Phosphorus recovery from aquaculture wastewater using thermally treated gastropod shell

Accepted Manuscript Title: Phosphorus Recovery from Aquaculture Wastewater using Thermally Treated Gastropod Shell Author: N.A. Oladoja R.O.A. Adelagun A.L. Ahmad I.A. Ololade PII: DOI: Reference:

S0957-5820(15)00168-8 http://dx.doi.org/doi:10.1016/j.psep.2015.09.006 PSEP 619

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

23-7-2015 19-8-2015 5-9-2015

Please cite this article as: Oladoja, N.A., Adelagun, R.O.A., Ahmad, A.L., Ololade, I.A.,Phosphorus Recovery from Aquaculture Wastewater using Thermally Treated Gastropod Shell, Process Safety and Environment Protection (2015), http://dx.doi.org/10.1016/j.psep.2015.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Phosphorus Recovery from Aquaculture Wastewater using Thermally Treated Gastropod Shell

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Oladoja, N.A. 2Adelagun, R. O. A. 3Ahmad, A. L. and 1Ololade, I. A. 1

Department of Chemistry, Adekunle Ajasin University, Nigeria Department of Chemistry, The Federal University, Wukari, Nigeria 3 School of Chemical Engineering, Universiti Sains Malaysia, Malaysia *

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Corresponding Author E-mail: [email protected] Phone No.: +2348055438642

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Abstract

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In tandem with the quest for the development of sustainable strategies for the recovery of P from P-rich aqua waste streams, thermally treated Gastropod shell (GS) was investigated as a reactive material for P-recovery from aquaculture wastewater (AQW). The enhanced defects in the surficial physiognomies, imparted by the thermal treatment process, accounted for the higher P-recovery efficiency. This contradicted the claim that the conversion of the carbonate form of calcium to the oxide form was the reason for the higher P-recovery efficiency of thermally treated calcium rich materials. The fittings of the time-concentration profiles of the P-recovery process to different kinetic models and the determinations of the thermodynamic parameters of the precipitation reaction showed that both adsorption and precipitation were the underlying mechanism of the P-recovery process, using the thermally treated GS. In addition to the removal of P, substantial amount of the total nitrogen in the AQW was also removed. The evaluation of the effects of the P-recovery process on the quality characteristics of the AQW showed that there was significant improvement in the overall physicochemical characteristics.

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Keyword: Phosphorus recovery; aquaculture wastewater; nutrients and eutrophication

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1.0: Introduction 1 Page 1 of 35

Aquaculture wastewater (AQW) is one of the major anthropogenic sources of phosphorus (P)

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pollution in both land and water systems. For example, the production of a tonne of channel

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catfish releases an average of 9.2 kg of nitrogen, 0.57 kg of P, 22.5 kg of BOD and 530 kg of

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settleable solids into the environment [1]. Amongst the constituents of AQW, P is one of the

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most scrutinized because it is the limiting nutrient for eutrophication onset. Premised on the

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negative environmental impact of P, different strategies are being developed for the removal

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and recovery from the waste stream. Furthermore, it has been shown that the use of P

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fertilizers is becoming more expensive and less sustainable because phosphate ores are

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limited, non-renewable resources; a fact that tends to increase the production costs of P

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derivates [2].

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On the basis of the operational simplicity and cost, the use of adsorption based water

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treatment technologies for P sequestration, has been pivotal in the quest for the development

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of sustainable strategies for P recovery. An overview of the array of P specific sorbents that

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have been studied showed that Al3+, Fe3+, and Ca2+ are the major elemental composition [3-

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5]. In order to design a sustainable P-recovery system, it was advanced [6] that the choice of

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the metal ion is important because P that is too tightly bound cannot readily be reused in

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industrial and agricultural applications. Although, the removal of P from aqua stream, using

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Al3+ and Fe3+ rich materials is common, but it is less appealing for the recovery purpose .

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This is because the P recovered from these solids are tenaciously bound to the metal phase

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and aluminum has been found to be toxic to many plants and some soil organisms [7].

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Consequently, magnesium and calcium rich materials are most commonly used for P

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recovery because of the potential to be recycled as fertilizer.

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In our Laboratory, the ability of a Gastropod shell (GS), African land snail (Achatina

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achatina), was screened as a potential low cost sorbent for phosphate removal from aqua

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system in a batch and column reactor [8-10]. Sequel to the promising findings from these

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investigations, the present studies aimed at optimising the potential of GS for P-recovery via

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thermal treatment. The GS has got the same basic construction as other Mollusk shells and it

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contains, mainly, CaCO3, as well as various organic compounds [11, 12]. In the present

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study, the adoption of thermal treatment protocol hinged on the report [13] that the common

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way to enhance the P removal efficiency of calcium-rich materials is via high temperature

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heating. It was advanced that during heating, CaO will probably form, which has a more

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reactive Ca2+ phase than commonly existing CaCO3. The assertion, that the high Ca2+ content

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of a sorbent is not a guarantee of rapid reaction between Ca2+ and P and the fact that the

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reaction is many times easier if the Ca2+ in the material is in the form of CaO instead of

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CaCO3, which has much lower dissolving ability, has been proven at different instances.

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Kaasik et al. [14] showed that hydrated sediment of oil shale ash consists of CaCO3 (calcite),

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Ca6 Al2 (SO4)3(OH)12·26H2O (ettringite) and Ca(OH)2 (portlandite). After the P sorption test,

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the calcite content remained the same, but the ettringite and portlandite content declined

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drastically. It has also been demonstrated that after the heating of Opoka, which naturally

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contained Ca2+ as CaCO3, the sorption capacity increased significantly, from 0.1 g P/ kg to

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39.0 g P/kg [15, 16]. Kwon et al. [17] showed that pyrolizing oyster shell above the 650 ◦C,

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the CaCO3 content fell and P-removal efficiency increased to 98% in the material, whereas

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the untreated material showed no P removal

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The aim of the present study was to study, the P-recovery efficiency of thermally treated GS

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from AQW. The influence of the thermal treatment temperature on the surface properties and

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the P-affinity of the GS was studied. The time-concentration profiles and the effects of

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hydrochemistry on the P-recovery process were determined. The P-recovery efficiency of the

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thermally treated GS in real AQW was evaluated and the effects of the process on the other

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physicochemical characteristics of the AQW were examined.

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2.0: Materials and Methods

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2.1: Material Preparation and Characterization

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The GS was prepared as previously described [8-12] and subjected to thermal treatment, in

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the furnace, at varying temperatures (100, 250, 500, 750 and 1000

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products were labelled TS100, TS250, TS500, TS750 and TS1000; the subscript shows the

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temperature at which each material was thermally treated.

