Expounding the role of interference on the recovery of nutrient fractions from aqua matrix using calcined gastropod shell

Journal of Water Process Engineering 27 (2019) 152–161

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Expounding the role of interference on the recovery of nutrient fractions from aqua matrix using calcined gastropod shell Saliu T.D.a, Akinyeye O.J.a, Ololade I.A.b, Unuabonah E.I.c, Oladoja N.A.a,

T

⁎

a

Hydrochemistry Research Laboratory, Department of Chemical Sciences, Adekunle Ajasin University, Akungba Akoko, Nigeria Department of Chemical Sciences, Adekunle Ajasin University, Akungba Akoko, Nigeria c Environmental and Chemical Processes Research Laboratory, Department of Chemical Sciences, Redeemer's University, PMB 230, Ede, Osun State, Nigeria b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nutrient Efficiency of interference Organic matter Phosphate Nitrate Eutrophication

The need to elucidate the influence of the presence of the major components (i.e. organic matter phosphate and nitrate) of nutrient-rich wastewater on the process of nutrient recovery, using calcined gastropod shell (CGS), necessitated this study. The time–concentration profiles of the recovery of organic matter, phosphate and nitrate were studied in synthetic feed water that contained single adsorbate of interest. Both the single and binary component equilibrium isotherm parameters were derived to explicate and quantify the impact of the presence of each constituent of the nutrient rich wastewater on the process of resource recovery. The time-concentration profiles of all the recovery processes showed that the process of nitrate recovery is the rate limiting step in the use of CGS as the reactive material. The pseudo second order kinetic equation gave the best description of both the nutrient and organic matter recovery but the processes were described by different equilibrium isotherm equations. The determination of the efficiency of interference on nutrient recovery showed that nitrate and organic matter exhibited minimal influence on phosphate recovery but the presence of phosphate and organic matter significantly impacted nitrate recovery. The recovery of organic matter was not vitiated by the presence of either phosphate or nitrate. The real life implications of the findings from this study showed that the use of CGS as a reactive material in resource recovery from a nutrient-rich wastewater would provide a double-pronged approach for resource recovery and appreciable treatment of wastewater for reuse.

1. Introduction The new concept of ecological sanitation which perceived nutrient rich wastewater as a valuable resource engendered the research into the development of sustainable nutrient (i.e. nitrogen (N) and phosphorus (P)) recovery protocols. Aside the high nutrient fraction, this wastewater is also replete with other constituents (e.g. organic matter, pathogens and inorganic salts) that limit the application as fertilizers in agricultural practice. Some of these non-nutrient fractions are nonecofriendly, thus the direct application on soils is discouraged to preclude the possible attendant challenges (e.g. salinity, aesthetic and odour concerns). The bids to develop efficacious nutrient recovery procedures have produced techniques that hinged on solid phase extraction procedures. This procedure is a simple conveyance of nutrient from the aqueous phase to a solid phase matrix. Array of materials that have been used in solid phase extraction of nutrient and nutrient fractions include: woodchip; acid mine drainage treatment residuals and steel slag [1];

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Polonite and blast furnace slag [2]; agar-alginate algal blocks [3]; porous MgO-biochar nanocomposites [4]: clinoptilolite [5]; raw and calcined shell of African land snail [6–14]. The affinity of the surface of a solid matrix for ionic specie and the extent of interaction between them are predisposed to the presence of other ionic species in the system. It has been posited [8] that the presence of other ionic species, aside the ion of interest impacts the process of removal, either synergistically, antagonistically or non-interactive. Synergistic and antagonistic actions of interfering ionic components in the adsorption of oxoanions from aqueous system have been reported [9–12,15]. In nutrient rich-wastewaters, suspended and dissolved organic matter are amongst the major constituents. Therefore, it is imperative that sustainable technologies for nutrient recovery should define how the presence of each of the major constituents of the wastewater influences the process of nutrient recovery. Generally, organic matter is a complex heterogeneous mixture consisting of plants and animal remains at various stages of decomposition. It comprises organic constituents such as humic acids (HA)

Corresponding author. E-mail addresses: [email protected], [email protected] (N.A. Oladoja).

https://doi.org/10.1016/j.jwpe.2018.12.004 Received 10 September 2018; Received in revised form 1 December 2018; Accepted 9 December 2018 2214-7144/ © 2018 Elsevier Ltd. All rights reserved.

