Of shells and tongues

Advancing biogenic materials for water purification

Ismaila Emahi is a man on a mission – to find low-cost solutions to providing cleaner, safer water. “Heavy-metal contamination, driven by industrial activities such as mining, threatens drinking water safety worldwide, from rural communities in Ghana relying on polluted rivers to urban centres facing groundwater contamination,” explained Iso Lomso fellow Emahi who is from the Department of Chemical Sciences at the University of Energy and Natural Resources in Sunyani, Ghana. “These toxic metals, including lead, arsenic and mercury, pose severe health risks, necessitating advanced, accessible technologies for monitoring and purification.” “My research has developed sustainable adsorbents from coconut shell and Crescentia cujete (calabash) shell powders, achieving over 90% removal of these toxic metals, and significant turbidity reduction in laboratory and field tests in Ghana. However, challenges such as material scalability, regeneration efficiency, and the lack of real-time detection limit their broader application.” Emahi explained that the United Nations declared clean, safe and accessible water as a human right as far back as 2010. It’s also Sustainable Development Goal 6 with an ever-approaching target of 2030. But, what happens if the desperate activities of some to earn a living puts others in danger? Emahi focused in on the devastating effects on water supplies of illegal gold mining – or galamsey - in Ghana. Showing videos, Emahi highlighted that the miners use mercury to separate the gold leaving mercury and other toxic metals in the water supply of nearby communities. The World Health Organization ranks mercury as among the ten most-hazardous substances. Even in small doses it can cause damage to the nervous, respiratory and cardiovascular systems leading to endothelial dysfunction, gastric and vascular disorders, liver, kidney, and brain damage, hormonal imbalances, miscarriages, reproductive disorders, affect newborn development and cause death. In some mining areas in Ghana mercury has been found in the water at 500 times the safe limit and kidney disease in these areas has increased 10 times. Emahi explained that potassium alum is used as the first step to purify such water. It’s an inorganic compound that binds to negatively charged, suspended particles and pushes the waste to the bottom. “It can clarify one litre of muddy water for 15 US cents,” he said. “But it only clarifies – not purifies.” The problem is so bad that the government is switching off treatment plants because they can’t clean the water. “Comprehensive drinking-water treatment needs integrated multiple steps and techniques. But in conventional treatments more advanced means more expensive,” added Emahi. And stopping the illegal mining and enforcing environmental laws is a huge challenge. “To miners it’s worth the risk because one small nugget of gold is the equivalent of a month’s salary,” he said. “They know the risks but are willing to pay the price.” Emahi and his colleagues are therefore looking for cheaper alternatives and specifically looking at plant materials as potential sources. “We believe plant biomass may be sustainable adsorbents for heavy-metal removal.” They have focused in on three options – plantain stems, calabash fruit (Crescentia cujete) shells and coconut shells. The first step has been to characterise the composition of the key compounds which varies from plant to plant and among the different parts of the same plant. This has been done using scanning electron microscopy and infrared spectroscopy. And results thus far have been promising but not without challenges. “The calabash shells are particularly promising,” said Emahi. “We get them to a powder form and use this as a chemical substitute. We have found that processed calabash-shell powder is able to reduce turbidity (cloudiness) of the water by 84.3% at an optimal dosage of one gram per litre.” Ongoing work with calabash is looking at adding chitosan (a polysaccharide) and testing different particle sizes and purification methods to improve filtration. They have also tested coconut shells for their effectiveness in removing both mercury and arsenic (which is found at higher than recommended levels in water across the globe) and have found that activated carbon from coconut shells show increased efficacy with removal of nearly 70% arsenic and over 50% of mercury. Further laboratory processing of the coconut shells enabled up to 99% removal of toxic metals with 500 ml of turbid water cleared using 50g of powder in five to 10 minutes. But removal of dangerous heavy metals can’t be done at the expense of the good metals. “Drinking water is not pure,” explained Emahi. “That’s why tap water tastes different in different countries. Good drinking water must contain recommended levels of essential metal ions like calcium, magnesium and iron.” “Currently the coconut-shell filter removes considerable amounts of magnesium and calcium while simultaneously leaching sodium and potassium into the water. This raises concerns about its suitability for safe water treatment without further modification or post treatment,” said Emahi. The work is also looking at the lifespan of the filters, and whether they and the metals removed can be rejuvenated. Obviously as plant shells they are biodegradable.

Artificial tongues

In addition to removing harmful substances, monitoring is also vital. Conventional monitoring methods include atomic-absorption spectroscopy, atomic-fluorescence spectroscopy and inductively coupled plasma while removal methods include ion exchange, ultra-filtration, nanofiltration, reverse osmosis, electrodialysis and electrochemical processes.  “These work but are very expensive and impractical at the point-of-use,” said Emahi. “We need cheaper options.” The group is therefore investigating an artificial tongue idea – a system they hope could quantify and remove the toxic metals simultaneously. In collaboration with researchers from the University of Cambridge and with funding from Cambridge-Africa Alboarada Trust Fund, they are investigating using DNA aptamers as modified sensors which they hope to be suitable for fast detection of toxic metals even at low concentrations. The preliminary work is about establishing optimal approaches and electrode-modification strategies for fabricating aptamer-based sensors for environmental monitoring – with an initial focus on arsenic and nitrates. Aptamers are single-stranded DNA or RNA molecules that bind to targets with affinity and specificity. They are identified by in vitro selection or SELEX (Systemic Evolution of Ligands by EXponential enrichment) but this is a laborious process with a low success rate. For small molecules like metal ions, the conventional SELEX process may not work, and they are therefore using a modified version called Capture or Structure-switching SELEX. “Aptamers don’t form by themselves and don’t necessarily bind for all metals,” said Emahi. “We are specifically targeting arsenic and mercury, and using water samples from different communities.” “We have had some challenges,” he continued, “Inconsistent signal responses to different concentrations of arsenic especially at low concentrations could compromise accuracy of sensor reading if fabricated on electrochemical or QCM platforms. We are therefore looking at new directions including an aptamer-modified biogenic filter. But we need to find more funding for this work.” “By combining aptamers’ high specificity with the eco-friendly adsorption capacity of biogenic materials, this technology aims to deliver real-time, cost-effective water purification solutions suitable for global contexts – from resource-limited regions to advanced water-treatment facilities. We believe it has promise as an innovative tool for improving global drinking-water safety.” The work has involved community engagement from the beginning as well as emphasising the need for solutions that can be locally produced. “We are not thinking about profit making in any of these ideas,” said Emahi. “The shells are already waste so using them could create jobs for communities.” And, of course, illegal mining is only one of many causes of water pollution which includes chemical pollution from agricultural activities as well as contamination of water sources with bacteria and viruses. “Mercury is banned but the miners can access it – so we must ask who is at the top of this supply line. The buyers and sellers are not in the open market.” “This is a big concern for Ghanaians – but it takes political will to change it. Ideally, we want to remove all contaminants and provide clean, healthy water. We hope our work will have broader application and can influence government policies.”   Michelle Galloway: Part-time media officer at STIAS Photograph: SCPS Photography