Clay Derived Zeolites for Removal of Nitrates, Phosphates and Heavy Metals From Water

Abstract

Access to clean and safe water for domestic and industrial use remains a significant challenge in developing countries of the Third World. In Kenya, safe drinking water is accessible to only 59% of the population. Multiple factors, including droughts, forest degradation, population growth, and inadequate water supply management, contribute to water scarcity in Kenya. Freshwater resources are unevenly distributed across the country and face heavy pollution from raw sewage, domestic and industrial waste, agricultural waste, and emerging pollutants like plastic, significantly limiting clean water accessibility. The main freshwater sources in Kenya encompass groundwater, water basins, dams, rivers, lakes, swamps, and springs. To address these water crises, cost-effective water treatment methods are essential. This study's focus was on water pollutants, specifically nitrates (NO-3), phosphates (PO43-), and heavy metals including copper (Cu2+), lead (Pb2+), and cadmium (Cd2+), originating primarily from agrochemicals like fertilizers and pesticides. This research investigated the potential application of clay-derived zeolites, produced from kaolinite clay material sourced from Mukurweini in Nyeri County, Kenya, for treating water contaminated with agricultural waste. The study employed wastewater containing each salt at a 1000 ppm stock solution, serially diluted to the desired concentration for evaluating zeolite efficacy in pollutant removal. Raw clay mineral samples weighing 3 kg each were subjected to thermal activation in a kiln, undergoing calcination at 600 °C and 700 °C for 2 hours. Following this, 50 g of the calcined clay mineral was mixed with NaOH (6 M, 7 M, and 8 M) using a mechanical shaker at 60 °C for 30 minutes to form a homogenous paste. The paste was then autoclaved, dried at 120 °C for 3 hours, and subsequently heated at 650 °C for 2 hours to fully activate the raw clay into zeolites. The synthesized clay-derived zeolites underwent comprehensive characterization using diverse analytical techniques: energy dispersive X-ray fluorescence (EDXRF), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray fluorescence (XRF), and Brunauer Emmett Teller (BET). These techniques provided insights not only into the zeolite morphology but also their structural composition, physical attributes, and chemical properties. FT-IR spectrum analysis revealed O-H stretching between 3732 and 3400 cm-1, Si-O stretching at 418 cm-1, and varied peak intensities. SEM displayed an irregular heterogeneous surface with pores smaller than 0.5 μm, and surface area ranged from 3.911 to 54.311 m/g. Batch adsorption studies determined optimal conditions for pollutant removal in wastewater: calcination temperature (550–900 °C), equilibrium contact time (30 minutes), pH (7), and temperature (25 °C). Under these conditions, a remarkable 99.2% removal of NO3-, PO43-, Cu2+, Pb2+, and Cd2+ was achieved for solutions with a concentration of 100 ppm for all ions. pH notably influenced adsorption capacities of the five substances. Kinetic studies demonstrated pseudo-second-order reaction models, and data aligned with the Freundlich isotherm. Reactions were exothermic and spontaneous. These findings highlight the potential of activated clay-derived zeolites in remediating agricultural waste-contaminated wastewater, offering promise for sustainable water treatment solutions.