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2.2. Nutrient and organic matter recovery process

and fulvic acids. These organic constituents are coloured, aromatic and hydrophobic in nature. It also comprises low molecular weight portions that are hydrophilic, including aliphatic and nitrogenous compounds such as amino acids, carbohydrates and proteins [1]. Using the hydrochemistry of natural organic matter (i.e. NOM) as a premise, it was postulated that the high adsorption propensity of organic matter on an adsorbent`s surface can significantly alter the surface chemistry of the adsorbent and possibly envelopes the reactive sites on the adsorbent [4–6]. It is also noteworthy that the deprotonation of carboxylic and phenolic groups on organic matter occurs within the natural pH value of water (i.e. in weakly acidic to basic medium) [7]. This ultimately conferred negative charges on the molecules of the organic matter. Thus, in the aqua matrix, the nutrient fractions and the deprotonated organic matter molecules bear similar charges. Consequent upon the high concentration of organic matter in nutrient-rich wastewaters, the predilection of organic matter to alter the surface chemistry of solid surfaces and the similarity in their surface charges with that of the nutrient fractions in the aqua matrix, we hereby hypothesize that competition may be induced between the nutrient and the organic matter fractions in the aqueous solution for the available reactive sites on the adsorbent, which vitiates the efficiency of the nutrient recovery process. The promise of the calcined shell of a Gastropod (CGS) as a green and sustainable reactive material in the recovery of phosphorus from aquaculture wastewater has been previously reported [11]. In order to verify our hypothesis, the influence of the presence of each of the major constituents of nutrient rich wastewater (i.e. organic matter, nitrate and phosphate) on the process of recovery of each fraction (i.e. organic matter, nitrate and phosphate) by CGS shall be elucidated. The timeconcentration profiles of the recovery of the nutrient fractions and organic matter shall be studied in a synthetic nutrient-rich water that contains single adsorbate (i.e. PO43−, NO3- and organic matter) and binary (i.e. PO43−/ NO3-, PO43−/HA and NO3-/HA) synthetic feed water. Both the single and binary component equilibrium isotherm parameters shall be derived to elucidate and quantify the impacts of the presence of each constituent of the nutrient rich wastewater on the process of resource recovery using CGS. The implications of the findings from this study shall be related to the real life nutrient recovery process from nutrient rich wastewater.

The study on the influence of organic matter on nutrient recovery efficiency was carried out by dissolving varying concentrations (mg/L) of HA in deionised water. Nutrient-rich water was prepared from the dissolution of appropriate potassium salt of the respective nutrient fraction (i.e. KH2PO4 and KNO3) in deionised water. The salts of similar cation were chosen to even out the influence of the index cation of the respective nutrient fraction on the recovery process. The rate of the recovery of the nutrient fractions and organic matter were determined by the addition of 0.5 g of CGS into 1.0 L of the synthetic feed water that contains a known concentration of any of the adsorbates of interest (i.e. PO43− NO3- and HA). The concentration range of the nutrient fractions (i.e. PO43- and NO3-) was between 5 and 50 mg/L, while that of HA was between 2.5 and 30 mg/L. The concentration range chosen for each adsorbate represents the low and medium range values of the adsorbate in real life system [16–20]. The adsorbate/adsorbent mixture was agitated at 200 rpm on a thermostatic shaker, and samples were withdrawn at predetermined intervals that ranged between 0 and 60 min for PO43− and HA, and between 0 and 8 h for NO3-. Shorter withdrawal time intervals were chosen for both PO43− and HA because of the faster rate of attainment of equilibrium time, relative to NO3-. The withdrawn samples were filtered using 0.45 μm polypropylene membrane and the residual concentration of the respective adsorbate (i.e. PO43- NO3- and HA) was determined using the appropriate standard methods highlighted below. In both the single and binary solution systems, the equilibrium isotherm analysis of the process of resource recovery was conducted by contacting 0.1 g of the CGS with 50 mL solution of known adsorbate concentration. The adsorbate concentrations ranged between 5 and 50 mg/L for PO32− and NO3-, and 2.5 and 30 mg/L for HA. The different concentrations of the adsorbate of interest and the interfering ionic species used in the preparation of the binary and single solutions systems are presented in supplementary information Table 1–6 (SI. Table). The mixture was stirred on a thermostatic shaker at 200 rpm until the attainment of the equilibrium time, samples were withdrawn, filtered using 0.45 μm polypropylene membrane and the residual adsorbate concentration was analysed in each case as highlighted below. The concentration of phosphate in the filtered sample was determined by the molybdenum-blue ascorbic acid method [21], nitrate was determined using the cadmium reduction method [21] and HA was quantified using the UV254 absorbance determination at λmax = 254 nm with a UV/VIS spectrophotometer [21]. The amount of each adsorbate recovered per unit mass of the CGS (in mg/g) was quantified using the mass balance equation.

2. Materials and methods 2.1. Preparation and characterization of GS Premised on earlier findings [11], pulverized shell of Gastropod, derived from African giant land Snail (Achatina achatina) was calcined at 750 ℃ in the furnace for 2 h to obtain the CGS. The BET surface area of the CGS was determined using an ASAP 2010 Micromeritics instrument, by Brunauer–Emmett–Teller (BET) method. The elemental and the mineralogical profiles were determined using X-ray fluorescence (XRF) and X-ray diffractometer (XRD), respectively. The surface architecture and elemental composition were determined using scanning electron microscope (SEM) coupled with energy dispersive analysis of X-rays (EDAX) while the chemical functionalities on the surface of the CGS were determined using FTIR spectrophotometer (Thermo Scientific, USA).