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The optimum temperature for the treatment of the GS, for P-recovery, was determined via

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batch sorption process viz: 50 mL of synthetic P rich solution, derived from potassium

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dihydrogen phosphate (KH2PO4) salt, of fixed concentration (40 mg/L) was contacted with

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0.1 g of each thermally treated material (i.e., TS100‒TS1000). The mixture was agitated at 200

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rpm for 2 h, samples were removed, filtered using 0.45 µm polypropylene membrane and the

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filtrate was analysed for residual P concentration, using the molybdenum-blue ascorbic acid

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method with a UV‒VIS spectrophotometer. The amount of P uptake was determined using

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the mass balance procedure in each case.

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The effects of the thermal treatment temperatures on the elemental composition and the

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mineralogical assemblage of the samples were determined using X-ray fluorescence (XRF)

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and X-ray diffractometer (XRD), respectively. The BET surface areas of the TS samples

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were determined using an ASAP 2010 Micromeritics instrument, by Brunauer–Emmett–

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Teller (BET) method. The surface architecture and elemental composition were determined

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by a scanning electron microscope (SEM) and the surface functional groups were determined

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using an FTIR spectrophotometer (Thermo Scientific, USA).

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2.2: P-Recovery in a synthetic P-Rich water system

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The time-concentration profiles of the P-recovery process were determined by the addition of

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0.5 g of TS into 1.0L of P solution of concentrations that ranged between 2.5 and 30 mg/L

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and stirred at a fixed agitation speed. Samples were withdrawn at intervals between 0 and 5 h,

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of stirring, centrifuged and the supernatant P concentration were determined in each case.

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The equilibrium isotherm analysis of the P-recovery process was evaluated by contacting 100

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mL solution of known P concentration that ranged between 2.5 and 30mg/L with 0.05 g of

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TS. The mixture was stirred at 200 rpm in thermostatic shaker at the equilibrium time,

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samples were withdrawn, centrifuged and the supernatant was analysed for the residual P

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concentration.

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The influence of hydrochemistry (i.e. solution pH, presence of inorganic and organic ionic

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species and ionic strength) on the P-recovery process was simulated viz: initial solution pH

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that ranged between pH 6.11and 11.12; presence of inorganic anions by the addition of

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varying concentrations (mg/L) (10, 50 and 100) of different anions (NO3-, Cl-, PO42-, CO32-

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and SO32-) derived from the potassium salts; organic load, simulated by the addition of humic

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acid (HA), of concentrations (mg/L) (5, 10, 20, 40 and 8 0); ionic strength (tested in NaCl

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solutions (%): 0, 0.05, 0.1, 0.2, 0.5 and 1, equivalent to ionic strengths (mol/L) of 0, 0.0085,

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0.017, 0.0342, 0.085 and 0.17). All the studies were conducted in duplicate.

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2.3: P-recovery in aquaculture wastewater

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Grab sample of the real AQW was collected from an aquaculture farm that breeds catfish,

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characterized. pending usage. The ability of the TS to recover P in the AQW was tested in a

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batch reactor by adding 0.5g of the TS into a liter of the wastewater and the mixture was

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agitated for a period of 30min. before sample was withdrawn to determine the residual P

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concentration. The effects of the P-recovery process on the physicochemical characteristics of

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the wastewater were also evaluated.

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3.0: Results and Discussion

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3.1: Preparation and Characterization of Materials

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The elemental composition of the raw GS and the thermally treated samples are presented in

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Table 1. The assertion, in our previous reports [8-12], that the GS is a calcium rich material

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was ascertained by the results of the XRF analysis. The diffractogram of the raw and

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thermally treated GS (Figure 1) showed that the raw GS is a crystalline material and that the

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crystallinity was retained over the entire thermal treatment temperatures adopted. The number

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of peaks, produced from the interactions of each sample (i.e. GS and TS100-TS1000) with X-

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ray, reduced with increasing thermal treatment temperature while the intensities of the

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prominent peaks was enhanced with increasing treatment temperature. The shells of

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Gastropods are known to be made up of CaCO3 and the form of CaCO3 in the shell of African

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land snail has been reported to be made up of largely aragonite [13 and 14]. Thus, the

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presence of the three polymorphs of CaCO3 (aragonite, calcite and vaterite) in the

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diffractogram of raw GS and thermally treated samples was determined using the following

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miller indices: 111 and 221 (for aragonite); 104 and 113 (for calcite) and 110,112, 114, 300,

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224 and 211 (for vaterite). In the GS, the aragonite peaks appeared at 26.26 (111) and 46.03

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(221), the calcite peaks appeared at 52.89 (113) and the vaterite peak appeared at 37.93 (112).

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It has been posited [18] that both aragonite and calcite have their highest-intensity peaks at

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different positions, and the general look of the two patterns is different. Aragonite has its

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greatest peak (111) at relatively small 2θ angle and has several lesser peaks, whereas calcite

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has a booming (104) peak a bit to the right of the aragonite large peak, and few and

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comparatively small other peaks. Other notable peaks of aragonite (221) and calcite (104)

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have also been used for easy identification of these two minerals. The changes in the

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mineralogical assemblage, caused by the thermal treatment of the raw GS, are shown in the

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different diffractogram presented in Figure 1 and the peak parameters of the salient peaks are

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presented in Table 2. The peak positions and parameters of the GS and the TS100, are the

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same but in the TS250, the disappearance of one of the Aragonite peaks, at 46.03 (221), and

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appearance of the vaterite peak, at 41.18 (211), were observed. The diffractogram of the

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TS500 and TS750 did not show any of the aragonite peaks while the presence of calcite ad

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vaterite peaks were observed. The TS1000 showed the presence of all the three polymorphs of

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CaCO3 but the appearance of CaO peaks at 22.89 (112) and 28.95 (200) were noted.

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It has been postulated [19] that when calcite crystals, are heated at a temperature of around

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900 °C, they will transform into CaO through the release of carbon dioxide gas. The

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calcination process of calcite begins when the partial pressure of CO2 in the gas above the

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solid surface is less than the decomposition pressure of the CaCO3. Evidences from the

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results of the XRD analysis showed that the formation of CaO was noted only in the TS1000,

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which is an indication that the formation of CaO in the thermally activated may not have

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occurred below the calcination temperature of 10000C. The presence of the three polymorphs

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of the CaCO3 in the diffractogram of the TS1000 (Figure 1) is a pointer to the fact that the

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CaCO3 constituent of the raw GS was not completely transformed to CaO at this temperature.

