Sorption Kinects and Equilibrium for The Removal of Cadmium and Lead from Aqueous Phase on Rice Husk in Inverse Fluidization Technique

The objectives of this study are to use the inverse fluidization technique to remove heavy metals from wastewater using inexpensive agricultural waste (Rice Husks) and to investigate the effects of operating factors on the dynamic behavior of the adsorption procedure in the inverse fluidized bed, such as the mass of modified rice husks, flow rate (Q), and particle size (dp ). During batch experiments, the best metal ion removal effectiveness was found to be at pH 5, which was discovered after investigating several pH values to achieve this goal. The ideal shaking speed for batch adsorption was 180 rpm. Adsorption efficiency was seen to rise as contact time in the process increased, and the ideal contact time was 3 hours. According to the findings, Cd and Pb had high removal efficiencies from aqueous solutions, 96.83 percent and 91.90%, respectively. Continuous column experiments (inverse fluidized bed) were used to confirm the adsorbent loading capacities for cadmium and lead, which were evaluated by batch research. The proposed adsorbent’s highest adsorption capacity in a batch system was determined to be 7.38 mg/g for Cd and 6.93 mg/g for Pb. Three models-Temkin, Freundlich, and Langmuir-were fitted to a series of equilibrium isothermal tests. The Freundlich isotherm model, with correlation coefficients R2 of 0.98 for Pb and 0.97 for Cd, offered the best fit to the experimental data for this system. The rice husk equilibrium isotherms were determined to be of a favorite kind. To investigate the impact of initial concentrations, bed depth, flow rate, and particle size at a temperature of 30 on the effectiveness of the adsorption process, numerous experiments were conducted in an inverse fluidized bed column. According to the results, rice husk appears to be a material that shows promise for cleaning wastewater of contaminants and toxins.


Introduction
Many regions of the world already have limited supplies of fresh water.Due to the rising population, increasing sources of water pollution, urbanization, and climate change, it will become increasingly constrained in the coming century, resulting in a water crisis and negative environmental effects.A serious environmental problem is the heavy metal poisoning of water from industrial effluents; the evolution of this research aims to reduce or eliminate the problem.One of the most widely produced and eaten grains worldwide is rice.In many developing nations, it is regarded as the most commercially significant product and has become a mainstay for billions of people.Husks, which make up roughly 20% of the weight of the rice, are the byproduct that produces the most waste during the processing of rice.Rice husk (RH), a fibrous fiber, mostly consists of cellulose, lignin, and organic and inorganic waste.About 20% of the burnt husk is made up of light, bulky, porous rice husk ash (RHA).The usage of rice husks and ashes have been linked in literature with a number of alternative technologies employing the ash as a filler in polymers, the creation of concrete, the synthesis of silicates and zeolites through the manufacturing of silica, and the fabrication of silicon carbide.Since it's affordable, abundant, simple to acquire, and able to be reused, it has additionally been employed in synthetic wastewater as a heavy metal adsorbent [1][2][3][4][5][6][7].
Optimization of process conditions.Investigation of heavy metal removal in both static and moving systems is required, as evidenced by studies on the removal of metal from different adsorbents, like clays [8,9].Therefore, the objective of this study is to contrast the various approaches.(In batch and inverted fluidization) for removing lead and cadmium using modified rice husk.By combining utilizing the inverse fluidized-bed adsorption column and the modified rice husk (MRH) by phosphoric acid rice husk, the heat treatment process carried out at temperatures of 300 °C, this study diverges from prior studies in this work.The effectiveness of MRH for cadmium (II) and lead (II) removal from aqueous solution at a temperature of 30 °C can be evaluated using the results of this research.

Adsorbate material
Because of the poor biodegradability, high toxicity, and accumulation of heavy metals in the food chain brought on by industrial and urban activities in the river, this problem is a major one on a global scale.Among the main sources of environmental pollution from cadmium are batteries, phosphate fertilizers, metal paints, pigments, and pigment stabilizers.And sources of lead pollution are liquid waste from metal plating and smelters and industries (mining, textile, preservatives, paint) [10].And for this reason, the heavy metals examined in this study were cadmium (Cd) and lead (Pb).