3. Results and discussion 3.1. Preparation and characterization of materials The results of the XRF analysis (Table 1) of the CGS affirmed the erstwhile claim that the CGS is a calcium-rich material [6–13]. The diffractogram of the CGS (Fig. 1) revealed the crystalline architecture. Consequent upon the fact that CaCO3 is the principal component of the shells of Gastropods [13 and 14], the miller indices of the three polymorphs of CaCO3 (aragonite, calcite and vaterite) were used as the diagnostic tools in the interpretation of the prominent peaks in the

Table 1 Results of the XRF analysis of CGS. Reactive Material

SiO2

Al2O3

Fe2O3

CaO

SO3

K2O

Cr2O3

NiO

P2O5

CuO

Cl

ZnO

TnO

SrO

CGS

1.18

0.39

0.24

97.19

0.05

0.16

0.11

ND

0.08

ND

0.01

0.03

ND

0.66

ND: Not detectable. 153

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possess partially filled valence orbitals, which provide them with rich active sites for the uptake of phosphate from the aqua matrix. The results of the EDAX analysis showed Ca, C and O as the major identifiable elements in CGS (Fig. 3B). This is a confirmation of the rich CaCO3 nature of the material. The specific surface area of the CGS obtained through BET method was 4.44 m2/g. The value of the single point adsorption total pore volume of pores was 0.012604 cm³/g while the value of adsorption average pore width (4 V/A by BET) was 113.5256 Å. 3.2. Time-concentration profiles of the recovery process 3.2.1. Phosphate and HA recovery in single solute solution system The time-concentration profiles of the recovery of each ionic species of interest from the synthetic feed water are presented in Fig. 4a-c. The appraisal of time-concentration profiles of the three ionic species showed that the rate and time of attainment of the state of equilibrium varied. The processes of recovery of phosphate and HA were faster than that of nitrate. Thus, the states of equilibrium were attained faster in phosphate and HA recovery than in nitrate recovery. Within the initial phosphate concentrations studied, the attainment of the state of equilibrium was almost instantaneous (Fig. 4a). The process of recovery of HA attained equilibrium in less than 30 min (Fig. 4c), while the equilibrium state was attained after six hours (6 h) in nitrate recovery (Fig. 4b (A)). In all the adsorbate studied, the attainment of the state of equilibrium was not initial concentration dependent. Rather, the magnitude of adsorbate recovered per gram of the adsorbent depended on the initial adsorbate concentrations. The rate parameters of the recovery process were derived from the fittings of the data obtained from the determination of the time-concentration profiles to the pseudo first order (Eq. (1)) [23] and pseudo second order (Eq. (2)) [24] kinetic equations.

Fig. 1. Diffractogram of the CGS.

diffractogram of the CGS (Fig. 1). The miller indices employed were 111 and 221 (for aragonite), 104 and 113 (for calcite) and 110,112, 114, 300, 224 and 211 (for vaterite) [22]. In the diffractogram of the CGS, the vaterite peaks appeared at 63.19 (2 1 1) and 72.68 (3 0 0) while the calcite peak appeared at 43.70 (1 1 3). None of the aragonite peaks was observed in the diffractogram of the CGS. Previously from the XRD analysis of the raw GS, the presence of the three polymorphs of CaCO3 at 26.26 (1 1 1) and 46.03 (2 2 1) for aragonite, 52.89 (1 1 3) for calcite and 37.93 (1 1 2) for vaterite was reported [11]. Juxtaposing the results from the XRD analysis of both the raw GS and CGS showed the total disappearance of the aragonite peaks in the CGS. This is an indication that the transformation of the polymorphs of the CaCO3 in one form to another occurred with thermal treatment. In the FTIR spectra of the CGS, the peaks of the N-Hstr that appeared at 2978, 2877 and 3452 cm−1 were attributed to ammonium ions and primary amines from complex protein (i.e. Conchin) in the outer shells of Gastropods. Conchin is amongst the organic macromolecules, primarily proteins and polysaccharides that form the microenvironment, where the CaCO3 crystals nucleated and grew. The multiple CO32− peaks (cm−1) at 2588, 2515, 2372, 2314, 2133, 1801, 1624, 1419 and 871 were attributed to the presence of the different CaCO3 polymorphs. In addition to the presence of multiple CO32− peaks that were identified in the CGS, a single CaO peaks was also identified at 3630 cm−1. The mineralogical assemblage of the CGS (Fig. 1) did not reflect the presence of CaO, which supports the claim that the conversion of CaCO3 to CaO occurs only at temperatures above 900 °C. The inability of the XRD to identify the presence of CaO in the CGS could be ascribed to the fact that the magnitude of this metal oxide in the CGS must be insignificant (Fig. 2) The SEM image (Fig. 3A) showed that the surface of the CGS is made up of a grooved surface with sharp and rough edges. Oladoja et al., [11] suggested from their studies, that the calcination of raw GS at 750 ℃ could enhance surface defects that increase the number of atoms with highly defective coordination environments on the surface of the material. These atoms are located on the surface of the CGS and they

k log[q e − qt] = log[q e] − ⎡ 1 ⎤ t ⎣ 2.303 ⎦

(1)

t 1 1 = + qt k2 qe qe t

(2)