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Amongst the CaCO3 polymorphs, vaterite is considered the less stable and it’s assumed to

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easily transform into either calcite or aragonite. Contrary to the claim that vaterite is

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thermodynamic unstable, the presence in both the raw GS and thermally treated samples were

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confirmed by the XRD analysis. Similarly, the presence of vaterite in some biological

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systems, sediments, and during oilfield drilling has been reported [20]. Consequently, it was

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deduced that since vaterite forms and persists in a number of natural systems, the claim about

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the thermodynamic instability may not be tenable, at all the time. It was advanced that the

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stability observed in some cases could have been influenced by some mechanism which

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prevent the transformation of vaterite to other CaCO3 polymorphs. In nature, aragonite (a

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CaCO3 polymorph that is less stable than calcite) is often found to be stable in the shell of

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biological organisms or in hydrothermal deposits of hot springs, etc. It was reported that less-

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stable polymorphs were preserved through kinetic effects [21-24] or were stabilized by

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impurities, such as some inorganic ions [25] and organic matters [26, 27]. At present, the

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material that is thermally treated, the GS, consist of three layers, namely Hypostracum,

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Ostracum and Periostracum. The Hypostracum is a form of Aragonite while the Ostracum is

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built by several layers of prism-shaped CaCO3 crystals with embedded proteid molecules.

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The Periostracum, the outermost shell layer, is not made of CaCO3, but of an organic material

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called Conchin, a mixture of organic compounds, mostly of proteids. Conchin not only makes

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the outer shell layer, but also embedded between the CaCO3 crystals of deeper layers. In the

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raw and thermally treated GS, the stability of the different polymorphs of the CaCO3 was

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attributed to the naturally occurring organic and inorganic (Table 1) constituents of the shell.

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Aside the differences in the intensities of the FTIR peaks, the spectra patterns of the raw GS,

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TS100 and TS250 were similar (Figure 2). Samples TS500 and TS750 also showed similar peaks

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with that of the raw GS and TS100 but the shape of the peaks were broadened and the

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intensities of the peaks were significantly reduced. The spectrum pattern of TS1000 completely

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was completely different, in terms of peaks positions and intensities, from the other samples.

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In both the raw and thermally treated GS, the presence of the peaks of the N-H stretch of

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primary amines and ammonium ions were ascribed to the presence of Conchin, a complex

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protein that is secreted by the GS outer shell. These proteins are part of a matrix of organic

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macromolecules, mainly proteins and polysaccharides that assembled together to form the

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microenvironment, where the CaCO3 crystals nucleated and grew. The different carbonate

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peaks, identified in the FTIR spectra, were ascribed to the presence of the different

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polymorphs (i.e. calcite, aragonite and vaterite) of CaCO3 identified using the XRD. Aside

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the carbonate peaks that were present in all the samples, sample TS750 and TS1000 also showed

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the CaO peaks. In the X-ray diffractogram of sample TS750, peaks synonymous with CaO was

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not identified; which informed the assumption that the transformation of CaCO3 to CaO did

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not occur below the treatment temperature of 10000C. The non-visibility of the CaO peak in

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the X-ray diffractogram could be attributed to the fact that the amount of this oxide in the

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sample matrix was very low. The low concentration of the CaO in the TS750 matrix was

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evidenced in the appearance of a single CaO peak at 3630cm-1.

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The results of the BET analysis showed that the surface area (m2/g) of the GS and the

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thermally treated samples (i.e. TS100-TS1000) were 1.9666, 0.0030, 2.8070, 1.3301, 4.4409 and

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50.0069, respectively.

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The effects of thermal treatment on the P-recovery efficiency of the materials (i.e. raw GS

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and TS100-1000) are presented in Figure 3. The amount of P recovered slightly increased with

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increasing thermal activation temperature (

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amount of P recovered (mg/g) ranged between 7.45 and 8.11. A substantial increase (from

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8.11mg/g at 5000C to 19.17mg/g at 7500C) in the values of P recovered was noted at

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temperatures of 7500C but the magnitude of P recovered when the thermal activation

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temperature was increased to 10000C remained the same (Figure 3). In the P-recovery

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systems, where TS750 and TS1000 were used as the reactive materials, the residual P

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concentrations were lower than the detectable limit of the P quantification procedure (<

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0.01mg/L).

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The profiles of the surface functional groups (Table 3) and the mineralogical assemblage

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(Table 2) of the thermally treated GS showed that in the TS750 and TS1000, the transformations

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of some of the CaCO3 into CaO occurred but the degree of this transformation was higher in

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the TS1000 than in the TS750. Premised on the higher P recovery exhibited by these two

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materials, relative to the other materials investigated, it could be inferred that the presence of

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the CaO enhanced the P recovery capacities of these materials. Considering the similarities in

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the P-recovery efficiencies of these two materials, within the limit of the initial P

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concentration studied, and the difference in the magnitude of the transformation of CaCO3 to

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CaO in these two materials, the assumption that the presence of CaO was solely responsible

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for the relatively higher P-recovery efficiencies they exhibited may not be tenable.

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Bearing in mind the trends in the values of the BET surface areas vis-à-vis the P-recovery

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efficiencies of the different reactive materials, it would be difficult to be categorical on the

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influence of BET on the P-recovery efficiencies.

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The effects of the surface architecture on the P-recovery efficiency were studied by

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comparing the SEM images of the raw GS with that of the TS750. The SEM image (Figure 4)

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showed that the thermal treatment caused a complete destruction of the original surface

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architecture of the raw GS. Originally, the surface architecture of the GS comprised of a

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systematic, closely knitted, fibrous strands of varying lengths, but the thermal treatment

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transformed it into a corrugated surface with sharp and rough edges. This showed that the

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thermal treatment boosted the surface defects in the TS750 and the effects of the surface

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defects manifested in the presence of abundance of atoms with highly defective coordination

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environments, on the surface of the TS750. Since these atoms have unsatisfied valences and

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reside at the surface, they provided abundance of active sites for the uptake of P from the

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aqua matrix. Evidence of structural defects on the high sorptive capacities of nanoparticles

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has also been provided [28]. Consequent upon the comparable P-recovery efficiencies of the

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TS750 and TS1000 but lower energy production advantage of the TS750 over TS1000, the TS750

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was chosen as the reactive material for the P-recovery process.

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3.2: Time-Concentration Profile of the P-Recovery Process

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The time-concentration profiles of the P-recovery process (Figure 5), at varying initial P

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concentrations, showed that the time for the attainment of equilibrium was influenced by the

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initial P concentrations in the aqua matrix and in all the initial P concentrations studied, more

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than 99% of the initial total P recovered occurred within the first two minutes of the process.

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In order to obtain the kinetic parameters of the P-recovery process, the time-concentration

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profile data were analyzed using the pseudo first order (Eq. 1) [29] and pseudo second order

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(Eq. 2) [30] kinetic equations. (1)

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t/qt = 1/kqe+ 1/qet

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Log [qe-qt] = log[qe] - [k1/2.303]t

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Where, qe and qt are the sorption capacity at equilibrium and at time t, respectively (mg/g). k1

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and k2 are the overall rate constants of pseudo-first order (g(mg/min)) and pseudo second

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order adsorption, respectively.

