Developing activated carbon adsorbent materials using cyanobacterial biomass as precursor to remove phosphate from surface waters

Abstract

High phosphate levels and warm temperatures in surface waters can stimulate the overgrowth of certain algae types, causing harmful algal blooms (HABs). HABs may adversely affect public health and have negative economic impacts due to associated healthcare costs, required clean-up activities, and tourism revenue loss. The overarching objective of this study was to convert algal biomass into chemically modified activated carbon adsorbent materials for phosphate removal. Cyanobacterial biomass was collected from Lake Okeechobee in Florida and processed before activation using fast and energy-efficient microwave heating. To improve phosphate removal, the surface of the adsorbents was modified using different mass ratios (1.0 to 2.0) of lanthanum chloride or zinc chloride to precursor. The adsorbents were evaluated for phosphate uptake to identify performant materials for further assessment. Multiple materials modified with lanthanum chloride achieved near-complete phosphorus removal efficiency (>99 %) over a wide range of initial concentrations (5, 10, and 20 mg/L). The best-performing material which was prepared with a lanthanum chloride to precursor mass ratio of 1.5 and microwave heating duration of 3 min achieved >90 % phosphorus removal using a low adsorbent dosage of 0.2 g/L and a short contact time of 30 min. This final candidate was studied in the presence of natural organic matter (25–62 mg/L), and the results showed that phosphate removal performance was not adversely affected. The research findings showed the potential of lanthanum-modified algae-derived activated carbon materials to mitigate HABs through phosphate adsorption from surface waters. The superior performance of La-modified materials for phosphate removal is due to the formation of LaPO₄.H₂O (also known as rhabdophane), which precipitates in water and permanently sequesters phosphorus.