8.5 L of a produced water (PW) sample was obtained from an oil extraction operation in South Kuwait. The PW had been submitted to oil-water separation with a three-way separator and with corrugated plate interceptors. Nevertheless, it still carried appreciable amounts of hydrocarbons. Spent coffee grounds (SCGs) were obtained from a selection of commercially available coffee brands. Pyridine (BDH) was dried over solid KOH and N, N-dimethylacetamide (Sigma Aldrich) was dried over MgSO₄. Acetyl chloride (Sigma Aldrich) was used without further purification. Commercial Clorox® was used as bleaching agent. It had a sodium hypochlorite (NaOCl) content of 3.5 w%. Potassium hydroxide (KOH pellets, Sigma Aldrich), sodium hydroxide (NaOH pellets, BDH Analr), anhydrous sodium carbonate (soda, eurolab), activated charcoal (Sigma Aldrich, acid-washed with hydrochloric acid), hexane (Panreac) and dichloromethane (CH₂Cl₂, Sigma Aldrich) were used as is. Water used to make solutions and to wash the precipitated salts was AlAinwater® of drinking water quality.

Torrefaction of SCGs was carried out in a Carbolite oven. Post-reaction, samples were dried in an MMM Ecocell drying cabinet. Reaction mixtures were stirred on WiseStir magnetic stirrers. The ash contents of the commercial activated charcoal (AC), SCG, the hydroxide extracted SCG, the bleached SCG as well as the bleached SCG-ester were determined by heating respective samples in crucibles (79C-00, Waldenwanger, Berlin) in a Carbolite oven at 600˚C for 3 h, where the mass of the resulting ash is expressed as weight % of the originally weighed-in sample. The preparation of the SCG material used as the starting material for the bleaching process was described previously [22] . The ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) analyses of the salt crystals were carried out by CCIC Middle East FZE-Fujairah Branch. The laboratory is accredited. For the ICP-OES measurements, an Agilent ICP-OES Model-5110 was used after the digestion of the samples with HNO₃ in a microwave digestor. Infrared spectra of the SCG materials and the derived esterified cellulosic materials were carried out as KBr pellets with a Perkin Elmer Spectrum 2 FT-IR spectrometer. TOC measurements were performed with a Shimadzu TOC-4200.

To a suspension of hydroxide extracted and dried SCG (10 g) in H₂O (150 mL) was given Clorox® (50 mL) and the resulting mixture was stirred for 5 h at rt. Thereafter, the mixture was filtered through a sintered glass filter. Thereafter, the filter cake was transferred to a metal sieve with a mesh size of 125 μm, and the obtained mass was thoroughly washed with 3 × 25 mL H₂O and dried for 10 h at 37˚C. Yield 5.55 g. ( Figure 3 )

Coffee grounds (extracted with NaOH, non-sieved, 1.0 g, 6.15 mmol, one cellulose unit, bleached) were added to a 250 mL round bottom flask, and dimethylacetamide (35 mL) was added. Thereafter, pyridine (2.6 g, 32.5 mmol) and acetyl chloride (2.0 g, 25.0 mmol) were added dropwise, and the resulting mixture was kept at 66˚C for 3 days. Then, the mixture was cooled to 0˚C, and 150 ml of cold water was added slowly to the solution (to avoid hydrolysis of the esters). The ensuing mixture was separated through a sintered glass funnel by vacuum filtration. The filter cake was then washed with dichloromethane (30 mL) and hexane (30 mL), and thereafter dried in the oven for 1 day at 37˚C. (Yield = 1.11 g).

Crushed leaves of Citrullus colocynthis (extracted with ethanol and with aq. NaOH, bleached, 1.0 g, 6.15 mmol, one cellulose unit) were added to a 250 mL round bottom flask, and dimethylacetamide (35 mL) was added. Thereafter, pyridine (2.6 g, 32.5 mmol) and acetyl chloride (2.0 g, 25.0 mmol) were added dropwise, and the resulting mixture was kept at 66˚C for 3 days. Then, the mixture was cooled to 0˚C, and 150 ml of cold water was added slowly to the solution (to avoid hydrolysis of the esters). The ensuing mixture was separated through a sintered glass funnel by vacuum filtration. The filter cake was then washed with dichloromethane (30 mL) and hexane (30 mL), and thereafter dried in the oven for 1 day at 37˚C (Yield = 1.10 g).