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The kinetic analysis (Table 4b) showed that the pseudo-second-order kinetic model gave

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better fit (r2 >0.99) to the experimental data than the pseudo first order kinetic model. In order

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to further confirm the kinetic equation that gave better description of the time-concentration

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profiles, the non-linear chi-square (χ2) error analysis was performed. The results obtained

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(Table 4b) also showed that pseudo second order model gave the better prediction of the

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experimental qe (mg/g) values.

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Consequent upon the calcium rich nature of the TS750 and the possible interaction between

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the Ca2+ content of the TS750 with the aqua P, to form insoluble calcium phosphate salts, the

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role of precipitation in the P-recovery process was determined. Considering the active ionic

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species in the P-recovery reactor (i.e. Ca and PO42-), the simplest insoluble phosphate species

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that can be produced from the interactions within the system were used as the model

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insoluble phosphate salts (i.e. Ca3(PO4)2 with Ksp values of 2.07 x 10-33). The SI values of

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Ca3(PO4)2 was estimated via the determination of the activities of the two ionic species in

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solution, at equilibrium, in a batch process, at different initial phosphate solution

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concentrations (mg/L). The SI values of the insoluble phosphate species were calculated

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using Eq. 3 [31]:

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-------------------------------- [3] The plot of the SI value against initial P concentrations, are presented in Figure 6a. The value

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of the SI, which ranged between 30.6 and 34.5, increased, linearly, with increasing P

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concentrations (mg/L) from 2.5 to 30. A positive SI value has been attributed to

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supersaturation of the ionic species in solution which lead to precipitate formation while, a

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negative SI is an indication of a dominant adsorption process [32]. Song et al., [33] opined

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that SI value is a good indicator to show the deviation of a salt from its equilibrium state, i.e.

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the thermodynamic driving force for precipitation to occur. But considering the kinetics,

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supersaturation does not certainly mean the quick occurrence of a spontaneous precipitation.

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Between the under saturated zone and spontaneous precipitation zone there is still a

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metastable zone, where the solution is already supersaturated but no precipitation occurs over

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a relatively long period [34]. The boundary between metastable zone and spontaneous

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precipitation zone can be called the critical supersaturation [35]. The thermodynamic driving

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force to a chemical reaction is the Gibbs free energy ∆G, and it is the criterion to judge

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whether a reaction is spontaneous, in equilibrium, or impossible, corresponding to ∆G < 0,

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=0, or >0, respectively. The Gibbs free energy of a precipitation reaction is given by [33]:

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cr

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(4)

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Where, R is the ideal gas constant (8.314 J/mol),, T is the absolute temperature (K), IAP and

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Ksp are, respectively, the free ionic activities product and the thermodynamic solubility

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product of the precipitate phase and, n, is the number of ions in the precipitated compound.

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In order to judge supersaturation, which is a measure of the deviation of a dissolved salt from

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its equilibrium value, the SI of a solution with respect to a precipitate phase is defined

Therefore SI

(6)

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(5)

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When SI = 0, hence ∆G = 0, the solution is in equilibrium; when SI < 0, ∆G > 0, the solution

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is under saturated and precipitation is impossible; when SI > 0,∆G < 0, the solution is

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supersaturated and precipitation is spontaneous. The ∆G values obtained (Figure 6) were all

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negative (range between -88764.5 and -100183.0) which indicated the spontaneity of

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precipitate formation in the system.

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Consequent upon the role of precipitation reaction in the P recovery process, the time-

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concentration profile was analyzed to determine the kinetics of the possible precipitate

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formation, using the Avrami fractional kinetic equation (Eq. 7). The theoretical basis of the

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Avrami fractional kinetic equation hinged on the description of changes in the volume of

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crystals as a function of time during the process of crystallization [36]. The linearized form of

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this equation by simple linear regression is presented in Eq. 7 below:

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ln[−ln(1 − F)] = ln(B) + k ln(t)

(7)

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Where F is the fraction adsorbed (qt /qe) at time, t, B, is a temperature dependent constant

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(similar to a rate constant), while, k, is the Avrami exponent (which reflects the

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dimensionality of crystal growth). The Avrami exponent, k, value can be 3 ≤ k ≤ 4 (for three-

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dimensional growth); 2 ≤ k ≤ 3 (for two-dimensional growth) or 1 ≤ k ≤ 2 (for one-

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The kinetic parameters obtained using Avrami kinetic equation are presented in Table 2. The

326

values of the linear correlation coefficient (r2) obtained were high (r2 value ranged between

327

0.8331 and 0.9615) and the error analysis using the non-linear chi-square (χ2) test showed a

328

good prediction of the qe values (Table 4b). The values of the Avrami exponent, k, which

329

ranged between 0.1779 and 0.7715, showed that the dimensionality of the growth of the

330

precipitate occurred in one dimension.

331

3.3: Equilibrium Isotherm Analysis of the P-Recovery Process

332

The affinity of the TS750 for P and the characteristics of the mode of interactions were

333

determined (Table 3) using the linear forms of the following three equilibrium isotherm

334

equations: Langmuir:

336

Freundlich:

337

Temkin:

(8)

(9) (10)

Ac ce p

te

d

335

M

an

us

cr

ip t

325

338

Where, Ce is the equilibrium concentration of P in solution (mg/L), qe is the amount of P

339

adsorbed at equilibrium (mg/g), qm is the theoretical maximum monolayer sorption capacity

340

(mg/g), and Ka is the Langmuir constant (L/mg). Kf is the Freundlich constant (mg/g)

341

(L/mg)1/n and 1/n is the heterogeneity factor.

342

The results of the equilibrium isotherm analysis showed that the TS750 possess heterogeneous

343

site for the interaction with the P species in the aqua system. This fact was ascertained by the

344

highest value of the correlations coefficient (r2 = 0.9377), when the experimental data were

345

analyzed with the Freundlich equilibrium isotherm equation (Table 5).

14 Page 14 of 35

The equilibrium adsorption curves, relating the TS750 and aqua phase concentration of P, at

347

equilibrium for each of the equilibrium isotherm equations are presented below: Langmuir =

349

Freundich =

350

Temkin

=

cr

348

ip t

346

3.4: Effects of Hydrochemistry on the P-Recovery Process

352

The results presented in the Supplementary Information section (see SIF 1-4) showed the

353

effects of hydrochemistry, simulated in the synthetic P-rich water, on the P-recovery ability

354

of the TS750. Considering the elemental composition of TS750 and the tendency for loss of

355

integrity in acidic medium, the effects of pH on the recovery process was monitored between

356

solution pH 6 and 11. The amount (mg/g) of P recovered were in the same range (47.30 and

357

49.09), despite the difference in the initial solution pH values (see SIF 1). The similarities in

358

the amount of P recovered were ascribed to the fact that the presence of the TS750 in the

359

synthetic P solution caused an increase in the solution pH and the equilibrium pH of all the

360

solution ranged between pH 10.11 and 11.18, irrespective of the value of the initial solution

361

pH. Since the sorbate speciation in aqueous system and the surface chemistry of an adsorbent

362

are all pH dependent, the similarities in the magnitude of the P recovered were attributed to

363

the fact that the surface chemistry of the TS750 and the speciation of the P species in the aqua

364

system were all the same in all the system studied, hence the mode of interactions between

365

the TS750 and the aqua P were similar, despite the difference in the initial solution pH.

