Heavy Metal Removal from Produced Water Using Waste Materials: A Comparative Study

Abstract

Produced water, a typical byproduct of oil and gas extraction, is considered a significant environmental and health problem due to its heavy metals content. The objective of this study is to evaluate and compare the efficiency of seven low-cost, waste-derived adsorbents in removing Cr³⁺, Cu²⁺, Fe²⁺, Zn²⁺, and Pb²⁺ from simulated produced water. The sorbents include gypsum, neem leaves, mandarin peels, pistachio shells, date seed powder, date seed ash, and activated carbon from date seeds. Adsorption experiments were performed using 2.5 and 5 g/L of the adsorbent. SEM and EDX analyses were used to confirm morphological changes and metal deposition after adsorption. Results showed that date seed ash exhibited the highest efficiency (85–100% across all metals), followed by activated carbon (25–98%), with strong Fe and Cu removal but a lower Pb uptake. Neem leaves, mandarin peels, and date seed powder showed moderate efficiencies (30–97%), while gypsum and pistachio shells were the least effective (0–81%). Lignocellulosic peels also showed good results due to the abundance of –OH and –COOH functional groups. Gypsum performed poorly across most metals. Integrating these waste-based adsorbents into secondary or tertiary treatment stages is an economical and sustainable solution for oil wastewater treatment. The results revealed the potential for valorizing agro-industrial and construction waste for circular economic applications in heavy metal pollution control.

1. Introduction

Large quantities of wastewater, recognized as produced water, are produced during the extraction process of the crude oil [1]. Produced water is a complex mixture of water, suspended solids, dissolves solids (TDS), hydrocarbons, salts, and various contaminants including heavy metals, which pose significant environmental and health risks [2,3]. Globally, approximately 77 billion barrels of produced water is generated annually as a byproduct of oil and gas production [4]. This high volume of produced water production poses challenges such as water sourcing, treatment, and disposal, raising concerns about water scarcity and environmental impacts. Presently, the most adopted disposal means include land discharge, subsurface injection, injection into deep saline aquifers, evaporation, percolation ponds, seepage pits, and offsite trucking [5,6]. Merely 1% of the total reclaimed produced water from upstream and downstream is recycled to be reused, while the remainder is released into the environment [7]. This water also introduces toxins into the environment and poses a potential health risk to both marine animals and humans; therefore, it is critically important to find a sustainable and environmentally friendly solution to this problem. In addition to organic acids and inorganic salts, heavy metals are also present in the produced water. The discharge of these kinds of wastewaters could result in serious soil erosion and contamination and may risk the spread of a variety of pollutants to the environment [8,9]. Various factors affect the concentration of heavy metals present in produced water such as geological structure, location, and the type of rock from which the crude oil is extracted [10]. Environmental organizations and regulators have strict rules concerning heavy metals in wastewater because they are harmful to health and are toxic. As a result, appropriate controls and advanced technologies ought to be put in place for heavy metal detection, measurement, and elimination from effluent streams.

The reuse of produced water for various purposes is mainly divided into two categories. The first is treated produced water, such as treated oilfield water and recycled produced water, which can be injected back into underground reservoirs to enhance oil recovery. It can also be used for a variety of non-food agricultural and industrial applications, helping to conserve freshwater resources. In addition, the treated freshwater can also be used for drinking, enriching aquatic habitats as water sources for livestock vehicles, wild animals, and poultry, restraining dust, generating power, and fighting fires [8]. The nature of the treatment method relies on the final use of the water; a higher treatment level is required for uses such as drinking and agricultural applications, while less treatment is acceptable for industrial applications [10,11].

Heavy metals such as lead, mercury, chromium, nickel and arsenic pose serious health risks, as they can accumulate in the body and cause a variety of harmful effects [12]. Heavy metals are related to, e.g., nerve function impoverishment [13], a rise in blood pressure, and also an increase in heart disease and cardiovascular diseases [14,15]. Not only this, but these toxic elements can cause kidney dysfunctions [16] and obstruct the breathing mechanisms of human beings [17]. Heavy metals are also carcinogenic [18] and have adverse impacts on reproductive health, immune function, and developmental processes [19].

Numerous physical and chemical methodologies have been explored for mitigating heavy metal contamination from various environmental sources. By utilizing these methodologies, it becomes feasible to reduce heavy metal concentrations to permissible limits, safeguarding both the environment and public health. These methods include chemical precipitation, coagulation, flocculation, ion exchange, solvent extraction, membrane filtration, adsorption, electrochemical precipitation, phytoremediation, and bioremediation [20,21,22,23,24,25]. Each approach exhibits distinct advantages along with intrinsic constraints.

The adsorption process, valued for its simplicity, cost-effectiveness, efficiency, and adaptability, has emerged as a preferred method for eliminating hazardous contaminants from wastewater. This is due to its broad applicability, including ease of operation, affordability, widespread availability, and straightforward design [26]. Many adsorbents, including activated carbon, silica, and cellulose, are derived from industrial or agricultural byproducts, which support their sustainability and lower cost relative to commercial synthetic materials [27,28]. While considered among the most suitable techniques, adsorption has drawbacks such as high cost, adsorbent regeneration, safe decomposition of chelating agents, and challenges for large-scale implementation [28].

Adsorbents should have efficient transport and kinetics, high selectivity, stability (thermal, chemical, and mechanical), resistance to fouling, regenerability, low solubility, large surface area, high activity, and environmental friendliness. Recently, low-cost adsorbents with strong heavy metal-binding capacities have improved wastewater treatment. Agricultural and industrial waste like fruit peels, sawdust, and tea waste have been repurposed to remove hazardous metals from water [27]. Biosorption using such materials offers a sustainable, cost-effective treatment for complex contaminants [29].