Where: qe is the adsorption capacity (mg/g) of the CGS at equilibrium and qt is the adsorption capacity (mg/g) of the CGS at time t. k1 is the overall rate constants (g(mg/min)) of pseudo-first order (PFO) equation and k2 is the overall rate constants (g(mg/min)) of pseudo second order equation (PSO). The fittings of the time-concentration profile data of the recovery of the nutrient fractions and HA to the two kinetic equations (Table 2) revealed that the PSO equation described the process better (r2 > 0.9) than the PFO equation (r2 < 0.90). The values of the qe (mg/g) increased linearly with increasing initial adsorbate concentration. The values of the PSO rate constant (k2) also increased with increasing initial adsorbate concentration but the increment was not directly related to the initial adsorbate concentration. The non-rectilinear profile of the k2 values with the initial adsorbate concentration contradicted the commonly reported trends. As a rule, the k2 value is expected to decrease with the increasing initial adsorbate concentrations. This is the common assumption that is related to the interpretation of k2 as a timescaling factor (i.e. the higher the initial adsorbate concentrations, the longer the system is required to reach the state of equilibrium) [25–28]. Despite the assumption that the values of k2 is strongly dependent on the initial adsorbate concentration, cases where k2 values are independent of initial adsorbate concentrations have also been reported [29–36]. Consequent upon the conflicting trends in the relationship between k2 and the initial adsorbate concentrations, a theoretical analysis of the kinetic models of adsorption for the system that obeys the PSO kinetic equation was performed Azizian et al. [37]. The results obtained from this theoretical analysis showed that the observed k2 values for any system is a complex function of the initial concentration

Fig. 2. FTIR spectra of the CGS. 154

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Fig. 3. Surface architecture (A) and EDAX (B) analysis of CGS.

the PSO kinetic equation. The values of the Avrami exponent, k, which ranged between 0.077and 0.72 for phosphate recovery and 0.427 and 0.558 for HA recovery showed that the propagation of the precipitate formed from the interaction of both phosphate and HA with the CGS occurred in one dimension.

of adsorbate [34]. Previously, it has been elucidated that one of the underlying mechanism of phosphorus recovery from aqua system using calcined GS was chemisorption (due to the higher fitting of the PSO kinetic equation) [11]. Other experimental and theoretical evidences were also reported to affirm the occurrence of precipitation reaction in addition to the chemisorption mode of adsorption [11]. Hitherto, a critical analysis of the mechanisms that underlie HA recovery using a multivalent cation rich material as a reactive material (i.e. nano MgO) revealed that the possible reaction mechanisms could be attributed to a series of reactions that involved precipitation, complexation and destructive adsorption [38]. It has been advanced that in a high ionic aqueous medium with multivalent cations (e.g. Ca2+ and Mg2+) the coagulation of HA molecule is activated [38,39]. Considering the fact that CGS is a predominantly multivalent cation (i.e. Ca2+), the core mechanism of HA recovery using CGS as the reactive material can be ascribed to HA precipitation and/or adsorption on the surface of CaO or hydrated CaO and the possible carbonation products, as enunciated previously [40]. On the strength of the evidences that precipitation reaction is one of the core mechanisms of HA and phosphate recoveries, the time-concentration profiles of the recoveries of HA and phosphate were also fitted to the Avrami fractional kinetic equation. This enabled the determination of the kinetics parameters for the possible precipitate formations. The theoretical basis of the Avrami fractional kinetic equation hinged on the description of changes in the volume of crystals as function of time during the process of crystallization [11,41]. The linearized form of this equation by simple linear regression is presented in Eq. 3: ln[−ln(1 − F)] = ln(B) + k ln(t)

3.2.2. Nitrate recovery A preliminary evaluation of the possible interactions between the aqueous phase nitrate and the CGS showed that nitrate recovery using CGS cannot be effected through precipitation reaction or any form of phase change reactions. Considering the theoretical basis for the derivation of the Avrami fractional kinetic equation and the possible underlying mechanisms of nitrate recovery from the aqua phase, it was considered inappropriate to fit the time-concentration profile data of the nitrate recovery process to the Avrami fractional kinetic equation [41]. Thus, the time-concentration profile data were analyzed using PFO and PSO kinetic equations only (Table 2). At present, the higher fittings of the pseudo second order kinetic equation (Table 2) to the process of nitrate recovery (r2 < 0.9) indicated the role of chemisorption or ion exchange as an important reaction mechanism in the recovery process. In order to define the possible modes of interactions between the CGS and nitrate in the aqua matrix, the speciation of the aqua phase nitrate and the surface chemistry of the CGS were elucidated using MEDUSA (Make Equilibrium Diagrams Using Sophisticated Algorithms) and HYDRA (Hydrochemical Equilibrium Constant Database) [42]. This software was used to calculate the chemical equilibrium data. Within the operational pH value (< 7.0) of the recovery process, the chemical equilibrium data (Fig. 4b(B)) showed that the prevailing ionic species in the reacting mixture were Ca2+ and NO3− but the products of interactions between these ionic species were CaNO3+ and Ca(NO3)2. At pH value < 8.8, the formation of CaOH+ occurred, which also provided an active site for electrostatic interaction with nitrate. Using the information derived from the chemical equilibrium data, the possible interactions between the CGS and NO3- is hereby proposed as a combination of both specific and non-specific adsorption. The chemical equations that depict the two modes of adsorption are presented below: Specific Adsorption

