Adsorbent Minimisation for Removal of Ibuprofen from Water in a Two-Stage Batch Process

9 Pharmaceutical products in water, frequently referred to as personal pharmaceutical products, PCPPs are 10 regarded as problematic emerging pollutants with the potential to cause damaging health and environmental impacts 11 to several ecosystems. In this study, an acid activated carbon has been produced from waste date stones, the waste 12 product obtained from the seedless date products manufacturing industry. This material has been used to remove the 13 pharmaceutical compound ibuprofen from water with a high adsorption capacity of 126 mg ibuprofen per g waste 14 date stone derived activated carbon. In order to optimise the use of the activated carbon, a design study has been 15 performed to minimise the quantity of carbon needed using a two-stage batch adsorption system. Several variables 16 have been inputted into the design model to test the model and compare the quantities of adsorbent required in the 17 two-stage and the single stage systems under various conditions.


22
In recent decades, a myriad of, PPCPs, pharmaceutical personal care products and endocrine-disrupting 23 substances (EDCs), has been frequently identified in the aquatic environment and various water bodies such as 24 treated sewage effluents, surface water, groundwater and drinking water (Tran et al. 2014;De Vargas et al. 2021).

25
The presence of these emerging contaminants, despite in trace amounts, is a growing concern due to their potential 26 adverse toxic effects on aquatic organisms and human health (Nassiri Koopaei (Rathner et al. 2017). It has also being reported that estrogenic compounds 34 could negatively affect and alter the self-endocrine regulation of human (Kishor et al. 2021). Another serious 35 concern caused by emerging contaminants is the development of antibiotic-resistant bacteria and genes (Rodriguez-efficient treatment process that can remove organic contaminants and does not produce any toxic by-products 50 (Golovko et al. 2020;Sadraei 2020).

51
In this study, ibuprofen is used as a reference compound to evaluate the performance of activated carbon 52 synthesized from local date stones. Almanassra et al. (Almanassra et al. 2020) reported that carbide-derived carbon 53 (CDC) can be effectively used as an adsorbent for the removal of ibuprofen from deionized water and treated 54 sewage effluent. Guedidi et al. (Guedidi et al. 2013) claimed that surface modification of commercial granular 55 activated carbon using hydrogen peroxide as an oxidant, led to the formation of carbonyl groups that aid the 56 adsorption uptake of ibuprofen. Another investigation by Guedidi et al. (Guedidi et al. 2017) that investigated the 57 performance of an activated carbon cloth in removing ibuprofen, showed adsorption kinetics of ibuprofen was 58 higher at lower pH. An investigation by Baccar et al. (Baccar et al. 2012) that studied the removal of ibuprofen 59 along with other pharmaceuticals such as ketoprofen, naproxen and diclofenac using olive-waste cake waste derived 60 activated carbon revealed that contaminant removal efficiency was lower at alkaline pH. In a comparative study

75
In the current study the activated carbon has been produced by thermally treating phosphoric acid impregnated 76 powdered waste date stone chars (by pyrolysis at 350-600°C) at 550°C for two hours. This carbon has been tested 77 experimentally for its adsorption capacity towards ibuprofen and the experimental equilibrium data were analyzed 78 using seven isotherm models. Finally, a model was developed and applied to determine the to optimum use of the 79 activated carbon using a design study performed to minimise the quantity of carbon required using a two-stage batch  Dates stones were provided by a stoneless date food products manufacturer (Bateel Ltd. Qatar). The raw date 85 stones were washed twice with water to remove the soluble impurities from the stones. These washed stones were 86 immersed in nitric acid (0.1 M) for 1 hour, and then in methyl alcohol for 60 min. in order to remove the organic and 87 inorganic material from the date stone surface. This processed date stone material was dried at 140°C for 12 hours, 88 crushed/powdered using a hammer mill and then segregated by sieving using BSS-25 sieves followed by storage in a 89 dessicator. Following this step it is suitable for charring and activation. This treated date stone was used as the raw 90 material for the preparation of the activated carbons.

