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Removal and recovery of phosphate from water using sorption
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Paripurnanda Loganathan
a
, Saravanamuthu Vigneswaran
a
, Jaya Kandasamy
a
and Nanthi S
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Bolan
b
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a
Faculty of
Engineering, University of Technology Sydney, New South Wales, Australia
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b
Centre for Environmental Risk, Assessment and Remediation, University of South Australia,
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Adelaide, Australia
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ABSTRACT
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Sorption is an effective, reliable, and environmentally friendly treatment process for the
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removal of phosphorus from wastewater sources which otherwise can cause eutrophication
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of receiving waters. Phosphorus in wastewater, if economically recovered, can partly
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overcome the future scarcity of phosphorus resulting from exhaustion of natural phosphate
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rock reserves. The authors present a comprehensive and critical review of the literature on
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the effectiveness of a number of sorbents, especially some novel ones that have recently
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emerged, in removing and recovering phosphate. Mechanisms and thermodynamics of
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sorption, as well as regeneration of sorbents for reuse using acids, bases, and salts, are
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critically examined.
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KEY WORDS: adsorption, phosphate sorption, phosphate desorption, phosphate recovery,
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sorption mechanism, sorption thermodynamics, sorbent, water treatment, wastewater
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treatment
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RUNNING HEAD: Phosphate removal and recovery from water
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CONTENTS
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1. Introduction 3
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2. Sorption mechanisms 7
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2.1. Ion exchange 8
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2.2. Ligand exchange 8
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2.3. Hydrogen bonding 9
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2.4. Surface precipitation 9
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2.5. Diffusion 10
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3. Sorption efficiency assessment methods 12
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3.1. Batch method 12
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3.2. Column method 13
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3.3. Continuous stirring tank reactor method 16
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4. Sorbents 16
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4.1. Inorganic sorbents 16
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4.1.1. Metal oxides and hydroxides 17
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4.1.2. Calcium and magnesium carbonates and hydroxides 23
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4.1.3. Layered double hydroxides 25
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4.2. Organic sorbents 28
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4.2.1. Activated carbon 28
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4.2.2. Anion exchange resins 31
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4.2.3. Other organic compounds 32
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4.3. Industrial by-products 35
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4.3.1. Red mud 36
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4.3.2. Slags 37
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4.3.3. Fly ash 39
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4.3.4. Other by-products 41
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4.4. Biological wastes 42
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5. Sorption thermodynamics 46
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6. Phosphate desorption and sorbent regeneration 47
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7. Phosphate recovery 49
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8. Summary and conclusions 51
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9. Acknowledgments 53
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10. References 53
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1. Introduction
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Globally, clean water for domestic, agricultural, and recreational uses, as well as for
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potable supply, is increasingly endangered due to water pollution, climate change and rising
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human needs. Stringent legislation and regulations exist in many countries to reduce
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pollutants bolstered by anthropogenic activities that thresten natural water bodies. Because of
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the limited availability of high quality water resources, reclamation and reuse of treated
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wastewater have become important in the sustainable management of this natural resource.
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Phosphorus (P) is a major nutrient contaminant in water. It enters water bodies
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through mining, industrial and agricultural activities, and sewage discharges. Excessive
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concentrations of P in water cause eutrophication (Hussain et al. 2011; Paleka and Deliyanni
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2009; Xu et al. 2010a), which is defined as the enrichment of water bodies by nutrients and
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the consequent deterioration of quality due to the luxuriant growth of plants such as algae and
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its repercussions on the ecological balance of the waters affected (Yeoman et al. 1988).
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Although both nitrogen (N) and P are considered to be the limiting nutrients for
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eutrophication, some algae are efficient in the fixation of atmospheric N and hence P often
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becomes the potentially limiting nutrient in freshwaters (Yeoman et al. 1988; Zeng et al.
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2004). In advanced stages of eutrophication, dissolved oxygen can become depleted to
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dangerously low levels causing fish death when algae decay (Awual et al. 2011; Long et al.
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2011). The large algae biomass produced by eutrophication can also affect water treatment by
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blocking filters or passing through them causing bad odour and taste in treated water
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(Collingwood 1977). Blue-green algae can produce compounds that are toxic to fish and
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other aquatic life (Davis 1980). These conditions are also potentially risky to human health,
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resulting from consumption of shellfish contaminated with algal toxins or direct exposure to
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waterborne toxins (EPA 2009).
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To control eutrophication the US EPA has recommended that total P should not
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exceed 0.05 mg P/L in a stream at a point where it enters a lake or reservoir and should not
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exceed 0.1 mg/L in streams that do not discharge directly into lakes or reservoirs (Mueller
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and Helsel 1996). The European Union (EU) considers that the cut-off for total P
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concentration between at risk and not risk of eutrophication in lakes is < 10 µg/L to > 100
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µg/L, and for rivers, total P concentration below 0.01-0.07 µg/L is considered excellent
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waters (European Commission 2009). The Australian and New Zealand water quality
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guidelines recommend that Australian upland rivers, depending on the region, should have a
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total P concentration < 0.01-0.02 mg P/L, low land rivers < 0.005-0.01 mg P/L, freshwater
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lakes and reservoirs < 0.01-0.025 mg P/L, and estuaries < 0.02-0.10 mg P/L (ANZECC
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2000). The corresponding limits for New Zealand are 0.026 mg P/L for upland rivers and
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0.033 mg P/L for lowland rivers (ANZECC 2000).
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Phosphorus commonly originates from human and animal wastes, food-processing
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effluents, commercial fertilisers, agricultural land runoffs and household detergents. In water
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and wastewater, P is present in the form of orthophosphate, polyphosphates and organic
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phosphorus. Polyphosphates and organic P are converted to orthophosphate by hydrolysis/or
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microbial mobilisation (Weiner 2008). Orthophosphate is soluble and considered to be the
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only type of P that is directly assimilated by most plants, including algae. Due to their strong
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adsorption onto inorganic particles they also occur adsorbed onto particulate
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matter/sediments in water. A proportion of P in detergents and cleaning compounds contains
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tripolyphosphate, which is slowly mineralised to orthophosphate (Zhou et al. 2011). Early
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work by Clescere and Lee (1965) showed that the hydrolysis of condensed phosphates to
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orthophosphate can take hours or days in the presence of various microorganisms. The
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chemical form and ionic charge of orthophosphate change with the pH of water in accordance
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with their pK values (pK
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= 2.15, pK
2
= 7.20, pK
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= 12.33) (Chitrakar et al. 2006b; Streat et
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al. 2008; Weiner 2008). The pH also influences the surface charge characteristics of soils and
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sediments, thereby controlling P sorption onto these materials. Raising pH decreases the
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positive charges and increases the negative charges on the surfaces of soils and sediments,
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affecting the energies (coulombic attraction vs coulombic repulsion) of P binding.
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Consequently, pH is expected to influence the physicochemical behaviour of P in water.
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Phosphorus in wastewater sources must be removed or reduced to avoid
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eutrophication of receiving waters such as rivers and lakes. The ecological recovery of the
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receiving water bodies after prolonged eutrophication can be very slow even after the
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solution P is reduced (Vollenweider 1968; Yeoman et al. 1988). This is because much of the
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P may be temporally trapped in the sediments in water bodies and released slowly into the
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water.
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Phosphorus removed from water can be a source of raw material for the phosphate
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industry, especially the production of phosphate fertilisers for agriculture. If a successful
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method is developed for effective and economical recovery of P, the current thinking that P is
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