(3)

Where: F is the fraction adsorbed (qt /qe) at time, t, B, is a temperature dependent constant (similar to a rate constant), while, k, is the Avrami exponent (which reflects the dimensionality of crystal growth). The Avrami exponent, k, value can be 3 ≤ k ≤ 4 (for three-dimensional growth); 2 ≤ k ≤ 3 (for two-dimensional growth) or 1 ≤ k ≤ 2 (for one-dimensional growth) The kinetic parameters obtained from the fittings of the Avrami fractional kinetic equation to HA and phosphate recoveries are presented in Table 2. In relation to the other kinetic equations fitted to the process of recoveries of HA and phosphate, this equation (the Avrami equation) gave a good description of the recovery of both phosphate and HA from aqueous streams. The values of the coefficient of determination (r2) obtained were higher than those obtained from the fitting of the PFO kinetic equation but fell below those obtained from

Ca2+ + NO3− → CaNO3+

(4a)

Ca2+ + 2NO3− → Ca(NO3)2

(4b)

Non-Specific Adsorption (pH value < 8.8) CaOH+ + NO3− → CaOH+ ——————— NO3− 155

(4c)

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Fig. 4. (a)The time-concentration profiles of the phosphate recovery (b)The time-concentration profiles (A) and the chemical equilibrium data (B) of the nitrate recovery (c) The time-concentration profiles of the HA recovery.

Freundlich: logqe = logkf + 3.3. Equilibrium isotherm analysis of the recovery process In order to define the characteristics of the mode of interactions of CGS with the different ionic species in the aqua system, the linear forms of the following equilibrium isotherm equations (Eq. (4)–(9)) were used to analyze the process of recovery of each of the ionic species. The theoretical basis of each equilibrium isotherm equation is presented in the Supplementary Information File (SI 1).

Langmuir:

ce 1 1 = + qe qm ce ka qm

1 logce n

(6)

Temkin : qe = B1 lnkT + B1 lnce

(7)

Dubinin–Redushkevich (D–R): lnqe = lnqm − k ′ ε 2

(8)

θ ⎞1⎤ Frumkin Isotherm : ln ⎡ ⎛ = ln k + 2aθ ⎢ ⎣ ⎝ 1 − θ ⎠ ce ⎥ ⎦

(9)

Harkins Jura:

1 B 1 = ⎛ ⎞ − ⎛ ⎞ logce qe2 ⎝ A⎠ ⎝A ⎠

(10)

The results presented in Table 3 indicated that the process of recovery of each adsorbate was described by different equilibrium

(5) 156

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Table 2 Kinetic parameters of nutrient fractions and organic matter recovery process. Kinetic Parameters for P Recovery Pseudo 1st order qe1 K1 (mg/g) (g/(mg min) x10−2 4.75 0.078 11.91 10.07 0.074 5.67 20.05 0.191 8.15 30.47 0.192 8.15 57.11 0.344 5.09 Kinetic Parameters for N Recovery B Pseudo 1st order Initial Conc. (mg/L) qe1 K1 (mg/g) (g/(mg min) x10−2 6.77 1.382 1.66 12.67 3.602 0.99 23.95 3.414 0.97 33.24 5.253 1.27 52.81 6.644 1.38 Kinetic Parameters for HA Recovery Pseudo 1st order Initial Conc. (mg/L) qe1 K1 (mg/g) (g/(mg min) x10−2 2.53 0.338 13.79 5.25 0.484 13.56 9.52 1.016 12.21 19.78 1.486 16.49 30.09 1.875 13.84

Pseudo 2nd order qe2 K2 (g/(mgmin)) (mg/g) 2.32 23.05 4.99 25.05 9.98 25.05 15.17 38.78 28.41 35.20

r2

Initial Conc. (mg/L)

0.8731 0.8444 08427 0.9566 0.8505

Pseudo 2nd order qe2 K2 (g/(mgmin)) (mg/g) 1.558 0.030 4.301 0.011 4.606 0.037 7.179 0.041 9.804 0.058

r2 0.9058 0.8968 0.9377 0.9329 0.9664

Pseudo 2nd order qe2 K2 (g/(mgmin)) (mg/g) 1.11 1.449 2.23 2.276 4.04 1.851 9.02 3.497 14.53 3.441

r2 0.8976 0.8486 0.8345 0.8682 0.6928

isotherm equations used. The process of phosphate (r2 = 0.9386) and HA recovery (r2 = 0.9782) were best described by the Harkins-Jura equilibrium isotherm equation while the process of nitrate recovery was best described (r2 = 0.9787) by the Freundlich isotherm equation. The description of the recovery process by different isotherm equations indicated that the mode of adsorbent-adsorbate interactions in each case was different. The Harkins–Jura adsorption isotherm can be expressed [43]