For the adsorptive filtration of PW, a glass column (length 35 cm, inner diameter 1.70 cm) was used. The column was filled with a well-mixed sorbent mixture of cellulose acetate (1.20 g) and activated carbon (3.60 g, 1:3 w/w). Sand (30 g) was added as a protective layer to the top of the sorptive material. The column was run with 1 atm pressure, initially at 500 mL per day. The de-oiled water was collected in plastic jerry cans, which thereafter were closed. PW (8.5 L) was filtered through the column. TOC measurements of the filtered PW were performed with a Shimadzu TOC-4200. The TOC of the filtered water was measured to be below 50 ppm. The metal cationic content of the filtered PW was determined by ICP-OES and is shown in Table 3 .

1) At room temperature, solid Na₂CO₃ (7.00 g) was added within 30 sec. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was washed with H₂O (50 mL) and subsequently dried in an MMM Ecocell drying cabinet at 37˚C (14 h) to give a white crystalline solid, (yield: 5.83 g). The filtrate and washings were kept separate, where the filtrate was analyzed further.

2) At room temperature, solid Na₂CO₃ (5.00 g) was added dropwise within 30 sec. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was washed with H₂O (50 mL) and subsequently dried in an MMM Ecocell drying cabinet at 37˚C (14 h) to give a white crystalline solid (yield: 4.95 g). The filtrate and washings were kept separate, where the filtrate was analyzed further.

3) At room temperature, solid Na₂CO₃ (3.00 g) was added dropwise within 30 sec. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was washed with H₂O (50 mL) and subsequently dried in an MMM Ecocell drying cabinet at 37˚C (14 h) to give a white crystalline solid (yield: 2.56 g). The filtrate and washings were kept separate, where the filtrate was analyzed further.

4) At room temperature, an aqueous solution of Na₂CO₃ (7.00 g in 50 mL H₂O) was added dropwise within 5 min. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was dried in an MMM Ecocell drying cabinet to give a white crystalline solid (yield: 5.77 g). This experiment was repeated with the difference that the filter cake was washed with H₂O (50 mL) to give a white crystalline solid (yield: 5.75 g). The filtrate and washings were kept separate, where the filtrate was analyzed further.

5) At room temperature, an aqueous solution of Na₂CO₃ (5.00 g in 35 mL H₂O) was added dropwise within 5 min. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was dried in an MMM Ecocell drying cabinet to give a white crystalline solid (yield: 4.91 g).

6) At room temperature, an aqueous solution of Na₂CO₃ (3.00 g in 21 mL H₂O) was added dropwise within 5 min. to stirred de-oiled PW (200 mL). The mixture was stirred for a further 10 min. at rt and then filtered through a glass filter. The filter cake was dried in an MMM Ecocell drying cabinet to give a white crystalline solid (yield: 2.47 g).

The samples from experiments 1.4.1.1-1.4.1.3 and 2.4.1.1-2.4.1.3 were stored in small plastic containers and sent within the week of the experiment to CCIC, Fujairah, for ICP analysis.

