Physicochemical Characterization of Natural Rocks and Their Applications for Wastewater Treatment

Abstract

Arid countries such as Arabian Gulf countries are suffering from a water shortage, especially with the recent high-water demand. The best solution for this shortage is the management of currently available water resources, through the reuse of treated wastewater for irrigation purposes. This solution also solves the problem of regularization of wastewater discharge, with positive impacts on the environment. This study aimed to apply an innovative, advanced method for treating wastewater with a favorable environment, low economic cost, and less energy consumption. The research investigated the possibility of using natural rocks such as volcanic and zeolite for advanced treatment of wastewater effluent. The research methodology relied on an experimental work in the lab scales and applied on materials available in Saudi Arabia. The experiments included a leaching batch test to first examine what leaches out from these rocks into water. Then, the materials were tested with wastewater effluent. The main mechanism of treatment was based on the absorption process. The results exhibited significant improvement in the water quality of treated wastewater. On the other hand, the results of the leaching tests showed many ions being dissolved from both rocks into water; thus, it is recommended to soak and flush these solid materials with clean water before using them for the treatment process. Further research is required to determine the best pretreated methods to be applied on these rocks to improve their performance as absorbents.

1. Introduction

Water is the main requirement for humans and all livings. Water availability is also the major element that influences civilizations throughout human history. Therefore, modern life and urbanization increase the water demand. Recently, many regions, especially arid ones, on the earth are suffering from water shortage. The best solution for this shortage is the management of current water resources, e.g., through the reuse of treated wastewater. The reuse of wastewater also solves the problem of the disposal of a huge quantity of wastewater generated annually [1,2].

For the main means of wastewater treatment, the two major biochemical processes used are termed aerobic and anaerobic. An anaerobic environment is one in which dissolved oxygen is available in sufficient quantity such that the growth and respiration of microorganisms are not limited by a lack of oxygen. An anaerobic environment is one in which dissolved oxygen is either not present or its concentration is low enough to limit aerobic metabolism [3]. This is the basic or mandatory treatment step for safe wastewater disposal; however, for the possibility of reusing this water, tertiary and advanced treatments must be applied. Various methods have been introduced for advanced wastewater treatment and mainly rely on the simulation of natural treatment processes [4].

The most common methods for advanced treatment of wastewater effluents are filtration and adsorption. Filtration is a technology utilized to remove particulate matter and microbial contaminants from wastewater effluents. It is based on retaining contaminants too big to pass through the water-filled pores of a filter [5]. Many types of filters are used, which can be categorized as soil filters and membrane filters. The sand filters are the most common soil filter adopted. They can be divided into filters with vertical, horizontal, and radial flow. For filtration by membranes, it is currently considered the most effective method applied for advanced wastewater treatment; however, it is regularly energy consuming and expensive in operation and maintenance. Filtration through membrane technology includes reverse osmosis, nanofiltration, ultrafiltration, and microfiltration processes [4,6].

On the other hand, the adsorption process by solid adsorbents shows potential as one of the capability methods for the treatment and removal of organic contaminants in wastewater treatment. Adsorption is a procedure by which desirable forces join an adsorbate, i.e., a particulate inside the solution, to the adsorbent, i.e., a solid surface. The suitable solid surface used for the adsorption process should have a high interior surface area and a high porosity [7,8]. Adsorption has advantages over the other methods because of the simplicity in its design, and it has a low initial cost and a lower area requirement for implementation. Natural rocks such as volcanic and zeolite are suitable candidates as adsorbents, and they may provide a low-cost and advanced treatment of wastewater effluent.

A volcanic rock forms from the cooling of lava, and most have several common elements. The differences in volcanic rocks are always in the silica content by fractional crystallization. Thus, more developed volcanic rocks tend to be richer in minerals, with a higher amount of silica pore, and to have different crystal sizes and shapes. Hydrothermal alterations can also vary the volcanic rocks widely and can affect the resultant porousness and mechanical behavior [9]. Volcanic soil can work as filters, and they are viable choices for eliminating chlorine, silt, and unpredictable natural mixtures (VOCs) from water. However, it may not be powerful enough for eliminating broken-up minerals, salts, and inorganic mixtures [10].