1 B 1 = ⎛ ⎞ − ⎛ ⎞ logce qe2 ⎝ A⎠ ⎝A ⎠

r2

Avrami kinetics B k

r2

1.000 1.000 1.000 1.000 1.000

3.524 4.351 4.084 4.549 4.449

0.9246 0.757 0.8244 0.8403 0.9413

r2

Avrami kinetics B k

r2

0.992 0.902 0.977 0.996 0.995

– – – – –

– – – – –

r2

Avrami kinetics B k

r2

0.999 0.999 1.000 1.000 1.000

1.205 1.243 1.042 1.329 1.328

0.9367 0.9604 0.9534 0.9377 0.8711

0.179 0.72 0.112 0.097 0.077

– – – – –

0.558 0.427 0.470 0.482 0.479

expressed thus:

logqe = logkf +

1 logce n

(12)

Where: kf and n are Freundlich coefficients, obtainable from the plots of logqe versus logce The fittings of both the phosphate and HA to the Harkins Jura equation is an indication that the adsorbent-adsorbate interactions occurred through a multilayer adsorption on the adsorbent and there is a distribution of the adsorbate on the heterogeneous pores on the adsorbent. Considering the theoretical basis for the derivation of Freundlich equilibrium isotherm, the process of recovery of nitrate must have occurred on the heterogeneous sites on the CGS.

(11)

It accounts for multilayer adsorption on the adsorbent and the distribution of heterogeneous pores on the adsorbent. The parameters of the isotherm equation are obtained from the plot of 12 versus logce .

3.4. Determination of efficiency of interference on the process of recovery

qe

The Freundlich equation for adsorption isotherm is purely an empirical isotherm having no theoretical basis and its validity extends to non-uniformity of the adsorption surfaces [44]. It indicates the surface heterogeneity of the sorbent. The linearized form of the isotherm is

In order to evaluate the effects of interfering ionic species on the resource recovery profiles of the CGS, the recovery of each of the major constituents of the nutrient-rich wastewater was simulated in aqua

Table 3 Equilibrium Isotherm Parameters of Nutrient Fractions and Organic Matter Recovery Process. Equilibrium Isotherm Parameters of Phosphate Recovery Process Langmuir qm = 0.978 Ka = 60.84 r2 = 0.9199

Freundlich 1/n = 3.644 Kf = 5.484 r2 = 0.5206

Equilibrium Isotherm Parameters of Nitrate Recovery Process Langmuir Freundlich 1/n = 0.950 qm = 49.02 Kf = 3.15 Ka = 0.01 r2 = 0.3201 r2 = 0.9787 Equilibrium Isotherm Parameters of HA Recovery Process Langmuir Freundlich 1/n = 1.422 qm = 8.19 Kf = 5.661 Ka = 0.369 r2 = 0.2148 r2 = 0.7006

D-R qm = 217 K’ = 8 × 10−8 E = 2500 r2 = 0.5166

Temkin-Phyzev B1 = 25.45 KT = 31.17 r2 = 0.234

Frumkin a = 11.345 k = 1.66 r2 = 0.8274

Harkins-Jura A = 1.134 B = 1.757 r2 = 0.9386

D-R qm = 6.25 K’ = 7 × 10−6 E = 188.98 r2 = 0.8572

Temkin-Phyzev B1 = 3.5539 KT = 3.695 r2 = 0.9498

Frumkin a = 5.945 k = 2.181 r2 = 0.8809

Harkins-Jura A = 1.499 B = 1.349 r2 = 0.7622

D-R qm = 10.1 K’ = 2 × 10−7 E = 845.15 r2 = 0.6422

Temkin-Phyzev B1 = 5.79 KT = 3.57 r2 = 0.3968

Frumkin a = 0.6141 k = 2.25 r2 = 0.345

Harkins-Jura A = 0.861 B = 0.09 r2 = 0.9782

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Fig. 5. Efficiency of interfering nitrate (A) and organic matter (B) on phosphate recovery at different initial phosphate concentrations (mg/L).

nitrate was < 5%. Previously, it has been established [11] using experimental and literature evidences that the essential mechanisms of phosphate removal from aqua matrix using CGS are adsorption (specific and nonspecific) and precipitation reaction viz: Specific Adsorption

matrix that contained varying concentrations of the constituent of interest while the presence of any of the other major constituents of no interest is regarded as interference. The efficiency of competition (%) of interfering ionic specie was evaluated by using Eq. (11), proposed by Deb and Datta [45]:

Efficiency of interfering ion ⎛⎜1 ⎝ sorbate uptake in the presence of interferring ion ⎞ − ⎟ x 100 sorbate uptake in the absence of interfering ion ⎠

Ca2+ + HPO42− → CaHPO4

(14a)

Ca2+ + PO43− → CaPO4-

(14b)

(13)

Non-Specific Adsorption

In each case, the values of the influence of the interfering ion versus the initial concentrations of the interfering ionic species in each case were plotted (Fig. 5–7).