Adsorbent Material
Around 740 million tons of rice are produced annually globally.China, India, Vietnam, Thailand, the United States, and Pakistan are the main suppliers.According to estimates, rice husk makes up to 23% of all mass output, or roughly 150 million tons annually, which is produced globally through rice processing.Due to its poor nutritional value, this material is difficult to use as animal fodder, and due to its silicon concentration and abrasive surface, it is difficult to naturally degrade.Due to its advantageous qualities, particularly its minimal carbon emission and environmental harm, this situation allows for the consideration of biomass as a novel component in the creation and use of renewable energy sources [11].The rice husk utilized in this investigation was procured from a nearby mill in the Iraqi city of Ghammas, Diwaniyah Governorate.To eliminate dirt and contaminants, the collected biomaterial was properly washed with tap water multiple times.It was then thoroughly cleaned once more with distilled water, and it was then dried in an oven (Binder drying oven ED series, Germany) at 105 °C for two hours.To expand the surface area, which boosts the capacity for adsorption, dry biomass was crushed by a high-speed multi-functional crusher (Model 100, China) and sieved to different particle sizes.The biomaterials of uniform size (1.18-2 and 2.35 mm) were then retained for additional uses.

Modification of Rice Husk
Acidic solution (phosphoric acid), H3, PO4, (1 percent v/v) is used to treat preserved rice husks that have been broken to pass through various sieve volumes.After that, it is rinsed multiple times with non-ionized water until the pH reaches 6 and dried to eliminate moisture at 80 °C for three hours.
When the biomass material has dried, it is activated and given a boost in adsorption efficiency by spending two hours in an electric muffle furnace (Shin Saeng Korea, model SEF-303) heated to 300 °C, after which it is kept for further usage Figure 1.

Preparation of the Aqueous Solution
In this experiment, the synthetic wastewater includes heavy metals.Lead and cadmium are the two heavy metals that are used.Metal salts Cd (NO3)2.4H20,and Pb (NO3) 2.2 H2O were dissolved in distilled water to achieve the target concentration of heavy metals in water.The following equation was used to get distilled water with a heavy metal content of 100 mg/L [12].
where W is the salt's heavy metal weight (mg).V stands for volume of solution (L) and Ci for metal ion initial concentration in solution (mg/L).At.wt is the metal ion's atomic weight (g/mole).M.wt.stands for the molecular weight of a metal salt (g/mole).Synthetic solutions with a pH of 6 were used for the experiment, and at a temperature of 25 ± 5 o C. adsorption tests were performed.

Batch experimentation procedure
The solutions of stock were diluted to the required concentration with distilled water before being used to create the working solutions.Synthetic solutions with a pH of 6 were used in the experiment.The batch experiments were conducted by varying the mass of the adsorbent.A total of 12 beakers containing a 500 mL volume of synthetic solutions 6 of them are for lead, with an initial concentration of 79.28 mg/l, and the other 6 are for cadmium, with an initial concentration of 87.4 mg/l, were utilized for all of the tests.The amounts of adsorbent ranged from (0.5, 1, 1.5, 2, 2.5, and 3 g).At first, 0.5 g of modified rice husk was added to every 100 ml of solution, and the pH of the mixture was assessed using a benchtop digital pH meter (model PHS-3C, China).By adding NaOH or HCL, the solution pH was kept at the desired level.The beakers were put on Hot Plate Magnetic Stirrers (Labinco L-82, Netherlands) that were constantly stirred by magnetic stirring for 3 hours at 25 °C.The concentration has reached equilibrium at this point.The two phases were separated by filtration equipment employing filtering papers of the Whatman No. 42 type after a predetermined amount of time, and the filtered solution was then collected to determine the concentration of metal ions.Atomic absorption spectrometry (model ASC-7000) was used to measure the initial and final metal concentrations.It operates at wavelengths of 228.3 nm for Cd and 283.8 nm for lead.The results were calculated using the following equation: proportion of heavy metals removed by the adsorbents during adsorption The adsorbed amount   was calculated using the flowing equation: Where:   is the equilibrium uptake (mg/g).V is the volume of the solution (l).  is the equilibrium concentration mg/l).C0 is the initial concentration(mg/l).W0 is the mass of adsorbent (g).The results are given in Tables 1 and 2.