366

The influence of variations in the ionic strength of aqua system on the P-recovery process

367

showed that the increase in the ionic strength of the aqua matrix, from a value that ranged

368

between 0.025 and 0.5M, caused reduction in the amount of the P recovered (see SIF 2). The

Ac ce p

te

d

M

an

us

351

15 Page 15 of 35

amount (mg/g) of P recovered reduced from a value of 57.93, when the ionic strength was not

370

enhanced (i.e. raw synthetic P-rich water sample), to a value of 21.19, at the maximum ionic

371

strength studied (i.e. 0.5mo/L). The reduction in the magnitude of P recovered is an

372

indication that the interaction between the TS750 and the aqua matrix P occurred via an outer

373

sphere complexation reaction or electrostatic attraction, which made it possible for the

374

anionic specie in the aqua matrix to compete with the P for the active sites on the TS750.

375

The effects of anionic interference on the P-recovery by the TS750 showed that the presence

376

of carbonate and chloride negatively impacted the process (see SIF 3) while the other anionic

377

species had minimal influence, within the limit of the concentrations of the anionic species

378

studied. The presence of organic ions, simulated via HA dissolution in the synthetic feed

379

water, showed that the amount of P recovered significantly reduced with increase in the

380

organic load (see SIF 4). The organic load simulator, HA, contains both hydrophilic and

381

hydrophobic molecules as well as many functional groups such as carboxyl, phenolic and

382

hydroxyl groups connected to a skeleton of aliphatic or aromatic units. It has been posited

383

that the carboxylic and phenolic group on the HA are deprotonated in weakly acidic to basic

384

media, thereby conferring negative charge on the HA molecule [37]. This negative charge

385

promotes competition between the HA and the aqua matrix P for the adsorption sites on the

386

TS750, thereby causing a reduction in the magnitude of P recovered.

387

3.5: Characterization of the P-laden TS750

388

The FTIR spectrum of the P-laden TS750 (PTS750) (Fig. 7) showed, in addition to the original

389

diagnostic peaks of the virgin TS750, strong phosphate band at 1041cm-1 and weak phosphate

390

bands at 578 cm-1. Increase in the intensity of the original peaks, identified in the TS750, were

391

observed in the PTS750. The comparison of the surface architecture of the virgin TS750 (Figure

392

4B) with that of the PTS750 (Fig. 8) showed the formation of thick mass of a new material,

Ac ce p

te

d

M

an

us

cr

ip t

369

16 Page 16 of 35

covering the surface in the PTS750. The presence of this surface coverage was confirmed by

394

the substantial reduction in the intensity of the original diagnostic XRD peaks found on the

395

virgin TS750, after the P- recovery process (i.e. in the PTS750). Aside the different diagnostic

396

peaks, originally reported in the TS750, no new peaks were detected in the PTS750. The

397

reduction in the XRD peak intensity in the PTS750 is an indication that the product formed

398

(i.e. the surface coverage) from the recovery of P by the TS750 is amorphous to X-ray. It has

399

been posited that phosphate and Ca2+ in aqueous system solution can form octacalcium

400

phosphate (OCP), dicalcium phosphate dehydrate (DCPD) and hydroxylapatite (HAP). It was

401

opined that the poorly crystallized or amorphous DCPD and OCP formed, as precursor

402

phases, in solution containing Ca and P, and they are recrystallized into thermodynamically

403

stable HAP over time [14, 38].

404

3.6: Mechanistic Insight into the P-Recovery Process

405

The affinity of metal oxides for P has been attributed to the presence of multiple charges on

406

the metallic species, high positive surface charge densities at near-neutral pH, and a

407

propensity to hydroxylate in aqueous systems [39]. Specifically, it was posited that the

408

reaction between P and Ca2+ rich materials (e.g. calcium oxide, calcite, or gypsum) surface

409

involves the adsorption of small amounts of P and subsequent precipitation of calcium

410

phosphate [40]. Initial adsorption is thought to occur at sites where lateral interaction with

411

phosphate ions produces surface clusters that then act as nuclei for subsequent crystal growth.

412

The kinetic analysis of the time-concentration profiles showed that chemisorption (due to

413

conformance with the pseudo second order kinetic equation) was the underlying mechanism

414

of the process but the occurrence of precipitation reaction, due to the thermodynamic

415

parameters (i.e. the -ΔG values) of precipitation also lend credence to the occurrence of

416

precipitation during the P-recovery process. Additionally, the effects of ionic strength on the

Ac ce p

te

d

M

an

us

cr

ip t

393

17 Page 17 of 35

P-recovery indicated that the mode of interaction between TS750 and P occurred via outer

418

sphere complexation (a form of non-specific interaction) while the effects of anionic

419

interference showed that the interaction occurred via specific adsorption. Thus, it could be

420

inferred that both that both the non-specific and specific adsorption were the underlying

421

mode of interaction in the P-recovery process.

422

In order to elucidate the possible mode of interactions, between the ionic species, during the

423

P-recovery process, the speciation of the aqua P and the surface chemistry of the TS750, was

424

elucidated, using the computer software, MEDUSA (Make Equilibrium Diagrams Using

425

Sophisticated Algorithms) and HYDRA (Hydrochemical Equilibrium Constant Database)

426

computer software [41] to determine the chemical equilibrium data. Taking the operational

427

pH for P-recovery into cognizance, the chemical equilibrium data (Figure 10) showed that the

428

ionic species that exist at the inception of the process included Ca2+, PO43-, HPO42- and

429

H2PO4- , while the interactions between these ionic species produced the following molecular

430

species, at varying concentrations: CaHPO4, Ca5(PO4)3OH and CaPO4-.

431

Taking into cognizance the experimental evidences that showed that both adsorption (specific

432

and non-specific adsorption) and precipitation reaction were the underlying mechanisms of P-

433

recovery, using the TS750 reactive material, the following reaction schemes are proposed as

434

the possible underlying mechanism of interaction in the P-recovery process viz:

436

cr

us

an

M

d

te

Ac ce p

435

ip t

417

Specific Adsorption

437

Ca 2+ + HPO42- → CaHPO4

438

Ca 2+ + PO43- → CaPO4-

439

Non-Specific Adsorption 18 Page 18 of 35

441 442

Ca2+ + HPO42- → Ca2+……. HPO42Precipitation Reaction 5Ca2+ + 3PO43- + → Ca5 (PO4)3OH

ip t

440

In the chemical equations presented above the specific adsorption process occurred via

444

covalent bonding while the non-specific adsorption process occurred via electrostatic

445

attraction between the oppositely charged species.