91
The production methods of activated carbons from the date stones has been reviewed (Hijab et al. 2020) and 92 the particular activation method of the current date stone powder has been described previously (Hijab et al. 2020).

93
The waste date stone derived activated carbon used in the present study was based on a modified phosphoric acid 94 activation (PAA) process used by Hijab et al. (Hijab et al. 2020).

95
The first stage treatment was thermal treatment to produce a char product. This was followed by the chemical 96 activation of the charred date stone powdered material by 30 % (w/w) orthophosphoric acid using a 1:1 (w/w) ratio 97 of acid: date stone char.

127
USA, and has been discussed in a previous paper (Hijab et al. 2018). Furthermore, the pore volume and BET surface 128 area of the raw date stones were measured in order to compare and assess the effect of the phosphoric acid activation 129 at 550°C on these characteristic properties. The characterization of the phosphoric acid activated carbon product was 130 performed by a Micromeritics ASAP 2020 surface area analyser (Micromeritics, USA) and the nitrogen based pore 131 volume and specific surface area were obtained and these values were compared with the raw feed date stone data.

132
The IBU adsorption capacities were determined by measuring the ibuprofen in solution concentrations by HPLC 133 analysis as previously mentioned in section 2.3.

144
Quantity of IBU adsorbed on carbon = Quantity of IBU removed from solution (1) Rearranging the equation for : where, is the quantity IBU adsorbed on the solid carbon at equilibrium in mg/g, 0 is the quantity adsorbed

216
Freundlich isotherm reduces to the Freundlich equation format at low concentrations, however, moving to higher 217 concentrations, the model, and its parameters approximate more towards the monolayer Langmuir model isotherm.

218
The SIPS or Langmuir-Freundlich isotherm is presented in Equation (9) and substituting for the equilibrium constant,

225
Consequently, the Temkin isotherm became a preferential isotherm to describe gas-phase equilibrium, but has not

239
Another model, particularly useful for isotherms of a high degree of rectangularity, such as ion exchange, has 240 been developed by Dubinin and Radushkevich. The Dubinin-Radushkevich isotherm (Dubinin 1960) is based on the 241 potential theory and is expressed in equation (14) as: Where can be expressed as: Substituting equation (15) into equation (14) produces equation (16): By taking the logarithm of equation (16) yields equation (17): and substituting KDB into BDRR 2 T 2 , equation (14) becomes equation (18): R is the universal gas constant and T is the absolute temperature; BDR is a constant depending on, , the 247 mean free energy of sorption per adsorbate molecule transferred to the surface of the solid from infinity in the 248 solution. The free energy is evaluated using equation (15) and BDR is obtained by rearranging equation (19):

Error Analysis
The subscripts "exp" and "iso" stand for the experimental and calculated adsorbed capacity values, respectively.

258
The results of the SSE analysis demonstrated that the SIPS, that is, the Langmuir-Freundlich isotherm describes the

324
Other studies (see Table 4.) for the adsorption of IBU using various adsorbents obtained a wide range of

336
Our optimisation study is based on minimising the amount of activated date stone carbon adsorbent using a two-337 stage batch process represented in the schematic in Figure 2. The IBU capacities will be determined using the best 338 fit equilibrium isotherm for the phosphoric acid treated date stone activated carbon, and the concentration, Ce, or C2,

339
is the IBU concentration in the treated effluent, to be discharged from the two-stage process. Typically this 340 concentration value is important because it is the maximum or effluent pollutant concentration limit allowed by the 341 legislative authorities in their effluent discharge limits for process licensing to be allowed into receiving waters. It is 342 of the utmost importance to determine the adsorbent IBU uptake capacity at this Ce/C2 valuebecause it is this 343 capacity that is controlling the quantity of adsorbent required in the operating wastewater facility. It is almost certain 344 that this corresponding qe value will not correspond to the maximum isotherm adsorption capacity, qmax, because its 345 value is determined by C2 and the system operating line gradient. The isotherm equation representing the adsorbate 346 uptake mass balance, provides the corresponding adsorption capacity value at C2. Consequently, it becomes of great 347 significance to evaluate this design capacity at the plant design stage, because the mass of adsorbent will be less than 348 the qe,max capacity and therefore the adsorber plant units sizes will need to be increased .

