Amine-grafted Walnut Shell for Ecient Removal Phosphate and Nitrate from Aqueous Solution

The presence of emerging pollutants such as PO 43 – and NO 3 – in water bodies has attracted worldwide 6 concern about their severe effects on water bodies and the health of humankind in general. Therefore, to preserve the 7 health of humankind and environmental safety, it is of the essence that industrial effluents are treated before they are 8 discharged into water bodies. To accomplish this, the walnut shell was functionalized (ACWNS) with amine for 9 effective removal of PO 43 – and NO 3 – . Characterization studies of ACWNS were conducted using FTIR, XRD, XPS 10 and BET techniques. Removal of both ions was enhanced at lower temperature (293 K). The maximum uptake 11 capacity of phosphate and nitrate, at 293 K, was 82.2 and 35.7 mg g – 1 , respectively. The primary mechanism by 12 which these ions were uptaken onto ACWNS could be electrostatic interactions and hydrogen bonding.Pseudo- 13 second-order kinetics fitted the PO 4 3̶ and NO 3- adsorption, while Freundlich and Langmuir models best fitted the 14 PO 4 3̶ and NO 3̶ adsorption, respectively. Furthermore, in the binary system, the uptake capacity of phosphate 15 decreased by 14.4% while nitrate witnessed a reduction in its uptake capacity by 10.4 %. So ACWNS has a higher 16 attraction towards both ions and this could be attributed to the existence of a variety of active areas on ACWNS that 17 exhibit a degree of specificity for the individual anions. Results obtained from real water samples analysis confirmed 18 ACWNS as highly efficient to be utilized for practical remediation processes. 19


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Water is an indispensable element for the continued survival of humankind and other organisms in the 23 ecosystem. Based on this, clean and quality water must be ensured for the survival of living organisms. However, 24 pollutants emanating from activities such as agricultural and mining affect the integrity of these water systems.  and nitrate in portable water are 0.1 mg L -1 and 50 mg L -1 , respectively. Consequently, efficient, inexpensive and ASAP2420-4MP, American ) was employed to determine the pore volume, specific surface area and average pore 96 diameter. X-ray photoelectron spectroscopy (XPS, Escalab 250xi, England) was performed to establish the alteration 97 and the uptake process. The scanning electron microscopy (SEM, Hitachi Su8020, Japan) was used to confirm the 98 surface formation of WNS and ACWNS. The absorbance and equilibrium concentration of phosphate (PO4 3-) and 99 nitrate (NO3 -) were quantified on a UV-Vis spectrophotometer (Persee TU-1900, China) at 700 and 220 nm, The removal experiment was conducted using a single component method. Preliminary experiments revealed 104 that the optimum adsorbent dose was 1.0 g L -1 (Fig. 4). 0.010 g ACWNS and 10 mL of adsorbate were placed in a 105 50 mL conical flask and agitated at 130 rpm. The adsorption kinetics was studied at 30 mg L -1 as the initial 106 concentration of PO4 3and NO3at 303 K, while uptake equilibrium experiments were performed at different PO4 3-107 and NO3concentrations (10 -100 mg L -1 ) at 293, 303, 313 K and the experimental time of 1 h was adequate to 108 bring the reaction to equilibrium. The samples were consequently centrifuged at 5000 rmp for 5 min and the clear 109 solution was taken. For the adsorption work in binary systems, 1.0 g·L -1 ACWNS was placed in a 50 mL flask 110 containing a solution mixture of initial concentrations of PO4 3and NO3at 1:1 ratio. The method and analysis for 111 the binary system was the same as illustrated above, and the experimental time was 1 h. All the works were carried 112 out three times and their means were put to data analysis.PO4 3− concentration was determined at 700 nm using a 113 UV-Vis spectrophotometer (via Mo-Tb anti spectrophotometry) while NO3 − concentration was directly measured at 114 220 nm using a UV-Vis spectrophotometer. The removal efficiency and amount of PO4 3− or NO3 − adsorbed on a unit 115 weight of adsorbent (qe or qt, mg g −1 ) was calculated from Eq. (1) and Eq. (2).

