A Universal Method for Flocculating Harmful Algal Blooms in Marine and Fresh Waters Using Modified Sand Liang Li and Gang Pan* Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 10085, China * S Supporting Information ABSTRACT: A universal environmental friendly method was developed to turn sand into effective flocculants for mitigating harmful algal blooms (HABs) in marine and freshwater systems. The isoelectric point of sand was largely increased from pH 4.5 to 10.5 after been modified by Moringa oleifera coagulant (MO) abstracted form MO seeds. However, when sand was modified by MO alone, maximum removal efficiencies of 80% and 20% for Amphidinium carterae (A.C.) and Chlorella sp. (C.S.) in seawater and 60% for Microcystis aeruginosa (M.A.) in fresh water were achieved in 30 min. The limited removal improvement was due to the form of only small flocs (20-100 μm) by surface charge modification only. Large flocs (270-800 μm) and high removal rate of 96% A.C. and C.S. cells in seawater and 90% of M.A. cells in fresh water were achieved within 30 min when the small MO-algae-sand flocs were linked and bridged by chitosan. High HAB removal rate is achievable when the sand is modified by the bicomponent mechanism of surface charge and netting-bridging modification using biodegradable modifiers such as MO and chitosan. The optimized dosage of modified sand depends on the property of algal cells and water conditions. ■ INTRODUCTION Harmful algal blooms (HABs) are one of the serious consequences of eutrophication in many parts of the world, for example, red tide in the oceans 1 and cyanobacterial blooms (Cyano-HABs) in the fresh water. 2 Such blooms pose a serious threat to aquatic life, human health, fish industry, local tourism, and water quality in lakes, rivers, reservoirs, and marine coastal environments. Over the past decade, attention has been received on the use of clay to flocculate and settle the HAB cells. 1,3 However, the efficiency of algae flocculation using clays is low and high loads of clay (0.25-2.5 g/L) 3-6 have led to various ecological concerns. 6 Large amounts of clay are often not immediately available in many cases and the transportation costs would quickly render this method uneconomical. 7 As a cheap and safe alternative to clays, chitosan-modified local soil/ sand (MLS) materials could largely enhance the flocculation efficiency and reduce the dosage, and hence minimize the costs and the use of exogenous materials to the aquatic environ- ments. 8 However, chitosan-modified soil/sand is less effective in marine systems than in fresh waters. This is because the positive charge and the netting-bridging function of the chitosan polymer chain are largely depressed as salinity increased in seawater. 9,10 Recently, Pan et al. found that a bicomponent modification method of chitosan and polyalumi- nium chloride (PAC) can turn beach sand or local soil into highly effective algae flocculant in seawater. 10 However, PAC is not biodegradable and less efficient (or need high dosage) at low salinities in fresh waters. 8,11 So far, there is no ecological safe and biodegradable modifier that makes sand or soil highly efficient for HABs mitigation in both fresh water and seawater. Efforts have been made to enhance the algae removal ability and reduce the loading of clay/sand by modifying them using chemical coagulants or flocculants. 3,4,7,8,10 Positively charged coagulants can neutralize the negative surface charge of algal cells and destabilize the cell suspension to promote the aggregation. 3 The high ionic strength of seawater is conducive to the aggregation between clays and algae and that of PAC- modified clays due to the reduction of the electrical double layer thickness. 3,10,12-14 However, to destabilize the algae cell suspension by the single mechanism of electrostatic interaction may be not sufficient to achieve a high removal efficiency because the flocs may be small and remain suspended in the water. 14 In some cases, the flocculation efficiency of clay or PAC-modified clay decreases dramatically as the water salinity decreases, making it difficult to use this technique for Cyano- HABs control in lakes and reservoirs. 8,11 There are also concerns that aluminum may affect aquatic ecological systems such as killing the plankton like Daphnia magna 15 and inducing Alzheimer’s disease through the food chain. 16 Moringa oleifera (MO) coagulant extracted from the MO seeds is known to be one of the most effective natural coagulants 17 in water treatment. The coagulant property is Received: December 21, 2012 Revised: April 11, 2013 Accepted: April 15, 2013 Published: April 15, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 4555 dx.doi.org/10.1021/es305234d | Environ. Sci. Technol. 2013, 47, 4555-4562