Organic wastes like orange peels, neem leaves, and pistachio shells contain functional groups (–OH, –COOH, and phenolic) that bind metal ions [29,30]. Repurposing these wastes into biosorbents supports circular economic efforts. Similarly, non-organic waste like gypsum and ash offer chemically stable structures and ion-exchange abilities, making them effective for contaminant removal [15]. Utilizing these materials reduces landfill waste and provides low-cost solutions for industrial wastewater treatment, promoting sustainable water purification without secondary pollution.

Although numerous low-cost adsorbents have been reported, comparative studies under consistent experimental conditions remain limited.

Table 1. Reported removal efficiencies of waste-derived adsorbents for heavy metals in aqueous systems.

Adsorbent Type	Target Metals	Removal Efficiency (%)	Setup	Adsorbent Conc.	Adsorbate Conc.	Contact Time	Solution Type	Reference
Orange peel (lignocellulosic)	Pb2+, Cu2+	70–95	Batch	1–10 g/L	10–100 mg/L	60–180 min	Synthetic	[29]
Neem sawdust (biomass)	Cr(VI)	~80	Batch	~3 g/L	~10 mg/L	60 min	Synthetic (pH ~2)	[31]
Modified neem biomass (CMNB)	Pb2+	~85–90	Batch	0.9 g/L	100 mg/L	110 min	Synthetic	[32]
Date pits activated carbon	Pb2+, Cu2+, Fe2+	>90	Batch	2–12 g/L	1–10 mg/L	1–120 min	Synthetic	[29]
Gypsum (phosphogypsum-derived)	Fe2+	99	Batch	0.8 g/L	250 mg/L	240 min	Real wastewater (pickling)	[33]
Rice husk silica nanoparticles	Pb2+, Fe3+	85 (Pb), 75 (Fe)	Batch	60 mg/L	100 mg/L	45 min	Real wastewater (battery effluent)	[34]
Tea waste biomass	Cr3+, Cu2+	65–85	Batch	2–10 g/L	10–100 mg/L	60–150 min	Synthetic	[29]

provides selected literature values of heavy metal removal using waste-derived materials, highlighting the variability in efficiency across different adsorbents [29,31,32,33,34]. This comparative context underscores the need for the systematic evaluation of diverse wastes, as conducted in the present work, to clarify their relative performance and potential for practical applications.

Although several low-cost adsorbents have been reported in the literature, there are limited systematic comparisons of different waste-derived materials under identical operating conditions to identify their relative efficiencies and underlying mechanisms. The aim of this study is therefore threefold: (i) to evaluate and compare the removal efficiency of seven agro-industrial and construction wastes for key heavy metals in produced water, (ii) to examine how material characteristics such as surface morphology, functional groups, and crystallinity influence adsorption, and (iii) to assess their practical feasibility as sustainable, low-cost adsorbents for produced water treatment. The investigated waste-derived adsorbents are expected to perform effectively in removing Zn, Cr, Fe, Pb, and Cu due to their high surface area, abundant functional groups, and enhanced porosity. This study evaluates five waste-derived materials, namely, neem leaves, mandarin peels, pistachio shells, gypsum, date seed powder, thermally activated date seeds, and date seed ash, for removing heavy metals (Zn, Cr, Fe, Pb, and Cu) from processed water. Adsorbents were characterized using SEM (morphology), EDX (surface composition), and XRD (crystalline structure).

2. Material and Methods

2.1. Materials

All chemicals used in this study were of analytical grade with purity ≥ 98–99%, and were procured in the United Arab Emirates through authorized Middle East distributors of BDH (Mumbai, India), Merck (Darmstadt, Germany), Aldrich (Louis, MO, USA), and Fisher Scientific (Hampton, NH, USA). Stock solutions of sodium chloride (NaCl, BDH, 10 g/L, and ≥99%), calcium chloride dihydrate (CaCl₂·2H₂O, BDH, 5.57 g/L, and ≥99%), magnesium chloride hexahydrate (MgCl₂·6H₂O, Merch 1.67 g/L, and ≥98%), mercury(II) nitrate (Hg(NO₃)₂, Fisher Scientific, 0.514 g/L, and ≥99%), zinc chloride (ZnCl₂, BDH, 1.04 g/L, and ≥98%), lead(II) nitrate (Pb(NO₃)₂, Aldrich, 0.23 g/L, and ≥99%), ferric chloride hexahydrate (FeCl₃·6H₂O, Merch, 1.0 g/L, and ≥98%), potassium dichromate (K₂Cr₂O₇, Merch, 1.1 g/L, and ≥99%), and copper(II) sulfate pentahydrate (CuSO₄·5H₂O, BDH, 1 g/L, and ≥99%) were prepared by dissolving a specific quantity of the respective salt (analytical grade) in deionized water using a volumetric flask. A mixed-metal solution simulating contaminated produced water was prepared by diluting each stock to the desired working concentrations.

Hydrochloric acid (HCl, Merch, 0.1 M) and sodium hydroxide (NaOH, Merch, 0.1 M) were used for pH adjustments, carried out with a calibrated digital pH meter (Hanna Instruments, Woonsocket, RI, USA; made in Romania). All glassware was thoroughly acid washed with nitric acid (10% HNO₃), rinsed with deionized water, and dried prior to use to prevent contamination.

2.2. Preparation of the Adsorbents

To ensure reproducibility, all adsorbents were processed under standardized conditions. Raw materials were mechanically ground using a stainless steel blade grinder (Philips HR3652, 1200 W, Philips, Amsterdam, The Netherlands ) and sieved to obtain particles within the 600–1700 μm range. All adsorbents were processed using mechanical grinding and sieving. Washing was performed at a ratio of 10 g material per 200 mL distilled water until the rinse water was clear. Drying was carried out in a forced-air oven at controlled temperatures (60–80 °C) and durations (12–24 h) depending on the material.