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where C0 means the initial adsrobate concentration (mg L −1 ) whereas C, V and m represent the concentration of the 119 adsorbate at any time t or equilibrium (mg L −1 ), the volume of adsorbate solution (L) and mass of adsorbent (g),

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The XRD analysis of WNS and ACWNS is exhibited in Fig. 2   The SEM of WNS and ACWNS are describeed in Fig. 2a   can uptake potential injurious ions. Therefore, it is indispensable to assess pHzpc of WNS and ACWNS. The 174 outcome is presented in Fig. 3c. It is observed from Fig. 3c that the pHzpc of WNS and ACWNS were 6.4 and 6.0 175 respectively. Below these values, the charge on the surface is positive, while above those values, the charge is 176 negative. Hence, it was anticipated that the removal would occur at pH lower than 6.4 and 6.0, since phosphate and 177 nitrate are anionic species. However, ACWNS is a novel adsorbent that efficiently adsorbed both nutrients 178 (phosphate and nitrate) over a broad array of pH (3-10) as exhibited in Fig 3 a   In the adsorption process, the degree of ionization, the adsorbent's surface charge and the speciation of 183 adsorbates can greatly be affected by the pH of the solution. Hence, the adsorption was studied at different pH (2-184 12) to arrive at the most favourable pH for the uptake process. The influence of pH on the uptake of PO4 3and NO3 -185 is shown in Fig. 3a and b. It was clearly observed from Fig. 3a and b that PO4 3and NO3adsorption capacities 186 were enhanced as the initial pH was raised from 2.0 to 3.0 and stayed considerably constant (up to pH 10.0 and 9.0 187 for PO4 3and NO3 -, respectively), and then reduced significantly as pH increased further from 10.0 to 12.0. The best 188 uptake occurred between pH 3 and 10 for PO4 3and 3 to 9 for NO3ions. The results also indicated that in a highly 189 acidic medium, removal of NO3 ̶ was slightly enhanced compared to the uptake of PO4 3as exhibited in Fig. 3  Generally, the mass of adsorbent used is relative to the amount of binding site available for nutrient absorption 211 (PO4 3and NO3 -). As seen in Figure 4a and b, the percentage reduction of PO4 3and NO3increased as the adsorbent 212 mass increased from 0.5 to 3.0 g L -1 . At higher doses, this dose-dependence may be attributed to increased surface where qm is the maximum adsorption quantity (mg g -1 ), KL is the adsorption constant (L mg -1 ), qe is the equilibrium 252 adsorption capacity (mg g -1 ), Ce is the concentration of nutrients at equilibrium (mg L -1 ). KF is the adsorption 253 quantity constant; 1/n is the adsorption intensity constant; A and B are the Temkin constants.

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The parameters of Langmuir, Freundlich and Temkin isotherms were obtained by evaluating the experimental 255 data based on nonlinear regression analysis. The results and the fitted curves are shown in Table 1 Table 1 indicated that uptake of phosphate onto ACWNS was  Table 1. Additionally, the Langmuir fitted curves were close to the experimental results relative to Freundlich and 266 Temkin curves (Fig.5 a and b). As a result, the Langmuir model was considered appropriate for describing nitrate 267 adsorption onto the surface of ACWNS, as it illustrates monolayer adsorption. Also from Table 1, the values of KL 268 were lower than 1 suggesting that the bonding forces were strong and that the uptake was favorable (Aryee et al.,  Table 2. The amine modified WNS (ACWNS) exhibited a higher performance at a dosage of 1.0 g L -1 In general, kinetic experiments were performed to further grasp the rate mechanism underlying the uptake phenomenon.The relationship between adsorption quantity of PO4 3and NO3onto ACWNS and the contact time at step, the reaction rate began to slow down in which the uptake processes were almost completed. The reaction rate 280 in the further slowed down in the third stage further slowed down until the equilibrium was attained within 60 min 281 for both anions. These occurrences could be explained by the fact that more active sites are available on ACWNS, as 282 well as the fact that the high concentration gradient of the contaminants (PO4 3and NO3 -) decreased over time until 283 equilibrium was reached within 60 minutes.Furthermore, as shown in Table 3, the total anions adsorbed by ACWNS 284 decreased with increasing temperature, as an increased qe value was observed at the lowest temperature for both

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To evaluate the parameters of kinetic models, nonlinear regressive regression was used.The calculated 295 coefficient (R 2 ) and errors (SSE) can be used to determine a model's adequacy.The obtained kinetic models 296 parameters are exhibited in Table 3   and Elovich kinetic models were quite high (Table 3). This depicts that the uptake of both PO4 3and NO3could 303 also be predicted by pseudo-first-order and Elovich kinetic models, showing the complexity of these adsorption 304 processes. Since the Elovich model could predict ion-exchange it reveals that PO4 3and NO3uptake behaviour on