Zeolite is a natural stone that was created thousands of years ago as volcanic ash. When this ash came into contact with an alkaline water source and was under pressure, it changed into a negatively charged rock with a porous honeycomb structure called zeolite. The zeolite behaves similar to both a sponge and a magnetic body. For example, ammonia is the primary product of odor and bacteria found in animal waste. The zeolite can absorb all kinds of odor producers such as ammonia through magnetism and then removes odors by exchanging the ions and neutral compounds. This process works with all similar compounds. Zeolite can absorb many elements. This makes it ideal for many applications that have large-scale cleaning projects [11,12].

When talking about using natural rocks or solid materials in treating wastewater, it is important to study leaching properties. Leaching is a mass transfer operation in which a solid material releases some of its components when it is in a contact with a solvent. It is a solid–liquid extraction process. Several factors affect the leaching process; these include but are not limited to the size of the solid particles, the porosity of rock mass, and the characteristics of the solvent. The fine materials leach out more than coarse ones. The porosity of the rock mass affects the infiltration process. The solvent or the liquid used is sensitive to pH value and its ratio to solid quantity. Finally, increasing temperature also activates the process of release from solid materials [13].

This work examines the possibility of using natural rocks such as volcanic and zeolite for advanced treatment of wastewater effluent. The research methodology relied on an experimental work in the lab. The experiments adopt leaching batch tests to first observe what percolates out from these rocks into water. Afterwards, the materials were tested for treating wastewater effluents. Several parameters and test procedures influencing leaching and treatment processes were investigated. The main mechanism of the treatment by these rocks was based on the absorption process.

2. Materials and Methods

2.1. Materials

The natural rocks used in this research are available in the Kingdom of Saudi Arabia (KSA). These rocks include zeolite and volcanic. The former rock is available in Al Madinah city, in the northwest of KSA at latitude 24.46 N and longitude 39.77 E, and the latter is available in the mountainous area called Jabal Shama close to the coastal city of Jeddah at latitude 20.75 N and longitude 39.57 E. The main contents of volcanic rock (VR) are silicon, aluminum, magnesium, and iron minerals. On the other hand, zeolite rock (ZR) is mainly composed of alumina-silicate minerals with different components such as sodium, magnesium, and potassium. Both rocks are considered porous media and have internal pores. The specific gravities (Gs) for volcanic and zeolite as determined in the laboratory are 2.40 and 2.41, respectively.

2.2. Leaching Tests

Leaching tests were adopted to examine the components released from the solid materials into the treated wastewater. The solid materials were soaked in distilled water for 24 h. Total dissolved solid (TDS), acidity or basicity of water (pH), and dissolved oxygen (DO) were monitored at different intervals, namely 1, 2, 4, 6, and 24 h. These intervals were adopted to follow the alteration of the pervious chemical properties and progression of ion release with time. Then, after 24 h, a water sample was collected and chemically analyzed to determine the released ions from solid materials into water. Water quality parameters were analyzed by standard methods as described by APHA [14]. Many cations, anions, and heavy metals were measured. TDS, DO, and pH were measured using Hach electrode meters. Ions such as NO₂⁻, NO₃⁻, SO₄², Fe²⁺ were observed using a Hach Spectrophotometer DR/5000. Flame photometry was adopted to determine the sodium (Na+) content, while chloride (Cl⁻) and total hardness were determined using a titration process. The amount of heavy metals were measured by Inductivity Coupled Plasma Mass Spectrometry (ICP-MS). Several parameters were investigated through experiments. The liquid-to-solid ratio (L/S) was adopted as 10 and 20, and three values of initial pH were used, namely 4, 6, and 10. Finally, the solid materials were used in two batches. They were ground to produce fine materials with a particle size of less than 1 mm. while they were crushed, passed through a 25 mm sieve, and retained on a 1 mm sieve to produce coarse materials.

2.3. Wastewater Treatment Tests

In this group of experiments, the two natural rocks volcanic, VR, and zeolite, ZR, were soaked with wastewater effluent for up to 24 h. The effluent was collected from the municipal wastewater treatment plant at Al Madinah city, KSA. The effluent consists of two types, namely, secondary, or biologically treated with aeration tanks (WII) and tertiary treated with sand filters (WIII). Besides previously mentioned measurements in the leaching tests, the biological oxygen demand (BOD); the chemical oxygen demand (COD); and CFU, i.e., microorganisms, were observed after 24 h. The studied parameters in these experiments included examining fine and coarse materials from the two rocks with both WII and WIII samples individually. In addition, another test for coarse materials was adopted after soaking them in distilled water for 24 h before testing with WII. This was to reduce the leached ions from solids into the water sample.

