Adsorption isotherm model of Hg2+ with biochar from young coconut waste

Biochar is a carbon-rich byproduct of biomass pyrolysis that may be used to restrict Hg mobility in soil by utilizing amelioration technology. This study examines the ability of biochar from young coconut waste to adsorb Hg in solution. Isothermal adsorption of Hg by batch equilibrium method. The basic principle of Hg adsorption behavior with biochar from young coconut waste (B-YCW) processed through the Kon-Tiki method at a temperature of 682 0C, moisture of 81.27%, and a yield ratio of 20.87% at a size of 0.5 mm. The adsorption of Hg2+ on B-YCW increased with increasing Hg concentration and decreasing pH. The capacity and adsorption coefficient of Hg2+ by biochar from young coconut waste was 312.88 mg g−1 and 69.64 L Kg−1 at a pH of 1.55 and a concentration of 100 mg L−1 Hg2+. The adsorption isotherm of Hg2+ occurs in the Freundlich and Langmuir models (Freundlich > Langmuir). The Freundlich model (y = 1.0375x - 1.2523; R² = 1) with a value of n of 0.96 and KF of 17.78 (L mg−1)1/n, and the Langmuir II model (y = 17.126x - 0.0244; R² = 1), with average Qm was 18.57 mg g−1; KL 68.198 L mg−1 and RL 0.0054 (favorable) from the Langmuir isotherm model.


Introduction
Mercury is an extremely poisonous heavy metal that is not required for life to exist.Even in low doses, mercury can harm human health [1] [2].The most common type of mercury found in the environment is inorganic mercury [3].This mercury gradually accumulates in water and soil solutions.Mercury quickly methylates with chemicals and bacteria in its normal state, generating more dangerous forms of mercury such as methylmercury or dimethylmercury [4].This is the basis for selecting potential materials that are stable.According to US EPA guidelines, Hg in treated wastewater (10 g l -1 ) and drink water (2 g l -1 ) [5].Meanwhile, the concentration in the soil should be less than 1 mg kg -1 [6].1297 (2024) 012093 IOP Publishing doi:10.1088/1755-1315/1297/1/012093 2 Thus, the a need for an appropriate technology that can be done in reducing the impact of Hg pollution in the environment through the adsorption process with the application of biochar-based amelioration technology.
Adsorption technology is a very effective method of reducing water and soil pollution.Hg as a heavy metal is polluting water and soil systems.Hence, the a need to identify the appropriate adsorbent material to use.Biochar has been recognized with diverse functions with carbon-rich materials, as a low-cost alternative in controlling inorganic pollution [7] [8] [9] [10] [11].The large surface area [12] [11], micro, meso, and macro pores [13] [14], and high pH [15] all contribute to the significant adsorption of pollutants into biochar.Based on the features of biochar, many studies have been conducted on biochar as an ameliorant material to remove contaminants in water and soil [16] [17] [18].However, the kind of feedstock will have variable effects on adsorption.This research focuses on the use of biochar from young coconut waste (B-YCW) in adsorbing heavy metals such as Hg.

Methods
This research was conducted from March to August 2023 at the Soil Laboratory of the Faculty of Agriculture and the Environmental Chemistry Laboratory of the Faculty of Mathematics and Natural Sciences, Andalas University, West Sumatra.

Biochar production
Adsorbent raw materials are sourced from Cocos nucifera L. The waste was then cut into pieces with a size of about 10*5 and dried for seven days at the Faculty of Agriculture Greenhouse, UNAND until the moisture was 18.20%.Next, biochar was produced using the Kon-Tiki method with the principle of pyrolysis, which is made of conical steel with a capacity of 827 liters, a top diameter of 100 cm, a height of 90 cm, and a wall slope of 63.5 degrees.The biochar product was sprayed with sufficient water to stop the carbonization process and then oven-dried at 40-70 0 C for 2*24 hours to achieve homogeneous biochar moisture content.Then, it was sieved using an EMS-8 electromagnetic sieve shaker on a 500 µm sieve.Basis for biochar particle size determination and biochar characteristics [19] [20] [21].