446

3.7: P-Recovery from Real Aquaculture Wastewater

447

The ability of the TS750 to recover P from real aquaculture wastewater (AQW) was tested in a

448

batch reactor. The physicochemical characteristics of the real aquaculture and the product

449

water (PW) from the P-recovery process are presented in Table 6. The TS750 removed 60% of

450

the original P content (0.15mg/L) from the AQW, which is equivalent to 0.18mg/g of the

451

TS750. The quality characteristics of the PW showed that the P-recovery process appreciably

452

improved the quality characteristics of the PW which was considered as a positive step

453

towards the save disposal of the AQW, after the P-recovery process. Aside the values of the

454

pH and EC that increased, appreciably, in the PW, the magnitude of other parameters reduced

455

after the P-recovery process. The magnitude of the solids (TS, DS and SS) in the AQW

456

substantially reduced after the P-recovery process. The reduction in the solids were attributed

457

to the effects of the occurrence of precipitation reaction during the P-recovery process. The

458

removal of the solid from the aqua matrix, via the precipitation reaction, was ascribed to a

459

phenomenon known as sweep coagulation, in coagulation-flocculation process.

460

reduction in the values of solids in the PW also manifested in the value of the COD, which

461

reduced from the value of 15mg/L to a value of 54mg/L in the PW. In the P recovery process,

462

appreciable amount of the TN content of the AQW was also removed (47.4% i.e. 18.72mg/g

463

of TS750) which showed that in the process of P recovery, substantial amount of the TN was

Ac ce p

te

d

M

an

us

cr

443

The

19 Page 19 of 35

also removed. The synchronous attenuation of both the P and N in the aqua matrix is an

465

indication that the solid fraction, recovered from the P-recovery process, can be used as a

466

very good source of these two nutrients in agricultural practice. The sodium and potassium

467

content of the PW was not influenced by the process but substantial amount of calcium was

468

added into the PW after the P recover process. The elevation of the magnitude of calcium in

469

the aqua matrix was assumed to be responsible for the higher values of the pH and EC in the

470

PW.

471

3.8: Characterization of P-laden TS750 from AQW system

472

The surface architecture of the P-laden TS750 in the AQW (AQWTS750) (Fig. 11) showed that

473

the particle surface remained crystalline after the P-recovery process and the deposition of

474

speckles of extraneous materials on the surface of the reactive material was observed. The

475

XRD patterns (Fig. 12) of the AQWTS750 were similar to that of the virgin TS750 but the

476

values of the miller indices showed that during the P-recovery process, significant structural

477

changes occurred in the CaCO3 crystallite. In the TS750, only the peaks of calcite (43.70

478

(113)) and vaterite (63.19 (211), 72.68 (300)) were observed while in the diffractogram of

479

AQWTS750, the vaterite peaks appeared at 21.09 (110), 37.93 (112), 41.20 (211), 69.14 (114)

480

and 81.05 (224), calcite peaks appeared at 52.50 (113), 67.96 (104) and aragonite peaks

481

appeared at 26.24 (111), 45.89 (221). The structural changes in the CaCO3 crystallites were

482

attributed to the hydration or solvation of the thermally treated material in the AQW during

483

the P-recovery process. It was noted that despite the similarities in the peaks positions, the

484

intensity of the peaks in the AQWTS750 were higher than that of the TS750. This showed that

485

the P-recovery process enhanced the crystallinity of the TS750. The enhanced crystallinity was

486

ascribed to the occurrence of the structural changes in the crystallites of CaCO3 in the

487

presence of array of ionic constituents of the real AQW. The comparison of the FTIR spectra

488

of the AQWTS750 and that of virgin TS750 (Fig. 13) showed that they were similar, aside the

Ac ce p

te

d

M

an

us

cr

ip t

464

20 Page 20 of 35

reduction in the intensity of the different peaks. In the AQWTS750, the disappearance of some

490

carbonate peaks, which appeared at 1624cm-1, 2314cm-1 and 2372cm-1, CaO peaks, which

491

appeared at 3630cm-1 in the TS750 and appearance of phosphate peak at 605cm-1 were noted.

492

4.0: Conclusion

493

The P- recovery ability of a GS can be improved significantly via thermal treatment and the

494

optimum treatment temperature can be achieved at a temperature of 7500C. The higher P-

495

recovery ability of the TS was caused by the enhanced defects in the surficial

496

physiognomies,. Kinetic analysis showed that chemisorption was the underlying mechanism

497

of the process while thermodynamic analysis showed that precipitation also contributed to the

498

P-recovery process. Avrami fractional kinetic analysis showed that the precipitate growth

499

occurred in one direction. In addition to the removal of P, substantial amount of the total

500

nitrogen in the AQW was also removed and the process produced water with significant

501

improvement in the overall physicochemical characteristics.

502

References

503

[1] Rodhe, W. 1969 Crystallization of eutrophication concepts in North Europe. In:

504

Eutrophication, Causes, Consequences, Correctives. National Academy of Sciences,

505

Washington D.C., ISBN 309-01700-9, pp. 50–64.

506

[2] Driver, J. Lijmbach, D. Steen, I. Why recover phosphorus for recycling, and how?,

507

Environ Technol. 20 (7) (1999) 651‒662

508

[3] Lindsay, W. L. Phosphate Chemical Equilibria in Soils; John Wiley & Sons: NewYork,

509

1979.

510

[4] Richardson, C. J. Mechanisms controlling P retention capacity in fresh water wetlands.

511

Science 1985, 228, 1424.

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[5] Faulkner, S. P.; Richardson, B. C. Physical and chemical characteristic of freshwater

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wetlands. In Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and

514

Agricultural; Hammer, D. A., Ed.; Lewis Publishers: Chelsea, 1989, pp 41−72.

515

[6] Morse, G.K., Brett, S.W., Guy, J.A., Lester, J.N. Review: phosphorus removal and

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recovery technologies. Sci. Total Environ. 212 (1998) 69–81.

517

[7] Johnston, A.E., Richards, I.R., Effectiveness of different precipitated phosphates as

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phosphorus sources for plants. Soil Manage. 19 (2003) 45–49.