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The intra-particle diffusion (IPD) equation was used to describe the kinetic effects of PO4 3and NO3 -where Kid is the IPD rate constant (g mg -1 min -1/2 ), C is the constant (mg g -1 ) that shows the thickness of the 310 boundary layer, i.e., the bigger the value of C, the greater the boundary layer effect. Fig. 6 shows a graph of qt 311 versus t 1/2 for PO4 3and NO3adsorption. If a graph of qt against t 1/2 as seen in Fig. 6a and 6b, indicates that the 312 adsorption mechanism had more than two stages. Conversely, IPD is the rate-controlling phase if the line moves 313 through the origin. Alternatively, the IPD may not be the sole rate-limiting step. The adsorption mechanism went 314 through three phases, as seen in Fig. 6 a and Table 3. The values of C 318 were not to equal zero. This means that surface adsorption and IPD may be expected to regulate the adsorption 319 mechanism in both cases (Aryee et al., 2020). Consequently, it indicates that external mass transfer followed by IPD 320 mass transfer could regulate PO4 3and NO3adsorption onto ACWNS. Table 3 shows that Kt1 is larger than Kt2, C1 321 is lower than C2, and the R 2 value in the first adsorption stage is higher than the R 2 value in the second process in  The uptake in a binary method was conducted and the results obtained are presented in Fig. 6. Noticeably, an 326 inconsequential decrease in the uptake of phosphate (from 21.15 to 15.2 mg g -1 ; reduction of 14.4%) and nitrate 327 (from 15.2 to 12.09 mg g -1 ; reduction of 10.4 %) onto ACWNS in the binary solution system was witnessed. This 328 may be attributed to the presence of a variety of active sites on ACWNS that exhibit a degree of anion-329 specificity.The outcomes infer that ACWNS to some extent exhibited higher affinity towards the adsorption of both 330 ions. Therefore, ACWNS presents itself as a prospective adsorbent for the uptake of PO4 3and NO3from 331 wastewater in a binary system simultaneously prior to its release into aquatic systems. From this study, it was established that the initial level of PO4 3in the tap and lake water was 0.09 and 0.108 334 mg L -1 , while NO3levels in the tap and lake water were 1.04 and 2.47 mg L -1 respectively, which are lower than 335 the permissible limits reported by World Health organisation, (2011). The adsorption efficacy of ACWNS for PO4 3-336 and NO3adsorption was found to be greater than 90% in all environmental media in the sorption experiments. From 337 this data, it was evident that ACWNS exhibits great prospects as an adsorbent for the practical remediation 338 processes. Table 6 presents the statistical analysis for the recovery of PO4 3and NO3from the real water samples. To test the usability of the exhausted adsorbent, recovery and reuse studies were undertaken. This process helps 341 to establish the economical, efficient practicability of the prepared adsorbent (ACWNS) with regard to adsorption desorption efficiency for both PO4 3and NO3 -(NaOH-NaCl was 94%, NaOH 88%, and HCl 70% for PO4 3-, and observed to be very low in an alkaline medium, indicating that, the occurrence of excess OHand Clin solution 352 disrupts the interactions that occur between the adsorbate and the binding sites on ACWNS resulting in a higher

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(2018), indicating that PO4 3-is easier to be removed. Moreover, the negative values of ΔH o for both PO4 3-(-9.46 kJ 377 mol -1 ) and NO3 -(-17.8 kJ mol -1 ) illustrate an exothermic uptake process; therefore high temperatures do not 378 promote their adsorption. It has been detailed that for uptake to be controlled by physisorption, ΔH° must be less

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To further understand the reaction mechanism involved in the adsorption of nutrients onto ACWNS, the XPS 385 technique was employed. The analysis revealed the elemental compostion of WNS, ACWNS before and after the 386 uptake of the PO4 3and NO3 -. Fig. 8 a, b, c and d illustrate the XPS wide spectrum results. It was seen from Fig. 7 a   387 that the elements C and O were present in the adsorbent before modification, which signifies that they are inherent 388 in agricultural based adsorbent. The emergence of N 1s on the adsorbent as shown in Fig. 8 b confirmed successful 389 modification and the appearance of P2p confirms the adsorption of phosphate ions onto ACWNA (Fig. 7 d). A

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There were no significant alteration in the binding positions of the O 1s peaks after adsorption (Fig. 8 h). However,       628 Fig. 9. Proposed schematic mechanism of PO4 3and NO3adsorption onto ACWNS.

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Captions of Tables   630   Table 1 Parameters of adsorption isotherms for PO4 3and NO3adsorbed onto ACWNS at varied temperatures 631 Table 2 Comparison of maximum adsorption capacities for uptake of PO4 3− and NO3using different adsorbents 632 Table 3 Parameters of Kinetic models for PO4 3and NO3adsorption onto ACWNS Table 4 Thermodynamic parameters for phosphate and nitrate adsorption on ACWNS