Table 1. Details of volcanic and zeolite samples showing studied parameters.

Volcanic Sample	Zeolite Sample	Material Size/Status	Water	Initial pH	Liquid to Solid (L/S)	Parameter Symbol
WII		-	Secondary treated wastewater	8.44	10	WII
WIII		-	Tertiary treated wastewater	8.38	10	WIII
V1	Z1	Fine materials (<1 mm)	Distilled water	6.00	10	F/D-ph = 6
V2	Z2	Coarse materials (<25 mm to 10 mm)	Distilled water	6.00	10	C/D-pH = 6
V3	Z3	Coarse materials (<25 mm to 10 mm)	Distilled water	4.00	10	C/D-pH = 4
V4	Z4	Coarse materials (<25 mm to 10 mm)	Distilled water	10.00	10	C/D-pH = 10
V5	Z5	Coarse materials (<25 mm to 10 mm)	Distilled water	6.00	20	L/S = 20
V6	Z6	Coarse materials (<25 mm to 10 mm)	Secondary treated wastewater	8.44	10	C/WII
V7	Z7	Coarse materials (<25 mm to 10 mm)	Tertiary treated wastewater	8.38	10	C/WIII
V8	Z8	Prewashed coarse (<25 mm to 10 mm)	Secondary treated wastewater	8.44	10	C/WII-prewashing

V (volcanic sample); Z (zeolite sample); F (fine materials); C (coarse materials); D (distilled water).

presents a summary of the studied parameters.

3. Results and Discussion

3.1. Leaching Properties

The leaching process is an important interaction between solid and liquid. The leaching process is the opposite of the adsorption process as, in the latter process, the solid particles are released from solid materials into water and, in the former, vice versa. Thus, in this work, the leaching properties of the two rocks were investigated via studying several influenced parameters including the effect of initial pH, material size, and L/S ratio, as described earlier. All results shown herein are the average of three identical experiments for each parameter. The statistical analysis showed that the standard deviations (SD) as a percentage from the mean for all measurements showed ranges of ±3% for pH values up to 8% for K⁺ and Fe²⁺ observations, as seen later in upcoming figures. The analysis showed also that the accumulated standard deviations (ASD) for all samples from ZR or VR for the same property demonstrated higher bias for VR than ZR from their means. For example, the ASD for pH values of all samples of ZR and VR was 0.99 and 1.17, respectively, with the percentages from the mean being 12.9 and 14.5%, respectively. For Na⁺, ASD was 9.22 and 17.2 mg/L with percentages of 33.9 and 98.8%, respectively.

3.1.1. Physicochemical Characteristics

Figure 1 presents the results of pH, TDS, and DO monitoring over 24 h after adding distilled water to the two rock samples. Five samples from each rock were examined, as detailed previously in Table 1. The results showed that soaking VR into water gradually increased the pH values with time. This increase continued up to six hours and, then at 24 h, slightly decreased, but the final pH was in general higher than the initial pH. This was found for all studied parameters except for the initial pH of 10 as it decreased with time up to 24 h. On the other hand, for ZR, pH exhibited a slight increase with time, and all samples sustained a value of around 8 for different parameters except the initial pH of 4 reached 5.4 only. Figure 2 summarizes the impact of the studied parameters on the final values of pH and TDS after 24 h. Figure 2 illustrated for both rock samples that the initial pH is more affected than the other parameters in the final pH values. Other parameters showed approximately the same pH values. In addition, VR exhibited a slight increase in pH values than those of ZR.

Form the results presented in Figure 1, the studied parameters showed notable influences on TDS measurements. For both rocks, the finer materials and higher L/S ratio showed the higher release of ions and, consequently, the higher values of TDS. As the particle size decreases, the surface area and porosity of materials increase, which leads to an increase in cation exchange with water [11]. A notable increase was also observed using both higher and lower initial pH. A significant increase was observed particularly when adopting a pH of 10. All measured values showed a gradual increase with time. Figure 2 shows also that higher values of TDS were detected for ZR samples than VR for all studied parameters.

For the results of DO, they exhibited a depletion in the oxygen content with time for ground materials compared with the coarse ones for both rocks. Furthermore, the test with an initial pH of 4 showed a decrease in DO content with time. This is perhaps because the amount of oxygen becomes less in water with increasing levels of dissolved or suspended solids in water. On the other hand, increasing the L/S ratio and initial pH to 10 slightly increases DO. In general, increasing DO indicates an improvement in the quality of the water.