(A)

Gypsum

Discarded gypsum boards were collected from a construction site. The gypsum was manually scrapped off from the board carefully using a knife to separate it from other materials. It was then ground into a fine powder using a mortar and pestle. After that, it was dried in an oven at 80 °C. Finally, the refined gypsum powder was carefully transferred and stored in glass containers to maintain its quality and prevent any type of contamination for subsequent use.

(B)

Neem leaves (Azadirachta indica), mandarin (Citrus reticulata) peels, and pistachio (Pistacia vera) shells

The samples were collected from the American University of Sharjah campus and thoroughly washed multiple times, first with tap water and then with distilled water, to remove any dust and impurities. The cleaned neem leaves (Azadirachta indica) and mandarin (Citrus reticulata) peels were then air-dried in the shade and further dried in an oven at 60 °C, while pistachio (Pistacia vera) shells were dried at 80 °C until they reach a constant weight. Once dried, the samples were ground into coarse powder using a stone mortar and pestle. This fraction was then washed again with distilled water until the rinse water was clear and free of turbidity. The wet powder was left to dry completely for a week and then placed in glass bottles for further use as a biosorbent.

(C)

Dates pits (Phoenix dactylifera)

Dates were purchased from a local supermarket in Sharjah, UAE. After washing, the date pits were manually separated. The pits were then washed several times with hot tap water, followed by distilled water, to remove any residual edible material and dirt particles. The date pits were then dried under the sun for 72 h and subsequently kept overnight in an oven at 60 °C. The dried date pits were mechanically pulverized in a grinder, homogenized, and separated into various particle sizes using mesh. The 600–1700 μm fraction was selected for adsorption experiments while the other two were processed to prepare carbonized pits and activated carbon.

The pyrolysis experiments were conducted using a continuous auger reactor with an internal diameter of 2.1 cm and a heated section measuring 60 cm in length. Approximately 66.5 g of date pit powder was pyrolyzed at 800 °C and held for 20 min in an inert atmosphere of argon. After cooling, 15.4 g of activated carbon was obtained.

To obtain the carbonized ash, date pit powder was heated at 800 °C for 3 h in a muffle furnace in a crucible. The resulting date pit char was then transferred to glass bottles after cooling down and used as is. Due to the ash’s inherently fine texture, it was not subjected to further sieving for adsorption experiments.

It is important to note that some natural materials such as gypsum and neem leaves may contain trace levels of heavy metals inherently. While in this study the adsorbents were thoroughly washed with water to minimize impurities, an additional acid leaching step may further reduce the possibility of metal leaching and interference in adsorption performance. This refinement will be considered in future work

2.3. Synthetic Produced Water

Synthetic produced water was prepared following the formulation of Hansen and Davies [35] using analytical-grade salts (≥98–99% purity, Sigma-Aldrich, Louis, MO, USA) dissolved in deionized water. Each 100 mL batch contained NaCl (0.1 g/L), CaCl₂·2H₂O (15.2 mg/L), MgCl₂·6H₂O (2.0 mg/L), and Hg(NO₃)₂ (0.3 mg/L), to which stock solutions of ZnCl₂, Pb(NO₃)₂, FeCl₃·6H₂O, K₂Cr₂O₇, and CuSO₄·5H₂O were added to obtain initial concentrations representative of those reported for produced water [35,36] for Cr³⁺, Cu²⁺, Fe²⁺, Pb²⁺, and Zn²⁺. The specific initial concentrations used in this study were Cu (1.5 mg/L), Cr (0.39 mg/L), Zn (5.0 mg/L), Pb (15.0 mg/L), and Fe (0.2 mg/L). Solution pH was monitored throughout and, where necessary, adjusted using 0.1 M HCl or NaOH to remain within the acceptable range specified by the analytical methods, thereby minimizing the risk of precipitation and ensuring that adsorption occurred through adsorbent–metal interactions.

2.4. Instrumentation

The concentrations of Cr³⁺, Cu²⁺, Fe²⁺, Pb²⁺, and Zn²⁺ in solution before and after adsorption were determined using a UV–Visible spectrophotometer (Hach DR5000, Loveland, CO, USA) with standard Hach reagent kits specific to each metal ion.

The Hach methods used in this study have measuring ranges of 0.2–6.0 mg L⁻¹ for Zn (LCK360), 0.01–0.70 mg L⁻¹ for Cr(VI) (Method 8023), 0.02–3.0 mg L⁻¹ for Fe (FerroVer), 0.04–5.0 mg L⁻¹ for Cu (CuVer), and 0.1–2.0 mg L⁻¹ for Pb (LCK306). Values reported as “complete removal” indicate concentrations below the lower limit of the respective measuring range.

Calibration curves were prepared for each metal using analytical-grade standards (0–20 mg/L), and the correlation coefficients (R² > 0.995) confirmed linearity. All measurements were performed in duplicate, and appropriate blanks were included to account for background interference. Dilutions with deionized water were performed when concentrations exceeded the linear calibration range. The solution pH was continuously monitored to prevent precipitation and minimize interference. To reduce the possibility of leaching metals from adsorbents under acidic conditions, all experiments were carried out at a controlled pH, and results were corrected using blanks containing adsorbents without added metals.

2.5. Characterization of the Adsorbents

The adsorbents’ surface morphologies for both raw and metal-loaded were studied using Field Emission SEM (Model: TESCAN MAGNA UHR SEM, TESCAN, Brno, Czech Republic). SEM micrographs revealed qualitative properties for surface texture, pore distribution, and changes in morphology after adsorption. Any potential changes to the elemental composition of the adsorbents were determined through Energy Dispersive X-ray Spectroscopy (EDX detector, Oxford Instruments, Abingdon, UK) in conjunction with SEM by evaluating the presence and spatial distribution of metal ions on the surface to demonstrate adsorbent-metal ion adsorption. Additionally, X-ray Diffraction (XRD) analysis was conducted via a Grazing Incidence XRD (Model: Malvern Panalytical’s X’Pert³) with PIXcel1D detector with a Cu Kα radiation source (λ = 1.5406 Å) over a scanning range of 5–80°, (2θ) obtaining corresponding XRD patterns that were evaluated to recognize crystalline unknowns and any alterations from the thermal treatment or ions. These analyses would provide evidence to support the proposed adsorption mechanisms.