Ca

2+

+ HPO42− → Ca2+……. HPO42−

(14c)

Precipitation Reaction 5Ca2+ + 3PO43− + → Ca5 (PO4)3OH

3.4.1. Efficiency of interference on phosphate recovery The efficiency (%) of interference of nitrate on phosphate recovery was studied at initial phosphate concentrations that ranged between 5.2 mg/L and 53.1 mg/L, while the concentrations of the interfering nitrate ranged between 6.2 mg/L and 53.4 mg/L. The influence of each interfering ion on the phosphate recovery process is presented in Fig. 5. Within the initial phosphate concentrations studied, the presence of nitrate showed no appreciable impact on the magnitude of phosphate recovered, especially at the low level initial phosphate concentrations that ranged between 5.2 and 21.5 mg/L. Above 21.5 mg/L phosphate concentration, the presence of nitrate minimally reduced the magnitude of phosphate recovered and the value of the efficiency of interfering

(14d)

In the present study the fitting of the time-concentration profiles of the nitrate recovery to the pseudo second order kinetic equation implied that the process is controlled by chemisorption. Considering the very fast recovery rate of phosphate in the single phosphate solution, which has also been previously reported [11], the role of precipitation is also assured in the present phosphate recovery process. An overview of the chemical equations (Eq. 4a-4c and 14a-14d) presented above that highlighted the fundamental mechanisms of phosphate and nitrate recoveries showed that the two ionic species interacted with the CGS through different reaction mechanisms. Albeit, both specific and non-specific adsorptions are common occurrences in

Fig. 6. Efficiency of interfering phosphate (A) and organic matter (B) on nitrate recovery at different initial nitrate concentrations (mg/L). 158

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Fig. 7. Efficiency of interfering phosphate (A) and nitrate (B) on organic matter recovery at different initial organic matter concentrations (mg/L).

recovery. It has been highlighted that the mechanisms of the recoveries of the two interfering ionic species were totally different from that of nitrate recovery. In precipitation reaction, the reaction mechanism is faster, thus the available reactive material (i.e. CGS) for nitrate recovery is quickly depleted from the aqua matrix. The precipitation reaction accounted for the very fast recovery rate of both phosphate and HA from the aqua phase. Subsequent to the fast depletion of the available reactive species in the aqua matrix in the presence of either HA or phosphate, the aqua phase nitrate has limited reactive species to bind with, thereby causing the appreciable impact of the interfering ion on the magnitude of nitrate recovery.

the recoveries of the two adsorbates, but phosphate precipitation is also a prime recovery procedure. It has been hypothesised [46] that the interaction between phosphate and the surface of calcium rich materials entails the adsorption of small amounts of phosphate and ensuing precipitation of calcium phosphate. Going by the understanding that phosphate can be recovered from the aqua phase through precipitation reaction and the minimal effects of interference by nitrate on phosphate recovery, it could be inferred that the phosphate precipitation reaction predominated during the phosphate recovery process, thus the competition between nitrate and phosphate was minimal. The effects of the interfering organic matter on the phosphate recovery process only manifested on the phosphate recovery at higher initial phosphate concentrations (i.e. < 20.6 mg/L). At this higher initial phosphate concentrations, the values of the efficiencies of the interfering HA ranged between 1.9 and 7.6%. A peep into the essential mechanisms of phosphate and HA recoveries revealed that precipitation reaction dominated the processes. This was also affirmed by the very fast rate of recovery of each species from the aqua phase. Due to the similarities in the underlying mechanisms of the processes of the recovery of these two ionic species, they are bound to compete for the available precipitating agent (Ca2+) in the aqua system for the precipitation reaction to take effect. The results presented in Fig. 5 showed that at relatively low phosphate concentration (i.e. < 20.6 mg/L), the available Ca2+ in the system could effectively precipitate both phosphate and HA, thus the efficiency of interference was insignificant. At high phosphate concentrations (< 20.6 mg/L), the available precipitating agent became insufficient for an effective synchronous precipitation of the two ionic species. The results presented in Fig. 5 indicated that the Ca2+ precipitating agent showed higher affinity for HA than for phosphate in the aqua matrix, thus, the precipitation of HA took precedence over the phosphate precipitation and the efficiency of competition of the HA became positive.

3.4.3. Efficiency of interfering ions on organic matter recovery The results of the determination of the efficiencies of interfering phosphate and nitrate, separately, on organic matter recovery are presented in Fig. 7. An overview of the results showed that the two interfering nutrient fractions did not show appreciable impact on the recovery of organic matter. At low HA concentrations (< 6 mg/L) both interfering ionic species reduced the magnitude of organic matter recovery. Nevertheless, higher concentrations of HA nullified the efficiencies of the two nutrient fractions to interfere in HA recovery. The time-concentration profiles of the recovery of HA showed that the rate of HA recovery was observed to be very fast. It has been suggested that high ionic strengths and the presence of multivalent cations such as Ca2+ and Mg2+ promote HA coagulation [47] in aqueous solution. Considering the nature of the adsorbent used, the role of precipitation reaction in HA recovery is assured. In the determination of the role of the presence of HA in phosphate recovery, it was glaring that the precipitation of HA took precedent over phosphate. The observation here also attested to this fact, thereby the efficiency of competition of phosphate for HA recovery was negligible (Fig. 7). In the case of the interference of nitrogen, since the mode of recovery is different, the fast precipitation reaction impeded the reaction that favoured nitrate removal, thereby nullifying the competitive ability of nitrate in HA recovery.