3.1.2. Ion Release

Figure 3 presents the results of some ions released into water from volcanic and zeolite rocks. Coarse materials exhibited lower extraction in comparison with ground materials for both rocks as the release was reduced in some cases by 92%. In addition, when the L/S ratio increased to double, i.e., 20, the ion concentrations decreased by from 38 to 88%. Decreasing the initial pH to 4 notably increased the release of K⁺, Mg²⁺, Na⁺, and Ca²⁺, while it decreased the amount of extraction for Fe²⁺. For the results of the initial pH of 10, the release of all ions was the lowest in comparison with the pH of 4 and 6 except for Na⁺. It showed the highest rate of release had a pH of 10. In general, ZR showed a higher rate of extraction than that of VR except for some cases such as Na⁺ and Fe²⁺, as the latter has the highest release for fine materials, as shown in Figure 3.

For cations, the amount of K⁺, Mg²⁺, and Na⁺ in water increased with the addition of ZR to water due to the ability of ZR to exchange cations/ions with water. The concentration of Ca²⁺ increased intensely in all water sampled compared with the other cations, resulting in a high concentration of Ca²⁺ in ZR or the location of Ca²⁺ on the surface of ZR influencing the rate of cation exchange. Cakicioglu-Ozkan and Ulku [15] stated that the location of the earth alkaline metal on the surface of ZR and the shape of ZR play vital roles in stabilizing the framework of ZR and the rate of adsorption. Si and Al influence the negative charge of ZR, which is balanced by the earth alkaline metal, and the negative charge generated comes from tetrahedrally coordinated aluminum [15]. Thus, the negative charge of ZR on the surface enhances the cation exchange between water and ZR [11].

For heavy metals, the amounts of iron, copper, lead, and cadmium were examined. Iron, Fe²⁺, showed high extraction in the case of ground materials for both rocks. Copper showed minimal release, and others were not detectable at all.

3.2. Treatment Effectiveness of the Rocks

In this section, the adsorption properties of the two rocks were investigated by soaking them in the WII and WIII wastewaters. All results shown herein are the averages of three identical experiments for each parameter. The statistical analysis showed that the standard deviations (SD) as a percentage from the mean for all measurements showed ranges of ±2.5% for pH values up to 7% for COD observations, as seen later in Figure 4, Figure 5, Figure 6 and Figure 7. The analysis showed also that the accumulated standard deviations (ASD) for all samples from ZR or VR for the same property demonstrated higher bias for ZR than VR from their means, as shown in

Table 2. Chemical analysis results for wastewater treatment by ZR and VR after 24 h.

Sample ID	W2	W3	Z6	Z7	Z8	Mean	ASD	V6	V7	V8	Mean	ASD
Description	WII	WIII	C/WII	C/WIII	C/WII-Prewashed	-	-	C/WII	C/WIII	C/WII-Prewashed	-	-
Turbidity (NTU)	3.30	2.10	3.70	2.30	1.72	2.57	1.02	3.80	2.60	2.00	2.80	0.92
TDS	1146.00	925.00	1237.00	1255.00	1060.00	1070.63	128.58	989.00	1013.00	940.00	980.67	37.21
pH	8.44	8.44	8.67	8.69	8.22	8.45	0.16	8.39	8.43	8.34	8.39	0.05
Hardness	190.00	170.00	460.00	350.00	260.00	301.88	104.54	420.00	320.00	245.00	328.33	87.80
CFU/100 mL	<1	<1	<1	<1	<1	1.00	1.00	<1	<1	<1	1.00	1.00
BOD	47.00	28.00	2.00	1.00	1.00	11.00	17.17	5.00	2.00	2.00	3.00	1.73
COD	22.00	19.00	9.00	4.00	3.00	8.25	7.89	2.00	3.00	4.00	3.00	1.00
Na+	236.40	228.50	272.00	284.10	230.00	253.86	22.27	269.70	270.20	240.00	259.97	17.29
K+	15.70	15.46	21.33	23.80	16.05	18.41	3.30	21.01	18.73	15.20	18.31	2.93
Mg2+	19.05	18.51	37.17	38.85	16.42	24.33	8.90	25.16	22.14	17.30	21.53	3.96
Ca2+	52.82	51.07	121.70	114.00	45.00	73.20	30.19	80.20	72.39	48.40	67.00	16.57
Cu2+	LLOQ	LLOQ	0.00	0.01	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.01
Fe2+	0.08	0.01	1.06	0.56	0.14	0.79	0.80	1.22	2.42	0.80	1.48	0.84
Pb2+	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ
Cd2+	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ	LLOQ
NO2−	0.44	0.41	0.32	0.17	0.09	0.25	0.15	0.37	0.20	0.04	0.20	0.17
NO3−	26.40	27.00	25.00	24.00	21.60	24.05	2.46	26.00	22.00	20.40	22.80	2.88
Cl−	435.00	405.00	389.00	340.00	325.00	399.63	45.93	450.00	438.00	415.00	434.33	17.79
SO4−	422.50	413.00	443.00	421.00	355.00	397.38	34.71	395.00	387.00	342.50	374.83	28.29