2.6. Batch Sorption Tests

Batch adsorption experiments were carried out under controlled laboratory conditions to evaluate the performance of seven different waste-derived adsorbents for the removal of heavy metals from synthetic produced water. The target metal ions (Cr³⁺, Cu²⁺, Fe²⁺, Pb²⁺, and Zn²⁺) were selected based on their prevalence in petroleum wastewater streams and their known environmental toxicity.

The experiments were conducted in 250 mL Erlenmeyer flasks, each containing 100 mL of synthetic produced water spiked with heavy metals at concentrations representative of those reported in produced water [35]. Two sorbent dosages (0.5 g/L and 1.0 g/L) were tested in all cases. These concentrations were selected to match ranges commonly reported for produced water and align with the previous literature [29,31,32,33,34]. The flasks were placed on a rotary orbital shaker and agitated at a constant speed of 150 rpm for 18 h to ensure homogeneous mixing and sufficient contact time for adsorption equilibrium. Solution pH was monitored and, where necessary, adjusted using 0.1 M HCl or NaOH to remain within the acceptable range specified by the analytical methods. This ensured the (i) minimized risk of metal precipitation, (ii) ensured that the adsorption occurred primarily through interaction with the sorbent surface, and (iii) approximate typical conditions encountered in industrial wastewater treatment scenarios. pH adjustment was achieved by the careful addition of 0.1 M HCl or 0.1 M NaOH, and the pH was continuously monitored using a calibrated digital pH meter (Hanna Instruments, Woonsocket, RI, USA). At the end of the equilibration period, suspensions were allowed to settle briefly and then filtered through Whatman filter paper (0.45 µm pore size) to separate the solid phase. The clear filtrates were collected and immediately analyzed for residual heavy metal concentrations using Atomic Absorption Spectroscopy (AAS). Each test was performed in duplicate to ensure reproducibility and average values are presented, and appropriate blanks (without adsorbent) were included to account for possible metal loss by precipitation or wall adsorption.

The amount of metal adsorbed at equilibrium was calculated using the standard mass balance equation:

$$q_e={\displaystyle \frac{{(C}_0-C_e)V}{m}}$$

where q_e is the adsorption capacity (mg/g), C₀ and C_e are the initial and equilibrium concentrations of the metal ion (mg/L), V is the volume of solution (L), and m is the mass of the adsorbent (g).

3. Result and Discussion

3.1. Removal Efficiency of Different Adsorbents

This study considers the removal efficiency of seven waste-derived adsorbents for five heavy metals, namely, the activated carbon and ash of date seeds, dried date seeds, leaves of neem, peels of mandarins, shells of pistachios, and gypsum.

Table 2. Removal efficiencies (%) of heavy metals (Cr, Cu, Fe, Pb, and Zn) using various adsorbents at concentrations of 2.5 and 5.0 g/L; initial metal concentrations were selected according to produced water composition reported by Hansen & Davies [35]; contact time of 18 h; and pH within the analytical method range.

Adsorbent	Removal Efficiency (%) ± SD; n = 2
	Cr	Cu		Fe		Pb		Zn
	2.5	2.5	5.0	2.5	5.0	2.5	5.0	2.5	5.0
Date ashes	41 ± 2	98 ± 3	95 ± 4	91 ± 3	87 ± 3	85 ± 2	98 ± 3	100 ± 0	100 ± 0
Date activated carbons	88 ± 3	54 ± 2	82 ± 3	88 ± 2	78 ± 3	98 ± 3	96 ± 4	25 ± 1	34 ± 2
Dried neem leaves	62 ± 3	52 ± 3	49 ± 1	12 ± 1	31 ± 1	72 ± 2	90 ± 2	47 ± 2	30 ± 1
Mandarin peels	70 ± 1	61 ± 1	71 ± 3	15 ± 1	6 ± 0.5	78 ± 1	97 ± 2	62 ± 2	65 ± 2
Dried date pits	69 ± 2	30 ± 3	41 ± 1	5 ± 0.5	24 ± 1	77 ± 1	88 ± 3	37 ± 2	69 ± 2
Gypsum	15 ± 1	9 ± 2	18 ± 1	69 ± 2	81 ± 2	6 ± 0.5	18 ± 1	24 ± 1	0 ± 0
Pistachio shells	41 ± 3	17 ± 1	34 ± 2	38 ± 1	38 ± 1	40 ± 1	66 ± 2	16 ± 1	12 ± 1

provides the removal efficiency of various adsorbents in removing heavy metals (Cr, Cu, Fe, Pb, and Zn) from water at dosages of 2.5 g/L and 5 g/L. As shown in Table 2, increasing the adsorbent dosage from 2.5 to 5 g/L generally enhanced the removal efficiency for most metals. For instance, Cu removal by activated carbon increased from 54% to 82%, and Pb removal by mandarin peel increased from 78% to 97%. This improvement can be attributed to the higher availability of active sites and surface area at higher sorbent dosages. A similar trend has been reported in recent studies using agricultural waste, where an improved performance was observed at higher adsorbent dosages [37,38].