Equilibrium Isotherm Rice Husk
In Table 2, it is evident: The outcomes demonstrate that the inexpensive adsorbent rice husk is remarkably effective at removing pb (II) from its solution.The highest amount of pb ions that rice husk can adsorb is 6.93 mg/g.The maximum absorption of Pb (II) was attained at a low mass of adsorbent, and the maximum elimination efficiency of Pb (II) was attained at a high mass of adsorbent.Table 1, demonstrates that as the dosage of the adsorbent was increased, the adsorption of cadmium on rice husk decreased.At adsorbent dosages of 0.5 g, where the Cd adsorption capacity was 7.38 mg/g, and 3 g, where it was 2.821 mg/g, the Cd adsorption capacity rapidly dropped.The overcrowding of adsorbent particles may have caused the overlapping of adsorption sites, which would explain these results.The adsorption increases with pH value until it reaches 5, at which point it falls because the amount of hydrogen ions in the adsorbate affects the adsorbate's level of ionization during the reaction, the degree to which metal ions are soluble in the solution, and the substitution of some of the positive ions at the active sites.The findings are similar to the outcomes of [14,15,16 as well as the Freundlich, Langmuir, Timken, and Langmuir adsorption isotherms.Each isotherm's fit to the experimental data was evaluated using the coefficient of determination ( 2 ) in order to assess their reliability and capacity to connect with experimental outcomes.According to Table 3, Freundlich has a greater coefficient of determination values than Langmuir and Temkin.With a strong correlation coefficient of  2 =0.971, it is obvious that the Freundlich isotherm fits the experimental data the best.An advantageous type 1/ < 1 The Cd (II) elimination equilibrium isotherm was present.An RL value of 0.028 (0<RL<1) was generated by the fitness of the Langmuir model, indicating favorable Cd (II) adsorption onto the rice husk surface.Figure 2 shows the relationship between     ⁄ (g/l) and,   (mg/l) for the Langmuir adsorption isotherm with a correlation coefficient of,  2 =0.92, Figure 3 shows the relationship between, log   .and log   .. for the Freundlich adsorption isotherm with,  2 = 0.97, and Figure 4 shows the relationship between,   (mg/g) versus, ln   for the Temkin adsorption isotherm with,  2 =0.95.

Adsorption Isotherm Models
Langmuir Isotherm Model: It presumes that sorption takes place in a structurally homogenous adsorbent and that the energy of each sorption location is the same [14].Surface coverage is a monolayer.This isotherm was expressed by the following equation [17]: The adsorption capacity of an adsorbent per mass of adsorbed adsorbate at equilibrium (mg/g) is known as   .X: The adsorbate's mass (mg).The adsorbent's mass is M (mg),   . is the solution's equilibrium concentration (in mg/l).K is the equilibrium adsorption constant, and   is the maximum adsorption capacity (rate of adsorption).The slope and intercept of the Langmuir plot's Figure 2 were used to get the values of, kl and   .According to the Freundlich isotherm equation, the absorption of metal ions happens on multilayer sorption surfaces that are heterogeneous.As the concentration of the adsorbate rises, more metal is adsorbed.[18] The following equation served as the expression for this isotherm [17,19]: : The mass of adsorbent per unit of adsorbate that has been absorbed (mg/g).x: represents the adsorbate's concentration.(mg/L).m: stands for the adsorbent's mass.(g/L).The equilibrium adsorbate concentration (mg/l) is given by   .Adsorption coefficient (mg/g.(L/mg)1/n),often known as   , is a rough measure of the adsorption capability of the adsorbent.Adsorption intensity or surface heterogeneity is represented by 1/n.The slope and intercept of the Freundlich plot's Figure 3 were used to get the values of   and 1/n.Temkin explored how indirect interactions between adsorbate and adsorbent can alter adsorption isotherms and postulated that, as a result of these interactions, all of the molecules in the layer's heat of adsorption would linearly decrease with coverage.The Temkin isotherm's linear form can be expressed as follows [20,21]: is the Temkin isotherm parameter (mg/g).T stands for the absolute temperature (Ko).
R: is the universal gas constant equal to 8.3145 (J.mol-1.K-1).  : is related to the heat of adsorption (J/mol).kt is the equilibrium binding constant (L/mg).The isotherm constants   and bt are calculated from the slope and intercept of the plot shown in Figure 4.The obtained experimental data of Cd (II) was fitted to the Langmuir, Freundlich, and Temkin models.The calculated parameters for each model are shown in Table 3.