371
Using the most accurate isotherm correlation, namely, Langmuir-Freundlich, as being the best fit isotherm, 372 the general mass balance equation for adsorber stage 1 is: Substituting the isotherm equation for q1 and since q0 = 0, then equation (22)  The total amount of adsorbent used is given by equation (26): Now this differential term,

389
The first target was to assess the amount of phosphoric acid treated date stone carbon required in order to 390 achieve the final discharge limit values in terms of percentage removals of IBU for 0.5 % C0, 1 % C0, 2 % C0, and 391 5% C0; and the second target was to determine the quantities of this carbon necessary to achieve certain fixed set 392 discharge concentration limits of IBU to meet the set values of 0.5, 1, 2 and 5 mg/g, have been analyzed. The

393
Langmuir-Freundlich (Sips) isotherm parameters, as the best fitting equation data, were applied for modelling the 394 process optimization and minimizing the quantity of the acid activated date stone carbon, which was used in the 395 model to optimize the system by minimizing the total date carbon to be used.

403
The trends in Table 5

410
In Table 5, it cab be observed that more date stone activated carbon is required for the second stage adsorber, 411 S2, than in the case of the first adsorber stage, S1. For example at Co = 1 mg/L and the removal target is 0.05 % C0, 412 then S1 = 1192 g and S2 = 3408 g. In all the cases tested, the ratio of the adsorbent used, namely, S2:S1 is in the range 413 of 2.0 to 2.5. Furthermore, for the same initial concentrations the target % C0 removal significantly affects the total 414 quantity of date carbon required. This requirement occurs because the final concentration, C2, required to be 415 achieved is steadily increasing as the % C0 increases; therefore, the percentage change in the C2 discharge is also 416 being relaxed as C2 is allowed to increase. Consequently, the percentage decrease requirement in the IBU 417 concentration is larger in the first condition (total carbon requirement = 4600 g adsorbent for 0.5 % C0 at C0 = 1 418 mg/L); than that in the second condition (total carbon requirement = 2700 g adsorbent for 1.0 % C0 at C0 = 1 mg/L);   Figure 6. Minimum total quantuty of acid treated date stone carbon, S1+S2, to remove IBU applying the Langmuir- provides an important reference point, for example, when there is a legislative or imposed effluent discharge 462 standard for treating the effluent, containing IBU, that must be achieved.

463
Inspection of Table 6, shows that more adsorbent is needed for the second stage adsorber, S2, than for the 464 first stage adsorber, S1, for example at Co = 10 mg/L and the removal target is 0.20 mg/L, then S1 = 353.0 g and S2

465
= 622.5 g. In all the cases tested, the ratio of the adsorbent used, namely, S2:S1 is in the range of 1.4 to 2.0. value decreases, therefore an increase in the total adsorbent mass, S1+S2, is required. The trend is seen in Figure 5 476 for the four C2 changes at a range of C0 values.

477
The variation in the total adsorbent quantity required as a function of the interstage concentration, C1,

489
Two data tables only, Table 5   In the case of the multistage adsorption system, smaller adsorber vessel reactors would be involved e.g. these 500 vessel units would contain the activated adsorbent and incorporate an interstage filter unit in order to separate the 501 spent adsorbent material. Hence, from a practical viewpoint, the multistage plant is better enhancing the process 502 efficiency by minimizing the total amount of adsorbent to be applied and reducing the overall bath processing time.