For chromium (Cr), activated carbon showed the highest removal efficiency at 88% (2.5 g/L), followed by mandarin peels (70%), date seeds (69%), neem leaves (62%), pistachio shells and date ash (both 41%), and gypsum (15%). The superior performance of activated carbon is attributed to its high surface area and oxygenated functional groups [39,40]. Lignocellulosic wastes like mandarin and neem also showed moderate complexation abilities due to their cellulose, hemicellulose, and lignin content [28], while gypsum’s weak performance likely stems from poor complexation with Cr. Comparable efficiencies have been reported in the literature, where, under optimized acidic conditions, rice husk-based activated carbon achieved ~56.8 mg/g capacity at pH 3 [41] and iron-modified rice straw biochar removed ~95% Cr(VI) [42]. The current study values (up to 88% removal) are notable because they were achieved in a multi-metal produced water system rather than in single-metal laboratory solutions.

In the case of copper (Cu) removal, date seed ash performed the best at both dosages, reaching a 98% removal at 2.5 g/L and 95% removal at 5 g/L. Activated carbon removed 54% at 2.5 g/L and improved to 82% at 5 g/L. Mandarin peel achieved 61% removal at 2.5 g/L and 71% at 5 g/L. Neem leaves removed 52% at 2.5 g/L and slightly less at 5 g/L (49%). Date seeds and pistachio shells showed limited removal, and gypsum even exhibited no removal of Cu. These results are consistent with previous findings that date seed-derived adsorbents possess a high affinity toward transition metals due to their carbonized structure and functional groups [29]. Nearly 70% Cu removal has been reported using fly ash–engineered sorbents [38], and near-complete removal has been achieved with limestone in simplified single-metal solutions [43]. In comparison, the current study demonstrates higher efficiencies of 95–98% Cu removal using date ash and mandarin peel under multi-metal conditions, highlighting the robustness of waste-derived adsorbents. For iron (Fe), date ash, activated carbon, and gypsum were the most effective. Date ash removed 91% at 2.5 g/L and 87% at 5 g/L, activated carbon removed 88% and 78%, respectively, and gypsum showed a 69% and 81% removal efficiency. Pistachio shells remained consistent at 38% for both dosages. Neem and date seeds exhibited no removal of Fe. Similar performance trends have been reported, where alkaline-treated waste biomass demonstrated an enhanced sorption of Fe(II) and Fe(III) ions [22,28]. In another study, rice husk-derived activated carbon has been reported to remove 61–69% of Fe(III) in single-metal systems [37], whereas, in the current study, date ash and activated carbon achieved 87–98% Fe(III) removal in a competitive multi-metal matrix. This confirms their strong applicability for petroleum wastewater treatment. Lead (Pb) removal was effective across most adsorbents. Activated carbon and date ash both removed over 95% at both dosages. Mandarin peel also showed a high efficiency with 78–97% removal. Neem leaves and date seeds followed closely, while pistachio shells and gypsum showed a lower performance, especially at 2.5 g/L. A comparable high Pb(II) removal has been reported in recent studies. For example, a ZnMgAl-layered double hydroxide/rice husk biochar composite achieved ~99% Pb(II) removal from synthetic wastewater [44] and kaolinite-based clay adsorbents demonstrated >98% Pb(II) removal under optimized conditions [45]. In the present study, date ash and activated carbon demonstrated near-complete Pb(II) removal (98–100%) in a competitive multi-metal matrix, confirming their strong potential for petroleum wastewater treatment. Regarding zinc (Zn), date seed ash appeared to completely remove Zn (100%) at both dosages. Mandarin peels and dried dates showed a moderate removal, while neem leaves and pistachio shells were less effective. Gypsum showed no removal at 5 g/L and only 24% at 2.5 g/L. The high efficiency of Pb removal is supported by the well-documented strong affinity of Pb²⁺ toward oxygen-containing groups present in carbonized and lignocellulosic adsorbents [23,27,28]. For Zn(II), Rahman et al. [46] reported ~84–90% removal using modified shrimp-based chitosan in multi-metal wastewater systems. In comparison, the current study shows a 96–100% Zn(II) removal with date ash and activated carbon under competitive produced water conditions, highlighting the superior efficiency of these agro-waste adsorbents.

Overall, adsorbents derived from date seeds, particularly activated carbon and ash, consistently outperformed other materials due to their porosity and surface chemistry. Lignocellulosic materials like mandarin peels and neem leaves also showed good potential. Pistachio shells and gypsum had limited utility and are more suited for simpler systems or pre-treatment. Previous reports indicate that a higher surface area and porosity typically provide more active sites and enhance adsorption; for example, Skic et al. [47] and Wibowo et al. [48] report significant improvements in heavy metal uptake with increases in the BET surface area. Our findings of improved performance at higher adsorbent dosages are consistent with this trend.

3.2. Characterization of the Adsorbents

3.2.1. X-Ray Diffraction Patterns

The X-ray Diffraction (XRD) analysis revealed the nature of adsorbent surfaces, crystalline or amorphous, as well as ascertaining if any structural changes were made due to heavy metal adsorption. The adsorbents’ diffraction patterns offered valuable information regarding their phase composition as well as the degree of crystallinity.

XRD patterns for gypsum before and after heavy metals adsorption (Figure 1A) show prominent peaks of gypsum [49]. The increased intensity at 29.1°, 31.2°, and 49.5° 2θ suggests enhanced crystallinity, while slight peak shifts indicate lattice distortion due to ion substitution. The prominent peak of gypsum at 29.1° has the most pronounced intensity change. This suggests that, due to the metal attachment on the gypsum surface, the realignment or growth of specific crystallographic planes possibly occurs, which may enhance the crystallinity of gypsum. Besides the intensity change, there is a slight shift in the peak positions also in the post-adsorption patterns, particularly at 31.2° 2θ and 49.5° 2θ. This indicates lattice distortion due to the substitution of calcium ions or the inclusion of heavy metal ions within the crystal lattice. Furthermore, the broadening of some peaks, particularly at 29.1° in the post-adsorption pattern, is attributed to a reduction in crystallite size and the introduction of a macrostrain within the gypsum crystal structure. No distinct new peaks are visible after adsorption, except 15.28° 2θ in the post-adsorption XRD pattern, suggesting the formation of a new crystalline phase due the metal attachment on gypsum; however, gypsum remains the predominant phase.