V (volcanic sample); Z (zeolite sample); WII (secondary treated wastewater); WIII (tertiary treated wastewater) C (coarse materials). All elements are measured in (mg/L). LLOQ is The lower limit of quantification.

. For example, ASD for pH values of all samples of ZR and VR were 0.16 and 0.05, respectively, with percentages from the mean being 1.9 and 0.54%, respectively. Table 2 presents a full comparison for all properties.

3.2.1. Physicochemical Characteristics

Figure 4 presents the pH and TDS results for ZR and VR at 24 h after adding them to the secondary treated wastewater effluent (WII) and the tertiary treated wastewater effluents (WIII). Based on the leaching results, the coarse materials were adopted to reduce the extraction of ions from rocks to the treated wastewater. For pH values, adding coarse materials of ZR to WII and WII showed an increase of 3%, while VR showed a trivial decrease. In the case of prewashed coarse materials by soaking in distilled water for 24 h before testing them with WII, the results showed a notable reduction in pH values particularly for ZR. The exchange cations between the water and ZR generate H⁺ (Lewis acid), which leads to a decrease in the system’s pH [15]. From the results in Figure 4, it can be concluded that both ZR and VR can exchange the cations with water, which makes these rocks viable materials in treating wastewater. For TDS, a slight increase was observed in the TDS content of water samples due to the addition of both rocks. ZR addition introduced more ions into the water than VR. In general, the prewashing case showed a decline in TDS content in comparison with the original content of WII and with coarse materials without prewashing. This is due to the impact of the prewashing process, which increased the absorption capacity of natural rocks. The results also showed a slight increase in the turbidity of the treated samples, which indicates an increase in the suspend particles as well, as seen in Table 2.

For dissolved oxygen (DO), the results showed fluctuations in the DO content with time after adding both ZR and VR into the WII and WIII effluents, as shown in Figure 5; however, the general trend was a decline in DO content with time. This is perhaps because the solubility of oxygen and any gas decreases markedly with increasing amounts of dissolved solids into water [14]. As discussed above, adding these rocks introduced some ions released into the water and increased TDS values. Prewashed samples of ZR and VR, i.e., Z8 and V8, showed increases in DO content after 24 h, indicating the decline of the dissolved ions from rocks into the water, in this case, in comparison with the case of the direct use of coarse materials, as discussed earlier. The DO content growth shown herein perhaps indicates that the introduction of oxygen particles from some chemical reactions occurred due to the addition of these rocks into water.

3.2.2. Ion Release

The results of the ion release showed a slight increase in the water content of some cations such as K+, Mg²⁺, Na⁺, and Ca²⁺, with both rocks added into wastewater effluents, i.e., WII and WIII, as seen in Figure 6. This increase declined in the case of prewashed materials Z8 and V8. There is also a notable increase in Mg²⁺ and Ca²⁺ due to the addition of ZR compared with that of VR. The concentration of Cu²⁺ and Fe²⁺ in the treated wastewater increased by adding both rocks, particularly for Fe²⁺, when adding VR, as presented in Table 2. The concentration of Ca²⁺ increased intensely in all water samples compared with the other cations, resulting in a high concentration of Ca²⁺ in ZR and VR, and the location of Ca²⁺ on their surfaces influenced the rate of cation exchange. Cakicioglu-Ozkan and Ulku [15] stated that the location of earth alkaline metals on the surface of ZR and the shape of ZR play vital roles in stabilizing the framework of ZR and the rate of adsorption. Si and Al influence the negative charge of ZR, which is balanced by the earth’s alkaline metal, and the negative charge generated comes from tetrahedrally coordinated aluminum. Thus, the negative charge of ZR on the surface enhances the cation exchange between water and ZR [11]. Hardness values increased as well in the treated samples due to Ca²⁺, as shown in Table 2.