519

[8] Oladoja, N.A. Ololade, I.A. Adesina, A.O. Adelagun, R.O.A. Sani, Y.M. Appraisal of

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gastropod shell as calcium ion source for phosphate removal and recovery in calcium

521

phosphate minerals crystallization procedure,

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[9] Oladoja N.A. Ahmad A.L. Adesina, O.A. Adelagun, R.O.A. Low-cost biogenic waste for

523

phosphate capture from aqueous system, Chemical Engineering Journal 209 (2012) 170‒179

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[10] Oladoja, N. A. Adesina, A. O. Adelagun R. O.A. Gastropod shell column reactor as on-

525

site system for phosphate capture and recovery from aqua system, Ecological Engineering 69

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(2014) 83–92

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[11] Oladoja, N.A., Aliu, Y.D., Snail shell as coagulant aid in the alum precipitation of

528

malachite green from aqua system. J. Hazard. Mater. 164, (2009) 1494–1502.

529

[12] Oladoja, N.A., Aliu, Y.D., Ofomaja, A.E. Evaluation of Snail shell as coagulant aid in

530

the alum precipitation of aniline blue from aqua system. Environ. Technol. 32 (6), (2011)

531

639–652.

532

[13] Vohla, C. Koiv, M. Bavor, H. J. Chazarenc, F. Mander, U. Filter materials for

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phosphorus removal from wastewater in treatment wetlands̶A review, Ecological

534

Engineering 37 (2011) 70–89

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[14] Kaasik, A., Vohla, C., Motlep, R., Mander, U., Kirsimae, K., Hydrated calcareous oil-

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shale ash as potential filter media for phosphorus removal in constructed wetlands. Water

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Res. 42 (2008) 1315‒1323.

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[15] Brogowski, Z., Renman, G., Characterization of opoka as a basis for its use in

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wastewater treatment. Polish J. Environ. Stud. 13 (2004)15‒20.

540

[16] Johansson, L. Industrial by-products and natural substrata as phosphorus sorbents.

541

Environ. Technol. 20 (1999) 309‒316.

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[17] Kwon, H.-B., Lee, C.-W., Jun, B.-S., Yun, J-d., Weon, S.-Y., Koopman, B., Recycling

543

waste oyster shells for eutrophication control. Resour. Conserv. Recycl. 41 (2004) 75‒82.

544

[18] LBR A&CmixturesXRD01.odg 7/2012, X-ray diffraction (XRD) of aragonite and calcite

545

[19] Stanmore B.R., Gilot P. Review-calcination and carbonation of limestones during

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thermal cycling for CO2 sequestration, Fuel Processing Technology. 86 (2005) 1707-1743.

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[20] G. Nehrke Calcite Precipitation from Aqueous Solution: Transformation from Vaterite

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and Role of Solution Stoichiometry, PhD Thesis (1971) Universiteit Utrecht

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[21]T. Ogino, T. Suzuki, K. Sawada, Geochim. Cosmochim. Acta , 51, 2757 (1987).

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[22] A. G. Xyla, P. G. Koutsoukos, J. Chem. Soc., FaradayTrans. 1, 83, 1477 (1987).

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[23] E. K. Giannimaras, P. G. Koutsoukos, J. Colloid Interface Sci., 116 , 423 (1987).

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[24] E. K. Giannimaras, P. G. Koutsoukos, Langmuir, 4, 855 (1988)

553

[25] Y. Kitano, Bull. Chem. Soc. Jpn., 35, 1973 (1962).

554

[26]Y. Kitano, D. W. Hood, Geochim. Cosmochim. Acta, 29, 29 (1965).

555

[27]G. Falini, S. Albeck, S. Weiner, L. Addadi, Science, 271, 67 (1996)

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[28] Mishakov, I. V.; Bedilo, A. F.; Richards, R. M.; Chesnokov,V. V.; Volodin, A. M.;

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Zaikovskii, V. I.; Buyanov, R. A.; Klabunde, K. J. Nanocrystalline MgO as a

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dehydrohalogenation. J. Catal. 2002, 206, 40–48.

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[29]S. Lagergren, About the theory of so-called adsorption of soluble substances. K. Sven.

560

Vetenskapsakad. Handl. Band. 24, (1898)1–39.

561

[30] G. McKay, The adsorption of basic dye onto silica from aqueous solution-solid diffusion

562

model. Chem. Eng. Sci. 39 (1), (1984) 129–138

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[31] S.K. Nath, R.K. Dutta, Acid-enhanced limestone defluoridation in column reactor using

564

oxalic acid, Process Saf. Environ. Prot. 90 (2012) 65–75.

565

[32] B.D. Turner, P.J. Binning, S.L.S. Stipp, Fluoride removal by calcite: evidence for

566

fluoride precipitation and surface adsorption, Environ. Sci. Technol. 39 (2005) 9561–9568.

567

[33] Song, Y., Hahn, H.H., Hoffmann, E., Effects of solution conditions on the precipitation

568

of phosphate for recovery: a thermodynamic evaluation. Chemosphere 755 48 (2002)1029–

569

1034.

570

[34] W. Stumm, J.J. Morgan, Aquatic Chemistry, third ed., John Wiley & Sons, Inc., New

571

York, 1996, p. 356.

572

[35] I. Joko, Phosphorus removal from wastewater by the crystallization method, Water Sci.

573

Technol. 17 (1984) 121–132.

574

[36] Avrami, M., Kinetics of Phase Change. I. General Theory, J. Chem. Phys. 7:1103–1112

575

(1939).

576

[37] T.S. Anirudhan, P.S. Suchithra, S. Rijith, Amine-modified polyacrylamide bentonite

577

composite for the adsorption of humic acid in aqueous solutions, Colloids Surf. A

578

Physicochem. Eng. Asp. 326 (2008) 147–156.

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[38] M. Koiv, M. Liira, U. Mander, R. Motlep, C. Vohla, K. Kirsimae, Phosphorus removal

580

using Ca-rich hydrated oil shale ash as filter material̶the effect of different phosphorus

581

loadings and wastewater compositions, Water Res. 44 (2010) 5232‒5239.

582

[39] Michael, J. B.; David, W. B.; Carol, J. P. Laboratory development of permeable reactive

583

mixtures for the removal of phosphorus from onsite wastewater disposal systems. Environ.

584

Sci. Technol. 32 (1998) 2308−2316.

585

[40] Moon, Y. H.; Kim, J. G.; Ahn, J. S.; Lee, G. H.; Moon, H. S. Phosphate removal using

586

sludge from fuller’s earth production. J. Hazard. Mater. 143 (2007) 41−48.