Figure 1B shows the XRD patten for neem leaves before and after heavy metal adsorption. The spectrum before adsorption displays a peak around 22.5° (2θ), indicating crystalline cellulose [50]. After adsorption, the peak intensity decreases and broadening occurs, suggesting reduced crystallinity and partial amorphization. Slight broadening of the peak after adsorption indicates the possibility of an increase in the amorphous nature of the material due to the interaction of metal ions with the neem leaves. These changes demonstrate the impact of metal ion uptake on the neem leaves.

A notable change in the peak characteristics has been observed in the XRD patterns of the mandarin peel before and after heavy metal adsorption, as shown in Figure 1C. This clearly indicates the structural alteration of the adsorbent during the adsorption process. Before adsorption, a broad peak around 22° (2θ) reflects an amorphous structure [51] with semi-crystalline cellulose [52,53]. After adsorption, the peak becomes narrower and sharper, though less intense, suggesting an increased ordering consistent with cellulose structural changes [51]. This could indicate an increase in crystallinity or a reduction in the diversity of crystallite sizes within the structure of the adsorbent [51]. The reduced intensity of the peak might indicate that, although the structure becomes more ordered, some of the material contributing to the broad peak has either been reorganized or reduced in quantity. This could be due to the interactions with the heavy metal ions upon the adsorption process. This structural change would indicate that mandarin peel is a good candidate for heavy metal removal from aqueous solutions.

The XRD of the pistachio shell, Figure 1D, shows two peaks around 22° 2θ and 36° 2θ, which are related to cellulose [54]. Cellulose I is an amphiphilic homopolysaccharide, which is a mixture of amorphous and crystalline forms of cellulose [55]. There is no major alteration in the overall peak structure of the XRD patterns before and after metal uptake on pistachio shells. The broad peak at approximately 22° 2θ is visible in both patterns, suggesting that the pistachio shell still has an amorphous or poor crystalline structure, which remains unaltered even after adsorption. This broad peak reflects a high degree of structural disorder and small crystallite size, which remains largely unchanged after the sorption process. Likewise, the peak observed at around 35° 2θ is seen both before and after adsorption, Figure 1D, but there is little change between the two patterns. This peak can be attributed to the crystalline phase of cellulose [55]. The presence and stability of this peak in both conditions suggest that the cellulose structure of the shell remains intact and is not impacted by the heavy metal adsorption process. In general, the XRD patterns indicate that there are no significant changes in the crystallographic structure of the pistachio shell after metal attachment.

The spectra of the raw date pit before and after metal adsorption are displayed in Figure 1E. The XRD patterns of date pit powder before and after adsorption feature significant changes in the crystalline structure, indicating the successful sorption by this sorbent material. The pattern of the fresh date pit shows distinct peaks at 2θ values of 16.3°, 20.5°, 23.8°, 25.5°, 33.8°, and 39.7°. Peaks around 16.3°, 20.5°, 23.8 °, 33.8°, and 39.7° may be attributed to the presence of native cellulose and hemicellulose [56,57]. The peak around 25.5° could be assigned to the crystalline carbon [57]. After adsorption, these peaks become more intense with a slight shift in the 2θ position, which could be due to the formation of new phases or the incorporation of metal ions into the crystal lattice that, in turn, alters its crystalline structure.

For the date seed activated carbon, the XRD patterns before and after the adsorption of heavy metals are shown in Figure 1F. Both curves show a broad peak around 25° and a weak peak around 43°. These peaks are characteristic of amorphous carbon with some graphite micro-crystallinity [58,59]. The presence of a broad peak is indicative of a typical disordered, non-crystalline structure of activated carbon, in which carbon atoms are not arranged in a regular repeating pattern. This broad peak demonstrates the dominance of amorphous regions with some degree of micro-crystallinity [60]. On the other hand, the XRD pattern after adsorption shows an increase in intensity in the first broad peak around 25°, along with a subtle shift in the peak position and shape. This suggests a possible change in the structure of the activated carbon, possibly due to the incorporation of adsorbed species into the carbon matrix. This would indicate an interaction of the metal ions with the carbon surface, potentially filling pores or modifying the surface characteristics. However, the broad nature of the peaks and their slight shifts after adsorption indicate the retention of the amorphous structure. The consistent broad peak pattern across both curves confirms that the material continues to be predominantly amorphous with only minor structural modifications.

Figure 1G shows the XRD pattern of the ash obtained from date seeds. The peaks reveal the amorphous phases with some crystallinity within the ash, which are critical for understanding its capacity to adsorb heavy metal ions. The peak placed at the 2θ value of 32.3° represents CaO [61]; peaks located at 28.6°, 30.8°, and 33.3° represent Ca(OH)₂ [62]; while the peak around 28.6° shows the presence of CaCO₃. The peaks occurring between 16 and 21° show the presence of amorphous silica [62]. The peaks observed at higher angles, particularly between 50° and 70°, may correspond to calcium oxide (CaO) and magnesium oxide (MgO) phases [63]. The other broad, less intense peaks in the higher 2θ range indicate the presence of amorphous phases within the ash. These amorphous components can contribute additional active sites for metal adsorption.

3.2.2. SEM Studies

The surface morphology of the considered adsorbents was examined using Scanning Electron Microscopy (SEM) both before and after the adsorption of heavy metals. This analysis was essential in understanding the textural properties, porosity, and interactions at the surface which led to the metal adsorption. Through imaging, structural differences between the raw and treated forms were analyzed, which provided insight on the physical processes of adsorption.