Experiments with Inverse Fluidization and Breakthrough Curves
Studies on continuous flow adsorption carried out in the Inverse Fluidized System's apparatuses and equipment are as follows: 50-liter circular steel tank used to feed wastewater into the column.To collect the discharge solution, a polyethylene container with a 30-liter capacity will be used.50 mm external diameter, 43 mm internal diameter, and 1000 mm height of Perspex column.Diverse fittings were used to connect pipes, PVC flanges, and hoses of various sizes.At the top and bottom of the inverse column, is a plastic distributor.Rota meters were used to measure the flow rate of the influent (ranging from 0-2000 ml/min of water).A centrifugal pump has a 0.5 hp capacity.The feed tank was used to feed the columns.To ensure consistent intake, a portion of the solution was cycled to the feed container.As illustrated in Figure 6: In a 50 L feed basin with distilled water that has a known concentration, the Cd ion solution was created.0.1N NaOH or HCl was used to modify the solution.
Through the flow meters, an adsorption column was pumped with the solution at the desired rate.
Samples will be taken from the bottom of the inverted fluidized bed column at this defined location.Periodically, samples were taken, and using a spectrophotometer, the concentration was calculated.pH equals 5 at 30 °C.Previous batch system experiments were done to study the impact of the solution's pH value.[22].The discharge concentration (    0 ⁄ ) vs time the drawing was used to create the breakthrough curves.

Effect of initial concentration
Figure 7 shows the experimental breakthrough curves for Cd (II) adsorption onto modified rice husk particles (1.18-2 mm particle size) at different initial concentrations, (Flow Rate) Q =16 l/hr, (Adsorbent Bed Height) L= 0.2 m, pH=5, T=30 °C.It is evident that as the inflow solute concentration increases, the time needed to reach saturation decreases.It will take more time for the diffusion rate to reach saturation when the initial solute concentration is low.It is also obvious that the adsorption capacity rises as the influent concentration does.This is brought on by a significant disparity in concentration between the concentration of the solute in the bulk solution and that in the solid phase.As a result, the solute will be able to mass transfer to the free site(s) on the solid phase of the rice husk more quickly.The concentration differential between the solute in the solution and the adsorbent is what drives adsorption.A high starting concentration will result in faster bed saturation and a steeper breakthrough curve.These outcomes support [23].

Effect of the Solution Flow Rate
Figure 8 shows the experimental breakthrough curves for Cd (II) adsorption onto Modified Rice Husk particles (1.18-2 mm particle size) at different flow rates Q = (10 l/hr.and 16 l/hr.)with a constant bed height of 0.2 m, an initial concentration of 30 mg/l of Cd (II), T= 30 °C, and solution pH 5. It shows that the breakthrough curves steepen as the flow rate rises.Due to the shortening of the contact period at high flow rates, the adsorbate solution exits the column before full equilibrium is reached.The surface film's thickness, which was regarded as the mass transfer resistance, will decrease as the flow rate increases.Because of this, the mass transfer rate will rise with an increase in the flow rate.By the increased disturbances (mixing) caused by the higher flow rate, it is simpler for the adsorbate molecules to penetrate and move through the particles and occupy one (or more) sites on the adsorbent.This outcome is consistent with that attained by [24,25].

Adsorbent Bed Height Effect
Effect of different bed heights (10cm, 20cm, and 30cm) on Cd (II) ions adsorption at constant flow rate Q= 16 l/hr.and starting concentration,  0 = 30 mg/l.The breakthrough curves for experiments are shown in Figure 9.As the height of the bed increases, it takes longer to reach the breakpoint.The process of adsorption will have access to a larger surface area as a result of the higher bed height.This shows that the ratio of effluent to adsorbate increases.quickly at a low bed height than at a higher bed height.Bed saturation is accelerated by a low bed height.A lower bed height is associated with less adsorbent and a reduced capacity of the bed to absorb adsorbate from the solution.The efficacy of solute removal will be improved when the bed height is increased while the flow rate remains constant because the solute will spend more time in contact with the bed.These findings align with those of [26,27].