The preparation of non-bleached, torrified (treated) SCG had been published previously [22] , see also ref. [49] [50] . The subsequent bleaching of the SCG was achieved easily by the addition of commercial Clorox® to a suspension of treated SCG. That reaction could be made out visually by the significant lightening of the color of the material. After stirring the mixture for 5h rt, it was filtered through a glass filter. Filtration proved to be slow, and the material needed to be washed copiously. For this, the material was transferred to a 125 μm metal sieve. Very often, organic materials have inorganic salts incorporated or adhering to the surface. A measure of the inorganic content of the material is the ash content, where the material is heated to 600˚C and the organic material is combusted, leaving the inorganics as ash. Here, it can be seen that the treatment of organic material with inorganic salts can contaminate the surface of the material, where the adhering inorganic salts cannot be removed completely by successive washings. In this regard, the ash content of the SCG material was measured after each treatment. At the outset, SCG, after torrefaction, had an ash content of 1.11 ± 0.06 w% ( Figure 4 ). The ash content of SCG increased significantly to 10.96 ± 1.23 w% ( Figure 4 ) after basic extraction of the material with aq. NaOH to remove the lignin fraction from the material. Here, it could be imagined that sodium ions still remain with the extracted SCG. After the bleaching process, the ash content of the material halved to 4.48 ± 0.28 w% ( Figure 5 ), most likely due to successive washings. Interestingly, the subsequent esterification of the bleached SCG decreased the ash content to a negligible level (0.45 ± 0.03 w%, Figure 5 ), indicating the ash content of the extracted SCG most likely is due to sodium ions associated with hydroxy groups of the sugar units of the cellulosic material. The esterification of these hydroxy groups releases the sodium ions. The commercial-activated AC was found to have an ash content of 8.00 ± 0.16 w%.

As a second source of cellulosic sorption material, the authors looked at the leaves of Citrullus colocynthis, a vine plant that grows on arid lands. It is known under a number of names such as Abu Jahl’s melon, colocynth, bitter apple and the vine of Sodom. In the UAE, it is also called the desert gourd or desert squash. Citrullus colocynthis leaves were collected in Tohwayya Wadi, Al Ain. The leaves were washed and subsequently dried for two weeks at rt and crushed in a mortar. Thereafter, they were stirred in ethanol to extract small organic molecules including chlorophyll. Then, the leaves were dried at 37˚C. Thereafter, they were subjected to basic extraction of the material with aq. NaOH to remove the lignin fraction from the cellulosic material ( Figure 6 ). Thereafter, the leaves were heated to 200˚C and ground. They were bleached with an aq. solution of Clorox®, filtered and washed with water. Subsequently, they were dried at 120˚C over 16 h. Finally, they were submitted to esterification with acetyl chloride (AcCl). The IR spectral data showed an incomplete esterification of the material ( Figure 7 ). Therefore, the material was submitted to a second esterification under the same conditions, however, it could be noted that esterification of all hydroxy functions of the cellulosic material could not be achieved. Therefore, further studies with this material were not carried out.

A number of papers have been reported on the crystallization of CaCO₃ from aqueous solutions [51] , including in the presence of other ions [52] . Crystallization can be induced by a liquid-liquid system where both Ca²⁺ and $C{O}_3^{2-}$ ions are present [53] or by the gas-liquid system where carbon dioxide (CO₂) is bubbled through a solution of Ca(OH)₂ or CaCl₂ [54] - [56] in the presence of ammonium hydroxide or amine-containing solutions [57] [58] .

In the current study, the de-oiled PW which had been filtered through a column of AC and bleached SCG-Ac, was treated with either solid Na₂CO₃ or a concentrated aqueous solution of Na₂CO₃. The major concentrations of earth-alkaline cation in the PW before the treatment with Na₂CO₃ are given in Table 4 .