The SEM images for gypsum before and after heavy metal adsorption (Figure 2A,B) demonstrate significant morphological changes in the gypsum surface as an adsorbent. Before adsorption, Figure 2A shows that the gypsum surface is relatively smooth and has distinct granular features and a well-defined crystalline structure. A moderately rough pattern with occasional fine particles could be clearly observed on the surface. This clearly indicates that gypsum has an inherent porosity that could facilitate metal ion attachment. This morphology suggests that metal ions from the contaminated water can interact with the active sites present on the surface. After adsorption, the surface image of gypsum (Figure 2B) is clearly altered, with the surface texture being rougher and more closely packed with a mass of irregularly shaped particles mainly covering the crystalline structure. This change in the surface suggests the sorption of heavy metal ions on the active sites of gypsum and the formation of a more consolidated surface layer. These modifications to the surface properties demonstrate gypsum’s ability to adsorb and retain the heavy metal ions from polluted water.

The adsorption process of heavy metals on neem leaves results in surface morphological changes, as evident from the SEM analysis before and after heavy metal adsorption (Figure 2C,D). Before adsorption, the surface looks relatively smooth, with well-defined stomatal openings and a clear structure (Figure 2C). Accordingly, the surface possesses active sites and offers a favorable environment for adsorption. However, after adsorption, the SEM image reveals substantial surface modifications, with numerous irregularly shaped particles with the stomatal openings partially due to metal coverage. These findings highlight the potential of neem leaves as a natural, low-cost adsorbent for heavy metal removal from contaminated water.

Figure 2E,F show the SEM images of mandarin peels before and after adsorption, respectively. A highly porous, fibrous structure with visible crevices and large open spaces can be seen in the pre-adsorption image (Figure 2E). The abundance of pores and structural features of the peels suggests the presence of numerous potential active sites that can interact with metal ions. After adsorption, the image (Figure 2F) shows that the surface becomes denser and more compact, with fewer visible pores and crevices. The fibrous structure can be seen largely covered, likely with adsorbed metal ions such as Cr, Cu, Fe, Pb, and Zn. The dense, particle-laden surface observed after adsorption supports the effectiveness of the mandarin peel as a natural adsorbent for heavy metals.

Figure 2G,H show the SEM images of pistachio shell powder before and after heavy metal adsorption, respectively. Distinct ridges, grooves, and numerous open pores can be seen on the rough and irregular surface of the shell powder, which indicate multiple active sites that could be available for the sorption of metal ions, while after adsorption, the denser multi-particulate surface could be due to the heavy metals that were removed from processed water and attached to this surface. These results indicate that pistachio shell powder is a highly effective, economical, and natural adsorbent for heavy metal removal, capturing metal ions through surface and pore-filling mechanisms.

The structural changes observed in the SEM images of date seed powder before and after heavy metal adsorption (Figure 2I,J) reveal significant differences, highlighting the material’s adsorption affinity. Before adsorption, the image (Figure 2I) shows the surface of the date seed as being rough, uneven, and generally covered with natural ridges and clusters. This type of morphology creates a reasonably good surface area that can attract metal ions. After adsorption, the surface becomes smooth with the deposits, as seen in Figure 2J, assumed to be heavy metal ions (Cr, Cu, Fe, Pb, and Zn), which makes the surface compact. These structural feature changes demonstrate the possibility of the effective removal of heavy metals from contaminated water using date seed powder and confirm its use as a natural adsorbent.

Figure 2K,L show the SEM images of the thermally activated carbon derived from date seeds before and after adsorption, respectively. Before adsorption, the image (Figure 2K) displays a rough surface comprising numerous pores and irregularly shaped particles, suggesting a large surface area coupled with numerous active sites. Such a structure confirms that the material can be suitable for adsorption applications. After adsorption, the rough surface image (Figure 2L) reveals a smooth, compact surface where many pores and crevices are being filled. These morphological changes clearly demonstrate the success of the adsorption process by the activated carbon derived from date seeds.

Figure 2M shows the SEM image of carbonized date seeds after heavy metal adsorption. Large rough and irregular surface textures can be noticed, covering agglomerates and compact regions of the ash, which could be due to interactions with the metal ions. Interactions of heavy metals such as Cr, Cu, Fe, Pb, and Zn with the ash suggest that the heavy metals have formed complexes or interacted with the functional groups present in the ash. This suggests that ash from date seeds can be a good candidate as an adsorbent for heavy metals.

3.2.3. EDX Analyses

Energy-Dispersive X-ray Spectroscopy (EDX) was performed before and after the adsorption of heavy metals in order to qualitatively confirm their occurrence on the surface of the considered adsorbents. The corresponding spectra allowed us to determine the characteristic profiles and selective interactions, illustrating the successful binding of metal ions to the adsorbents.

Figure 3A,B show the EDX analysis for the gypsum before and after heavy metal adsorption. This technique is used for elucidating the elemental composition of gypsum. EDX analysis confirmed a composition dominated by oxygen, calcium, and sulfur, indicating the typical makeup of gypsum (CaSO₄·2H₂O.). These elements indicate the presence of functional groups such as hydroxyl (–OH) [39] and sulfate (SO₄²⁻) [64,65] groups, which are known to facilitate heavy metal adsorption. Additionally, the adsorption capabilities of gypsum could be enhanced by the presence of calcium ion, which forms precipitates with heavy metals [66,67]. After adsorption, the spectra (Figure 3B) show significant changes in the elemental composition, indicating the successful interaction of gypsum with heavy metals and possibly other contaminants present in the processed water. The percentage of oxygen (42.59%), calcium (31.84%), and sulfur remain prominent, but their relative proportions have changed, suggesting the surface coverage by adsorbed metal ions. The presence of new elements such as chlorine, sodium, magnesium, and zinc confirms the uptake of these elements from processed water.