Determination of HgCl 2 Concentration
The Hg 2+ concentration solutions used in the adsorption determination were 1, 10, and 100 mg L -1 , which used the reagent material of HgCl 2 in 0.1 M HCl.Determination of concentration by dissolving 0.13 g of HgCl 2 in 1000 ml of 0.1 M HCl, while the concentrations of 1 and 10 mg l -1 Hg 2+ were made by dissolving the parent solution of 100 mg l -1 Hg 2+ concentration [22].

Isothermal adsorption by batch equilibrium method
Adsorption of Hg focused on isothermal adsorption with the Batch equilibrium method.Adsorbent was weighed 0.5 g of B-YCW and mixed with 20 mL of 1, 10, 100 mg l -1 concentration solution Hg in a 25 mL glass cylinder tube.Isothermal adsorption was carried out at a contact time of 1*24 hours using a rotary shaker at a speed of 300 rpm and a temperature of 25 C in a Laminar flow chamber.Determination of adsorption pH was carried out after the contact time carried out at each concentration.Equilibrium filtrate concentrations of Hg were measured using CV-AAS [23].The adsorbed Hg was calculated by formulas 1, 2, and 3.

Adsorption isotherm model
The adsorption isotherm models used consisted of one-parameter (Henry) and two-parameter (Freundlich and Langmuir) isotherm models.The adsorption isotherm models used linear and nonlinear forms of each model (Table 1).

Experimental data modeling
The Batch Equilibrium Method was used to explain the mechanism in determining the applicability and stability of the adsorption process.Microsoft Excel 2016, SPSS 16, and OriginLab were used to process the data.

Hg 2+ adsorption behaviour
The equilibrium concentration of Hg adsorption increased with increasing Hg adsorption capacity.However, the adsorption pH decreased with increasing Hg concentration (Table 2).The highest adsorption capacity occurred at a concentration of 100 mg L -1 Hg 2+ of 312.88 mg g -1 at pH 1.55.This is influenced during the adsorption process that occurs in an acidic solution using 0.1 M HCl at different Hg concentration levels of 1, 10, and 100 mg l -1 , where the pH of the pure solution is respectively.The removal efficiency of Hg at different concentration levels decreased, where the highest adsorption capacity had a removal efficiency of 78.22% at a concentration of 100 mg l -1 Hg.The higher concentration of Hg added will affect the adsorbent efficiency in adsorbing Hg.The mass of adsorbent used remains at dynamic equilibrium with increasing levels of Hg concentration.The maximum capacity of the adsorbent in adsorbing Hg ions occurs at the equilibrium time which affects the capacity and sorption coefficient of Hg.Increasing Hg concentration in the adsorption process increased the Hg sorption coefficient by B-YCW by 69.64 and 63.32 at 100 and 10 mg l -1 concentration, respectively (Table 2).The adsorption coefficient can explain the increase in the ability of a substance to attract or absorb ions from a solution.
The pH value plays a very important role in the adsorption process.It affects the surface value and charge of the adsorbent as well as the speciation form of Hg in solution.The speciation diagram of 1*10 -4 M HgCl 2 is presented in Figure 1., where the speciation of 1*10 -4 M HgCl 2 solution, Hg is present nearly 100% in the form of HgCl 2 (aq) in the pH range between 1 and 5. Adsorption of Hg(II) from 0.1 M HCl solution using B-YCW is at acidic pH (1.55; 1.60 and 1.62).The adsorption process can be ineffective at very acidic pH, due to the competition between H 3 O + ions and Hg 2+ cations in solution for sorption sites and a decrease in the available surface [25].However, in the adsorption of B-YCW carried out in 0.1 M HCl solution, the adsorption capacity was in the range of 3.24; 31.92, and 312.88 mg g -1 .This shows that the solution used in adsorption also affects the adsorption capacity of Hg.The adsorption mechanism of Hg by biochar can be physical adsorption, chemisorption, electrostatic interaction, simple diffusion, intra-particle diffusion, hydrogen bonding, redox interaction, complexation, ion exchange, precipitation, and pore adsorption are all possible processes.The adsorption mechanism forms a layer of adsorbate (Hg ions) on the B-YCW surface.Adsorption on solid B-YCW consists of three main steps: (1) transport of Hg from the aqueous solution to the B-YCW surface; (2) adsorption onto the solid surface; and (3) transit within the B-YCW particles.Physical adsorption occurs when the adhesion of the Hg to the B-YCW surface is non-specific.Chemisorption occurs when chemical bonds create a strong force of attraction.The B-YCW has a wide variety of functional groups, most of which are negatively charged, including hydroxyl and carboxyl groups.B-YCW is also porous, having multiple spaces and surface locations where Hg ions can bind.The presence of these holes and gaps increases the surface area accessible for adsorption.Hg ions have additional opportunities to interact and be retained by B-YCW.This porous nature has a high ion adsorption capacity and is efficient in soil and water system management applications.
Adsorption of Hg by B-YCW consists of (a) dispersion of Hg molecules; (b) electrostatic attraction to the surface; (c) electrostatic attraction between positively charged Hg ions; and (d) negatively charged functional groups of B-YCW.This successfully increases the adsorption capacity.Attractive forces such as van der Waal aya and hydrogen bonding may be involved in the adsorption of Hg on the B-YCW surface.In the complexation process, Hg, surrounded by ligands, occupies a central position and forms a mononuclear complex.When two or more Hg atoms are linked together at the central location by the ligand, a polynuclear complex is formed.Polydentate ligands can also be used in chelation to aid in the development of stable structures through various bonds.Based on all adsorption mechanisms by biochar, it is predicted that the complexation reaction of the OH group with Hg 2+ is the main mechanism that occurs between Hg and B-YCW.The adsorption contribution of the -OH functional group rate is more dominating than the -COOH functional group, reacting with Hg 2+ to form the (-O)2Hg complex (Reaction 1 and 2).This is based on the high pH value of B-YCW of 10.09 [21].