Effect of particle size
The impact of particle size was investigated; figure 10 displays the experimental breakthrough curves.The breakthrough curves were obtained at a constant bed height of 0.2 meters of rice husk and a constant flow rate of 16 liters per hour with 50 mg/l of Pb (II) ion's initial concentration for two distinct particle sizes.The experimental findings demonstrated that, as depicted in the figure, the removal of lead ions by fine particle sizes was greater than that of coarse particle sizes.The more surface area that was available made it possible for smaller metal particles to be bonded to.It is possible to see the penetration curve's slope for large particles that are smaller than the steepness of the penetration curve for small particles.Their smaller adsorbent particles diffuse more readily from solutions due to their smaller size and higher surface area-to-volume ratio.This was attributed to the fact that for tiny rice husk particles, the metal ion molecules are expected to be more easily able to enter the micropores, and the transport is predominantly because of film diffusion, which has a greater effect than intraparticle diffusion.Intraparticle diffusion will become more prevalent as particle size increases since it is a slow and ineffectual process.As a result, as particle size grows, the adsorption capacity decreases, and as particle size decreases, the mass transfer rate rises.Additionally, these outcomes support [28,29].

Conclusions
This study was conducted to examine the adsorption behavior of lead and cadmium from an aqueous solution onto rice husks.Aqueous solutions can be removed from cadmium by using MRH as an inexpensive adsorbent in this study.According to the results, contact time and Cd (II) concentration affect cadmium elimination.There is a difference in the concentration of the solute in the adsorbent that fuels adsorption and the solution.The adsorption process accelerates with an increasing adsorbent dose.Rice husk has the maximum capacities of adsorption of 6.93 mg/g for Pb (II) and 7.38 mg/g for Cd (II) at an adsorbent dose of 0.5 g/100mL in batch systems, and rice husk displayed favorable equilibrium isotherms.The experimental data for this system can be best represented by the Freundlich isotherm model, with a coefficient of determination  2 equal to 0.98 for Pb and 0.97 for Cd.The removal efficiency for heavy metals by rice husk is as follows: 91.90% for Pb and 96.83% for Cd.
A Freundlich isotherm was discovered to match equilibrium adsorption data better than a Temkin model.Continuous flow (Inverse fluidization) tests' findings indicate that as the flow rate rises, less time is needed to obtain adsorbent saturation.(C/Co) the ratio rises more quickly for lower bed heights than for higher bed heights.Higher adsorbate starting concentrations result in steeper breakthrough curves and a faster time to break point.The findings of the experiment demonstrated that, in addition to increasing the adsorption rate while reducing the flow rate, fine particles provided greater removal than coarse particles.Because rice husk is readily available, low-cost, and possesses a high sorption capacity, it can be used as a substitute adsorbent to treat wastewater that contains cadmium (II) and lead (II) ions.

Recommendations for Future Studies
• Researching rice husk's capacity to absorb various pollutants, including pesticides, hydrocarbon solvents, and organic pollutants, and examine the ability of rice husk for competitive adsorption in the event that more than one pollutant is used.• Since physicochemical parameters like specific surface area, pore volume, and active sites have a significant impact on the performance of the adsorbent, further processing of rice husk particles can be carried out to increase these features.• A comparison of the effectiveness of removing water contaminants using fluidized versus fixed beds (using heavy metals as pollutants).• It is advised to remediate the contaminated water using rice husk in a three-phase inverse fluidization bed.

Figure 1 .
Figure 1.A Photograph of (A) Modified Rice Husks and (B) Unmodified Rice Husks.

Figure 7 .
Figure 7. Adsorption of Cadmium (II) onto modified rice husk particles with varying initial concentrations: Experimental Breakthrough Data.

Figure 8 .
Figure 8.The Experimental Breakthrough Data for Cd (II) Adsorption onto Modified Rice Husk Particles at Various flow rates Q.

Figure 9 .
Figure 9.The Experimental Breakthrough Data for Cd(II) Adsorption onto Modified Rice Husk Particles at Various bed heights.

Figure 10 .
Figure 10.The Experimental Breakthrough Data for Adsorption of Pb (II) onto Modified Rice Husk particles at Different particle sizes (1.18-2mm and 2.3-3.5 mm)

Table 3 .
Modeling parameters for the adsorption of Cd (II) on modified rice husk using the Langmuir, Freundlich, and Temkin models.