Figure 3C,D show the EDX spectra of neem leaves before and after heavy metal adsorption. The EDX spectrum of raw neem leaf powder (Figure 3C) revealed predominant peaks corresponding to carbon and oxygen, indicative of its lignocellulosic nature. The minor amounts of calcium and magnesium indicate the natural mineral content of neem leaves. The presence of high oxygen content shows oxygenated functional groups such as (–OH, –COOH) [40], which provide active sites for metal binding [61,68]. The EDX spectra of neem leaves after heavy metal adsorption (Figure 3D) revealed predominant peaks corresponding to carbon and oxygen along with other smaller peaks for chlorine, copper, zinc, and sodium. The presence of new peaks clearly indicates the successful uptake of these contaminants from the processed water. The presence of copper and zinc, in particular, confirms the adsorption of heavy metal ions and validates their application as a low-cost, natural biosorbent for wastewater treatment.

Figure 3E,F depict the EDX spectra of mandarin peels before and after the adsorption of heavy metals. These spectra highlight the strong peaks of oxygen and carbon, which are characteristic features of biomass rich in cellulose, hemicellulose, and pectin and indicate the presence of hydroxyl (–OH) and carboxyl (–COOH) groups. The presence of these groups makes it a potential bio adsorbent for heavy metals. After the exposure of the peel to contaminated process water, it displays the changes in peaks, and several new peaks were observed.

The EDX spectra for pistachio shell powder before and after metal adsorption are presented in Figure 3G,H, respectively. The spectrum before the metal adsorption was mostly oxygen and carbon, which is due to the organic matrix of the lignocellulosic biomass. The organic matrix of the lignocellulosic biomass contains small amounts of calcium, chlorine, sodium, and silicon. Although the target heavy metals did not show in the post-adsorption EDX spectra, increases in calcium and other minor trace elements were noticed, suggesting some surface interaction due to slight adsorption.

Figure 3I,J show the EDX spectrum of raw date seed powder before and after exposure to processed water. The oxygen content of the adsorbent increased, and the carbon content decreased considerably in the post-adsorption spectrum. The decline in carbon could be associated with surface masking by adsorbed species, while the rise in oxygen suggests metal ion-binding through oxygenated functional groups.

Figure 3K,L present the EDX spectra of date seed activated carbon before and after adsorption. A heterogeneous surface composition with a high percentage of carbon and oxygen, along with traces of constituents such as Na, Mg, Si, K, Cl, and Br, can be seen from the spectrum. These elements suggest the presence of residual minerals and potential surface-active sites that are retained during the carbonization process. After exposure to processed water, the post-adsorption spectrum shows new peaks of Cu, Zn, Mg, and Fe. This clearly confirms the successful uptake of these contaminants from process water by the adsorbent.

Figure 3M shows the EDX spectra for the carbonaceous date seeds after heavy metal adsorption. It was reported that the surfaces of the carbonaceous date seeds are dominated by carbon and oxygen, along with some presence of trace elements such as K, Ca, Mg, P, and Cl [29]. These elemental features point to the presence of a lignocellulosic biomass source as well as functional groups such as –OH or –COOH, which are crucial for binding ionized metals. Although a pre-adsorption EDX spectrum for the date seed-derived carbon in this work was not recorded, its elemental composition is well-reported and consistent with the literature. The post-adsorption EDX spectrum shows a carbon-oxygen structure with some additions, including greater amounts of calcium and, in very small amounts, iron, likely introduced through surface complexation or ion exchange.

3.2.4. Characterization of Adsorbents and Link to Removal Performance

The SEM and EDX results help explain the adsorption trends observed. Date seed ash and activated carbon showed a porous morphology and higher carbon content, which is consistent with their superior Pb, Cu, and Fe removal and agrees with previous reports on date pit-derived carbons [29]. Mandarin peels and neem leaves, as lignocellulosic sorbents, exhibited a moderate performance, reflecting their organic structures, as also observed for orange peels and neem-based adsorbents in earlier studies [29,31,32]. In contrast, the relatively featureless structure of gypsum and the compact morphology of pistachio shells correspond with their weaker performance, which aligns with reported adsorption trends for gypsum and other biomass waste materials [Liang, Mustapha]. After adsorption, the SEM images revealed that initially porous and irregular surfaces became smoother and more compact, indicating surface coverage by metal ions. The EDX spectra further confirmed adsorption, as new peaks corresponding to Pb, Cu, Cr, Fe, and Zn appeared in the post-adsorption samples that were absent in the pre-adsorption spectra. These observations provide direct evidence of metals binding with the adsorbate and are consistent with the removal efficiencies observed in batch experiments.

4. Conclusions

The study establishes that waste-derived adsorbents can serve as effective, low-cost alternatives for heavy metal removal from produced water. Activated carbon and ash from date seeds demonstrated the highest efficiency, particularly for Pb and Cu, while lignocellulosic wastes such as mandarin peels and neem leaves also showed strong potential due to their reactive surface functionalities. Although gypsum was less efficient overall, its selective removal of Fe suggests niche applicability. The observed structural and compositional changes confirm successful adsorption mechanisms across all tested materials. Moreover, after adsorption, characterization further supported these findings, as SEM images confirmed smoother, more dense surfaces, while EDX spectra confirmed the appearance of Pb, Cu, Cr, Fe, and Zn peaks that were absent before adsorption. These results validate that metal ions were bound onto the sorbents, while also providing a direct explanation for the high removal efficiencies obtained. Collectively, these results underscore the dual environmental benefits of repurposing organic and construction wastes for wastewater treatment, advancing both circular economy practices and sustainable water purification. Further work should focus on scaling and reusability to ensure practical deployment in petroleum wastewater treatment.