Adsorption isotherm model
Hg adsorption was investigated using isotherm models to estimate the optimal adsorption process based on high R 2 values [29].The adsorption of Hg with B-YCW, the R 2 of the linear Henry, Freundlich, and Langmuir model isotherms can be used.The R 2 value in each model is Henry < Freundlich = Langmuir II (0.99 < 1 = 1).

One-parameter isotherm model (Henry's model)
The Henry adsorption isotherm model describes the accuracy in the adsorption process of Hg at different concentration levels.It isolates all Hg molecules from nearby surface sites at a biochar surface partial pressure proportional to the Hg concentration (Ce) [30].The equilibrium concentration of Hg in the adsorbed phase can be calculated through Henry's linear equation (Table 1 and Fig. 2).The linear plot of the Henry isotherm model has the equation y = 0.07x -0.1208; R² = 0.9999 (Fig. 2).The shape of this type of type explains that the adsorbate-adsorbate has an interaction is greater than the adsorbate-adsorbent interaction.

Two-parameter isotherm model (Freundlich and Langmuir model)
The Freundlich adsorption isotherm model can explain reversible adsorption processes that are not ideal.The Freundlich model is also not limited to single-layer formation, which can be used for multilayer adsorption.The expression of the Freundlich isotherm model is surface heterogeneity and exponential distribution of active sites.This model shows the ratio of Hg to solute at different solution concentrations is not constant, where the adsorbed amount is the total adsorption at each site.This illustrates that when the adsorption process is complete, Hg is on stronger binding sites and the adsorption power decreases exponentially.Freundlich isotherm model in Hg adsorption with B-YCW in heterogeneous systems.The linear equation of Hg adsorption with B-YCW in Freundlich models: y = 1.0375x -1.2523; R² = 1 (Fig. 3).