587

[41] Chemical Equilibrium Diagrams, .

an

us

cr

ip t

579

M

588

te Ac ce p

590

d

589

25 Page 25 of 35

Tables

591

Table 1: Results of the XRD Analysis of Raw and Thermally Treated GS TS250

TS500

TS750

TS1000

0.37 1.66 0.12 0.06 0.02 0.19 96.38 0.11 0.36 0.20 0.06 0.20 0.69 ND

0.29 1.33 0.06 0.04 0.01 0.16 97.08 0.94 0.23 0.02 ND 0.09 0.69 ND

0.39 1.34 0.09 0.05 0.02 0.19 96.93 ND 0.29 0.16 ND 0.01 0.68 ND

0.53 1.85 0.11 0.05 0.01 0.20 96.22 ND 0.24 0.01 ND 0.01 0.68 0.24

0.39 1.18 0.08 0.05 0.01 0.16 97.19 ND 0.24 ND ND 0.03 0.66 ND

0.11 0.36 0.01 0.03 0.01 0.05 98.62 ND 0.24 ND ND 0.07 0.52 ND

Table 2: XRD Peak parameters of Raw and Thermally Treated GS

TS100

TS750

TS1000 594 595 596

Calcite 52.89 (113)

Ac ce p

TS250 TS500

Aragonite 26.262 (111) 46.03 (221) 26.26 (111) 46.03 (221) 26.20 (111)

d

Samples Raw GS

52.89 (113)

te

593

M

592

cr

TS100

us

Raw GS

an

Metal oxides Al2O3 SiO2 P2O5 SO3 Cl K2O CaO Cr2O3 Fe2O3 NiO CuO ZnO SrO TnO

ip t

590

30.55 (221)

52.55 (113) 29.46 (104) 39.41 (113) 43.70 (113) 31.36 (113)

Vaterite 37.93 (112)

CaO

37.93 (112) 41.18 (211) 35.97 (110) 54.64 (300) 56.55 (211) 63.19 (211) 72.68 (300) 15.15 (10)

22.89 (112) 28.95 (200)

597 598 599 600 601 26 Page 26 of 35

Table 3: FTIR peaks of Raw and Thermally Treated GS Raw GS

TS100

TS250

TS500

TS750

TS1000

(N-H) Primary amine, (N-H) ammonium ion

3433, 3460,

3456

3421

3460

3452

3460, 3425

2981, 2924

2981, 2924,

2974, 2920,

2970, 2870

2978,2877

CO32-

2627, 2522, 1786, 1473, 1083, 1037, 860

2627, 2522, 1786, 1473, 860

2623, 2522, 2360, 2330, 1786, 1477, 1068, 860

2515, 2434, 1797, 1427, 871

603

606

cr

2981,

1793,1631, 1481, 1408, 1056 428, 864 3637, 3687, 3738

M

Table 4a: Kinetic parameters of the P-Recovery Process Pseudo 1st order

Pseudo 2nd order

Initial Conc. (mg/L)

qe1 (mg/g)

K1 (g/(mg min)

qe2 (mg/g)

2.5 5.0 10.0 20.0 30.0

0.815 0.809 1.607 15.649 2.023

2.755 0.038 0.042 5.489 0.041

4.973 9.960 19.763 40.000 57.803

K2 (g/(mgmin))

d

te

Ac ce p

605

2588, 2515, 2372, 2314, 2133, 1801, 1624, 1419, 871 3630

an

CaO

604

ip t

Functional Groups

us

602

100.54 6.32 7.67 83.33 17.30

Avrami kinetics B

k

3.946 2.373 2.170 6.422 3.107

0.3052 0.1892 0.243 0.7715 0.1779

Table 4b: Error analysis for the kinetic data Pseudo 1st order

Pseudo 2nd order

Avrami kinetics

2 Initial qe(exp) qe(Mod) R χ2 Conc. (mg/g) (mg/g) (mg/L) 2.5 4.97 0.815 0.9238 21.18

2 qe(Mod) R (mg/g)

4.97

1.000 0.00

5.0 10.0 20.0 30.0

9.96 19.76 40.00 57.80

1.000 1.000 1.000 1.000

9.97 19.79 39.95 57.93

0.80 1.60 15.65 2.01

0.7721 0.627 0.8999 0.6491

105.11 206.80 37.73 1555.74

χ2

1.00-5 4.55-5 1.28-3 2.92-4

qe(Mod) (mg/g)

R2

χ2

4.96

0.8331

2.01-5

9.43 19.37 39.95 54.63

0.900 0.8714 0.9615 0.874

0.03 9.11-3 0.00 0.20

607 608 609 27 Page 27 of 35

610

Table 5: Equilibrium isotherm parameters of P-recovery process Langmuir isotherm qm = 84.746 Ka = 1.192 r2 = 0.6374

Freundlich Isotherm 1/n = 0.551 Kf = 39.976 r2 = 0.9377

Temkin Isotherm B1 = 10.732 KT = 46.353 r2 = 0.7443

Table 6: Physicochemical parameters of the raw AQW and Product Water Raw AQW

Product Water

pH Turbidity (NTU) EC (µs/cm) P TS DS SS COD TN K Ca Na

6.61 20.2 92.5 0.15 643 460 183 135 19.76 6.01 5.43 3.64

10.11 8.70 192.5 0.06 261 191 70 54 10.4 6.88 23.10 3.86

616

us

an

te

615

Ac ce p

614

d

613

cr

Parameters

M

612

ip t

611

28 Page 28 of 35

Figures

617

Figure 1: XRD pattern of raw and thermally treated GS

Ac ce p

te

618

d

M

an

us

cr

ip t

616

619 620

Figure 2: FTIR pattern of the raw and thermally Treated GS 29 Page 29 of 35

ip t cr us an

621

Figure 3: Evaluation of P recovery capacity of raw and thermally Treated GS

M

622 623

B

Ac ce p

te

d

A

624 625

Figure 4: Surficial architecture of Raw (A) and Thermally Treated (B) GS at 7500C

626 627 628 30 Page 30 of 35

ip t cr us an Figure 5: Time-concentration profiles of P-recovery at different initial P concentrations

Ac ce p

te

d

630

M

629

631 632

Figure 6: Determination of the occurrence of precipitation in the P-recovery process

633 31 Page 31 of 35

ip t cr us an

634

M

Figure 7: Comparison of the FTIR spectra of TS750 with PTS750

636 637

Ac ce p

te

d

635

Figure 8: Surficial architecture of P-laden TS750 in synthetic feed water

638 639

32 Page 32 of 35

ip t cr us an Figure 9: Comparison of the XRD pattern of TS750 and PTS750

642 643

Ac ce p

te

d

641

M

640

Figure 10: Chemical equilibrium data of interaction of TS750 with Aqueous P

33 Page 33 of 35

ip t cr us an Figure 11: SEM image of P-laden TS750 in real AQW

Ac ce p

te

d

645

M

644

646 647

Figure 12: Comparison of the XRD pattern of TS750 and AQWTS750

34 Page 34 of 35

ip t cr us an

648

Figure 13: Comparison of the FTIR spectra of TS750 and AQWTS750

M

649 650

654 655 656 657 658 659 660

te

653

Ac ce p

652

d

651

661 662 663 664

35 Page 35 of 35