Treated rice husk as a recyclable sorbent for the removal of microcystins from water

Abstract

Microcystins (MCs) appear during harmful algal blooms (HABs) in water sources worldwide, and represent a threat for humans and animals ingesting or inhaling MCs from the environment. Herein, treated rice husk (RH) was tested as a recyclable sorbent for removal of six MCs (MC-RR, MC-LR, MC-YR, MC-LA, MC-LF, and MC-LW) from water. RH was refluxed with hydrochloric acid and heated to 250 °C to produce the sorbent material. Twenty milligrams of treated RH removed >95% of the MCs from a 30 mL solution containing 25 μg/L of each MC. The adsorption of MCs onto RH follows the Freundlich isotherm model (R² ≥ 0.9612) and pseudo-second-order kinetics (R² ≥ 0.9996). More than 90% of MCs were removed within 5 min, and >95% were removed at equilibrium (in <40 min). Performance of the RH sorbent was evaluated by removing MCs from Lake Erie water collected during an algal bloom in 2017. The total concentration (extracellular plus intracellular) of six tested MCs in lake water ranged from 3.7 to 13,605.9 μg/L, and removal of MCs by treated RH ranged from 100.0% to 71.8%, respectively. The removal capacity of RH for the six MCs from the lake water sample containing 13,605.9 μg/L of MCs was 586 μg per g of treated RH. After being used to extract MCs, the RH was heated to 560 °C to produce silica nanoparticles. Therefore, treated RH enables rapid and efficient removal of MCs from water and it can be recycled for use as a raw material. Overall, treated RH can contribute to mitigation of environmental and health effects caused by MCs and reduce concerns for toxic waste disposal.

Keywords: Lake Erie; Microcystins; Recyclable material; Rice husk; Silica nanoparticles; Water purification.