3.4.2. Efficiency of interference on nitrate recovery The results of the determinations of the efficiencies of phosphate and organic matter competition on nitrate recovery are presented in Fig. 6. The presence of phosphate appreciably impacted the magnitude of nitrate recovery. This impact was visible within all the interfering phosphate concentrations studied. The values of efficiency of interference ranged between 20.3%, at the lowest initial nitrate concentration (6.2 mg/L) and 70.4% at the highest initial nitrate concentrations (53.4 mg/L) studied. The presence of interfering organic matter also significantly influenced the process of nitrate recovery. The efficiency of the interfering organic matter was positive and ranged between 14 and 55%, within the initial nitrate concentration range studied. Appraisals of the impacts of the interference of both phosphate and HA on the magnitude of nitrate recovery showed that the presence of these two interfering ions significantly reduced the magnitude of nitrate

3.5. Implication of interference on the process of resource recovery The evaluation of the recoveries of both the nutrient fractions and organic matter from the synthetic feed water showed that the recovery of organic matter took dominance over any other constituents of the nutrient rich water. Amongst the nutrient fractions the recovery of phosphate took dominance over nitrate. The recovery of organic matter and phosphate are very fast while process of nitrate recovery is rather long. Thus, the recovery of the nitrate fraction of the nutrient-rich wastewater may be considered as the rate limiting step if CGS is used as the reactive material. 159

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References

Albeit, in nutrient recovery, nitrate and phosphate are usually the ionic species of interests in nutrient rich wastewater but the recovery of organic matter cannot be jettisoned. Taking into cognizance the surface chemistry and the hydrochemistry of organic matter, they naturally provide veritable sites for nutrient fractions and the essential microelements to bind, thereby serving as a reservoir of nutrient and microelements required for agricultural operations. Albeit the high binding capacity of the organic fraction of the wastewater can be advantageous, but this binding capacity can also be a disadvantage if there are undesirable ionic species present in the wastewater. Amongst the shortcomings of struvite crystallization for nutrient recovery that have been identified [46] is the issue of securing the quality and safety of the recovered nutrients for fertilizer applications because of the coprecipitation of possible undesirable constituents (e.g. pharmaceuticals) of the nutrient-rich wastewater. Further research is recommended to evaluate the extent of uptake of undesirable fractions of nutrient-rich wastewater in the use of CGS as the reactive material in nutrient recovery. Amongst the different processes for nutrient recovery, struvite precipitation is the leading procedure. One of the major limitations of struvite precipitation is that it does not allow for optimal recovery of nitrogen from the nutrient rich wastewater [49]. It has been reported that the current implemented struvite recovery technologies only recover less than 3 percent of the N and up to 15 percent of the P found in wastewater [46]. For example, the concentration of N in urine far exceeds that of P, but the struvite precipitation only removes N and P in a 1:1 M ratio, necessitating the combination with adsorbent materials to increase the N recovery [46]. In the present procedure, using synthetic nutrient-rich wastewater, the nitrate recovery efficiency of the CGS is relatively low when compared with the phosphate and HA recovery. Nonetheless, the procedure still showed higher nitrate recovery efficiency, when compared with that of the struvite crystallization procedure, because it recovers more than 50% of the available nitrate in the aqua matrix. The optimal nitrate recovery capacity of the CGS needs to be properly appraised in real life nutrient-rich wastewater to ascertain this claim. In terms of the quality characteristics of the treated water, the high organic matter recovery propensity of this procedure is also an added advantage. Consequent upon the high organic matter recovery potential of the CGS, a clearer and cleaner water is produced from this procedure, thus this procedure have the potential to serve a dual purpose of resource recovery and improved water quality characteristics. Furthermore, the procedure can be a step towards the development of a dual purpose procedure for resource recovery and the development of water recirculation system, especially in aquaculture practice were large quantity of water is required for the operations.

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4. Conclusions

• Different reaction mechanism(s) underlie the recovery of nitrate, phosphate and organic matter from aqua matrix using CGS. • Phosphate and organic matter recovery capacities of the CGS were far higher than that of nitrate. • The process of nitrate recovery is the rate limiting step in nutrient recovery using CGS as the reactive material. • At low HA concentrations, the recovery is minimally impacted by the presence of either phosphate or nitrate. • The CGS showed higher affinity for HA than either phosphate or nitrate. Thus, organic matter is preferentially recovered over either phosphate or nitrate from aqueous solution onto the CGS.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jwpe.2018.12.004. 160

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