503
Therefore making the multistage process potentially attractive in economic terms. The larger the number of stages, 504 the greater will be the saving in adsorbent costs, but the total capital equipment costsfor multiple unitswill 505 increase and the handling costs will also be increased. Due to these competing influencing factors, the number of 506 batch stages is usually limited to two or three in order to optimize the economic benefits. For this reason, it is 507 usually required to evaluate the minimum quantity of adsorbent in a two-stage process and the associated savings.  Table 7 for the treatment of 1000L, 1 m3, of solution. For a 514 single-stage adsorption unit the quantity of the treated date stone carbon required is in the range of 43.3 to 21.2 kg as 515 shown in Table 7 using equations (6) and (22): The extra quantity of adsorbent necessary to perform for the single stage treatment ranges from 9.4 to 8.8 517 times more than the optimized two-stage batch adsorber amountthis is quite substantial.
518 Table 8. presents the total quantity of date stone derived activated carbon applied for the two-stage batch

522
For the single-stage batch system equivalency, the quantity of date stone carbon is 9.04, 4.47, 2.19, 0.822 523 and 0.365 kg using equations (6) and (21). The additional quantity of the activated carbon adsorbent necessary for 524 the single-stage batch adsorption system ranges 6.1 times to 1.8 times, demonstrating the advantages for a two-stage 525 process over a single-stage process in terms of adsorbent utilization. 526 527

533
In Table 8, the effect of a lower effluent discharge concentration has a major impact on the additional 534 amount of adsorbent required in the case of the single stage batch adsorption process, that is, at a discharge limit 535 concentration of 0.005 mg/L, then 43.3 kg adsorbent are required, that is, 9.4 times the quantity required for the 536 optimized two stage system. In Table 8, the influence of initial concentration is demonstrated and shows that the

563
Phosphoric acid has been used to treat waste date stone derived char and heated to 550°C to produce activated 564 carbon. The BET-N2 surface area of this date stone derived activated carbon is 727 m 2 /g, a pore volume of 0.70 565 cm 3 /g and a pore diameter size = 3.89 nm. The adsorption capacity the active carbon was tested for the adsorption of 566 the ibuprofen and the maximum adsorption capacity carbon product was 126 mg/g, which compared favourably with 567 the very few reported IBU capacities in the literature. Experimental equilibrium data have been modelled applying seven isotherm equations, namely, Langmuir, Freundlich, Langmuir-Freundlich, Redlich-Peterson, Temkin,

569
Dubinin-Radushkevic and Toth isotherm models. The Langmuir-Freundlich isotherm provided the optimised best fit 570 correlation based on the application of an SSE error analysis. This correlation was then applied for minimising the 571 amount of date carbon adsorbent needed in designing a two-stage batch adsorber system for the removal of IBU.

572
The quantity of the date stone derived activated carbon was determined for a two-stage batch adsorber and for an 573 equivalent single stage process. The quantity of activated carbon adsorbent require was greatly reduced in the two-574 stage system, sometimes by a factor of more than 9-fold. This optimization study also demonstrated the amount of 575 adsorbent strongly depends on both the IBU concentration and especially the effluent discharge limit value adopted

586
The authors declare that they have no competing interests" in this section.   Five isotherm models compared with the experimental IBU equilibrium data.

Figure 2
Schematic gure of a two-stage system Figure 3 Interstage concentrations, C1, to remove different xed % C0 IBU using the Langmuir-Freundlich model.

Figure 4
Minimum quantity of acid treated date stone activated carbon, S1+S2, against the interstage concentrations, C1, to remove IBU using the Langmuir-Freundlich model for different removal values, % C0.

Figure 5
Minimum total quantity of acid treated date stone activated carbon, S1+S2, against the initial IBU concentration, C0, to remove a xed % C0 IBU using the Langmuir-Freundlich model.

Figure 6
Minimum total quantuty of acid treated date stone carbon, S1+S2, to remove IBU applying the Langmuir- Minimum quantity of acid treated date stone carbon, S1+S2, to remove IBU against the interstage IBU concentration, C1, using the Langmuir-Freundlich model for the set values of nal IBU concentrations in the e uent, C2.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. SupplementaryInformation.docx