The results showed that ZR exchanged cations with water Na⁺, K⁺, Mg²⁺, and Ca²⁺ since the concentration of these cations increased in water. The results in Figure 6 agree with the results published elsewhere [16]. That study found that ZR can exchange cations with wastewater, which removes NH³⁺ from the wastewater. Furthermore, Aguiar et al. [17] informed that ZR removed NH³⁺ from wastewater via cation exchange or by adsorption into the pores of the aluminosilicate system. Moreover, Sprynskyy et al. [18] investigated the removal of heavy metals (Pb²⁺ and Cd²⁺) from wastewater by using ZR (clinoptilolites). They found that the concentrations of Pb²⁺ and Cd²⁺ decreased from 800 to 27.7 and 25.76 mg/L, respectively [18]. The decrease is due to the cation exchange between wastewater and ZR [18]. ZR donated Na⁺ and Mg²⁺ to water and adsorbed Pb²⁺ and Cd²⁺. The results of Figure 6 present the ability of both ZR and VR to donate Na⁺ and Mg²⁺ to water, which makes ZR an viable material in removing heavy metals. As the water molecules are attached to the pores of ZR or VR and bonded to framework ions and exchangeable ions [19], anions such as NO²⁻, NO³⁻, Cl⁻, and SO⁴⁻ showed a decline in treated samples in comparison with the original WII sample, as seen in Table 2, which shows the ability of these rocks in removing such these anions. This is perhaps due to the ion exchange capability of ZR and VR. This agrees with the work conducted elsewhere using Zeolite in a column experiment. That research concluded that natural Zeolite was more effective for nitrate removal from groundwater than activated carbon [20].

3.2.3. BOD and COD Results

BOD and COD have traditionally been used to measure the strengths of effluent released from wastewater treatment plants to natural streams. Figure 7 presents the BOD and COD measurements for wastewater effluents after treatment by ZR and VR. The results exhibited notable improvements for both wastewater effluents. WII and WIII was observed as values of BOD and COD declined significantly after adding both rocks to the water. This refers to the powerful capacity of these rocks in absorbing contaminants from the water. Based on criteria for surface water discharge, the secondary treatment standard for BOD has been set at 30 mg BOD/L, i.e., 30 mg of O₂ are consumed per liter of water over 5 days to break down the waste [21]. The results of the treated wastewater for BOD and COD, showed values of 1 and 3 mg/L, respectively, as seen in Figure 7. For microorganism monitoring, the results confirmed the absence of fecal streptococcus, as seen from the CFU observations in Table 2.

In summary, the results demonstrated herein showed the promising ability of ZR and VR in the purification of secondary treated wastewater. Both rocks showed approximately similar behavior; however, VR exhibited relatively less release into water. Both showed good ability in the removal of nitrates. The main limitation of using such rocks is the relatively high extraction of some ions from the solid into water, which increased the TDS content of water.

Further research needs to be carried out to overcome this problem and to increase the feasibility of using such natural rocks in treatment applications. A prewashing process for the rock samples before use improved their treatment performance. Therefore, different pretreatment process such as thermal treatment need to be investigated.

4. Conclusions

The leaching results showed that many ions are released from natural rocks, ZR, and VR into distilled water due to the ability of these rocks to exchange ions with water. The negative charge on the surface of ZR and VR enhances the cation exchange between water and these rocks.

Coarse materials showed fewer extractions of ions into the water than ground materials; thus, it is recommended for applications treating wastewater, particularly the coarse format, to be more practical and economical.

The prewashing process for coarse materials of ZR and VR before using them to treat wastewater increases the absorption capacity of natural rocks.

The results presented the ability of both ZR and VR to donate some cations such as Mg²⁺ to water, which makes these rocks viable materials used to remove heavy metals. As the water molecules are attached to the pores of ZR or VR and bonded to framework ions and exchangeable ions, they also showed a high capability in removing nitrate from wastewater.

Future work is needed to investigate the full capacity of these rocks in removing different types of pollutants dissolved inside wastewater by testing some triggering processes such as heat treatment of the rocks before use in the treatment applications.