0.103 mg/g for the carboxylated, sulfonated and aminopropylsilylated MSPPs, respectively. The amount of Cd2+ exchanged by the carboxylated, sulfonated and arnimpropylsilylated MSPPs was 0.062 mg/g, 0.093 mg / g, and 0.040 mg/g. The results suggested improvement in Pb2+ and Cd2+ exchange capacity with chemical modifications of the CNSL based polymer particles. A strong correlation between Pb2+ and Cd2+ uptake of the polymer particles and the pH of the solution is also observed. The low values of Pb2+ and Cd2+ exchanges, call for further study to optimise the resins' heavy metal ions uptake capacity

The paper is available at: https://journals.nust.ac.zw/index.php/zjst/article/view/225