In the first series of experiments, solid Na₂CO₃ (3 g, 5 g, 7 g, all for 200 mL PW) was added to PW within 30 seconds. The precipitate formed was filtered after 10 min., washed with H₂O (50 mL) and dried over 37˚C. Filtrate and washings were kept separate, Thereafter, the cationic composition of the precipitated salt was obtained by ICP-OES analysis ( Figure 8 ). Tables 5-7 and Figures 9-11 show the remaining content of earth-alkaline metal ions in the treated PW after the filtration of the precipitated metal carbonates. After the addition of 3 g Na₂CO₃/200mL PW, the calcium ion concentration dropped from 11,300 ppm to 4413 ppm. After the addition of 5 g Na₂CO₃/200mL PW, the Ca²⁺ concentration in PW diminished to 1647 ppm, dropping to 84.0 ppm after the addition of 7 g Na₂CO₃/200mL PW. Interestingly, magnesium as the second most important earth-alkaline metal found in the studied PW is not as easily crystallized as CaCO₃ so there is some separation of calcium from magnesium possible, although magnesium remains the most dominant contaminant in the precipitated CaCO₃ crystals. The solubility products of calcium carbonate, magnesium carbonate, and strontium carbonate are 4.96 × 10⁻⁹, 6.82 × 10⁻⁶, and 5.60 × 10⁻¹⁰, respectively [59] . Clearly, magnesium carbonate is more soluble in water than calcium carbonate, with the solubility products of the two magnesium carbonate hydrates MgCO₃∙3H₂O and MgCO₃∙5H₂O tabulated as 2.38 × 10⁻⁶ and 3.79 × 10⁻⁶, respectively [59] . Only after most of the calcium content in PW has crystallized as CaCO₃ and added sodium carbonate delivers further carbonate ions does magnesium start crystallizing more abundantly ( Figure 11 ).

Tables 8-14 show the purity of the crystallized CaCO₃ obtained under different conditions. The purity of the crystallized CaCO₃ is high with the first additions of solid Na₂CO₃ and then drops: 94.41 w% (upon addition of 3 g Na₂CO₃, 89.47 w% (upon addition of 7 g Na₂CO₃) as also shown in Table 8 , Table 10 and Table 12 . The same holds true when adding a concentrated aqueous solution of Na₂CO₃: 92.26 w% (upon addition of conc. aq. Na₂CO₃ [3 g in 21 mL H₂O]), 89.76 w% (upon addition of conc aq. Na₂CO₃ [7 g in 35 mL H₂O]) as shown in Table 9 and Table 13 .

Apart from magnesium, the calcium carbonate precipitate also showed a significant amount of sodium. As sodium salts are quite soluble in water, CaCO₃ crystallized with 5 g solid Na₂CO₃/200mL PW was taken up in water (75 mL) after filtration, and the resulting slurry was stirred at 75˚C for 30 min and subsequently filtered. Interestingly, the CaCO₃ precipitate before and after this treatment gave the same CaCO₃ content (92.57 w% vs. 92.58 w%), as shown in Table 10 and Table 11 . The authors believe that this could be due to encapsulation of sodium salts by the crystallizing CaCO₃, where the sodium carbonate crystallites serve as seeding points for the crystallization, and where a concentration gradient in carbonate would be expected from the dissolving sodium carbonate crystallite outwards.

To have a more uniform concentration of carbonate throughout the solution during the crystallization process, a conc. aq. solution of Na₂CO₃ was added dropwise to PW in a separate series of experiments ( Table 9 , Table 13 , and Table 14 ). However, the purity differences between the CaCO₃ crystallized by addition of solid Na₂CO₃ and by addition of a concentrated aqueous solution of Na₂CO₃ were not markedly different (92.26 w% vs. 94.41 w%), as seen from Table 11 and Table 12 . Washing of the filter cake with minimal amounts of H₂O in the latter procedure did change the purity of the precipitated CaCO₃ (95.48 w% purity vs. 92.58 w% purity), as can be seen in Table 13 and Table 14 . Here, most likely Na₂CO₃ attached to the surface of the CaCO₃ crystals is removed, decreasing the Na₂CO₃ content from 4.65 w% to 1.30 w%.

Apart from being used as construction material, calcium carbonate is used in the production of paints, paper and textiles. It is also used as a filling material in plastics [60] [61] and rubber products such as tires, influencing the mechanical properties of the materials [61] . Furthermore, it finds applications in the adhesive industry and the pharmaceutical industry [62] . Also, in the oil and gas industry calcium carbonate is used as a constituent of drilling fluid [63] . The purity of calcium carbonate is an important factor in the pricing of the material, with typical prices for bulk CaCO₃ for non-industrial uses in the UAE ranging from US$ 0.4/kg CaCO₃ to US$ 9.6/kg CaCO₃. The average import price of CaCO₃ into the UAE stood at US$ 0.22/kg CaCO₃ in 2022.