Figure 3. Linear plot of Freundlich isotherm model
The value of n indicates the type of isotherm, with the parameters K F and n dependent on temperature (25 0 C), with n = 0.96 and K F = 17.78 (mg g -1 ) (L mg -1 ) n obtained in the adsorption of Hg with B-YCW.The value of 1/n indicates the adsorption intensity or surface heterogeneity.It indicates the relative distribution of energy and the heterogeneity of adsorbate sites.Adsorption is favorable when 1/n is greater than zero (0 < 1/n < 1); unfavorable when 1/n is more than one; and irreversible when 1/n = 1 [29].The value of 1/n in Hg adsorption was determined to be 1.04.The Freundlich isotherm model equation is empirical with surface heterogeneity where the adsorption energy is spread over the surface topography.This indicates all surface sites and the same adsorption energy are in the   The Langmuir adsorption isotherm model has been developed in various studies.However, it is also necessary to explain the adsorption of Hg with B-YCW.The adsorption process on B-YCW is based on the kinetic principle with a contact time of 24 hours that intermolecular interactions occur continuously on the surface with a zero accumulation rate.The Langmuir isotherm model aims to quantify and compare the adsorption capacity of Hg with B-YCW as a result of changes in surface chemistry and structural geometry of B-YCW.Langmuir discovered and classified six unique and simple adsorption processes: (a) single-site Langmuir adsorption, this type of adsorption occurs between Hg and the most basic B-YCW, in which the B-YCW surface has identical elementary adsorption sites capable of holding one adsorbed Hg molecule; (b) Multi-site, in which the B-YCW surface has several types of elementary adsorption sites, each of which can hold one adsorbed Hg molecule.The binding sites are independent, and the interaction between Hg and B-YCW is negligible.(c) Langmuir adsorption in general, can describe B-YCW as having many diverse adsorption sites with different Hg affinities when viewed as a continuum.This explains the limited interaction of Hg with B-YCW, the adsorption temperature follows the binding energy distribution of the adsorption sites.(d) Adsorption is cooperative, involving identical surface binding sites that can accept multiple molecules.(e) Adsorption is dissociative, which is a two-step adsorption process, where chemical bonding causes residence on the surface adsorption site and dissociation of Hg molecules (f) Adsorption is multilayer, where each adsorption site is believed to be intermolecularly dependent and identical [31].The Langmuir isotherm assumes the thickness of the adsorbed layer in one molecule (monolayer adsorption) and adsorption processes on the same and equivalent sites.The Langmuir isotherm model also describes homogeneous adsorption, with each Hg molecule sharing a constant activation energy and enthalpy of sorption.All sites must have equal affinity for Hg, and there is no movement of Hg on the surface plane.An increase in distance leads to a rapid decrease in the force of attraction between molecules.Tables 1 and 3 and Fig. 4 show the linear equation of the Langmuir isotherm model.The linear equation of Hg adsorption with B-YCW in Langmuir models I (y = 2035.9x-121.99);II (y = 17.126x -0.0244); III (y = -0.0072x+ 16.58) and IV (-117.21x+ 1960.9) (Fig. 4).This confirms that the Langmuir type II model is suitable in the process of Hg adsorption with young coconut waste biochar, as seen from the R 2 value, where the Langmuir II model has a value of R 2 = 1, compared to the others (R 2 = 0.8769 and 0.8485).The Langmuir model also explains the separation factor (R L ) which can be related to the K L constant as a dimensionless constant expressed as a variation of the area and porosity of the relevant adsorbent, indicating that higher surface area and pore volume can increase the adsorption capacity.The splitting factor determines whether adsorption is linear (R L = 1), irreversible (R L = 0), unfavorable (R L > 1), or favorable (0 < R L < 1) [29].The RL value for Hg adsorption with B-YCW is 0.0054.The adsorption of Hg with B-YCW is favorable (Table 3).The Langmuir model assumes a homogeneous adsorbent surface with the same adsorption energy at each location.However, the model and its coefficients are used to assess the distribution of contaminants in water and soil.The Langmuir equation can only compare different adsorbents without explaining the chemical mechanism.The site energy distribution (SED) was developed to explain the heterogeneity

Conclusion
The adsorption of Hg 2+ on young coconut waste biochar increased with increasing Hg concentration and decreasing pH.The capacity and adsorption coefficient of Hg 2+ by biochar from young coconut waste was 312.88 mg g -1 and 69.64 L Kg -1 at a pH of 1.55 and a concentration of 100 mg L -1 Hg 2+ .The adsorption isotherm of Hg 2+ occurs in the Freundlich and Langmuir models (Freundlich > Langmuir).The Freundlich model (y = 1.0375x -1.2523; R² = 1) with a value of n of 0.96 and K F of 17.78 (mg g - 20 0.02 Remarks: Co = the initial concentration of the adsorbate; Ce = Equilibrium concentration; R = removal efficiency; Qe = adsorption capacity; K d = Coefficient adsorption; CV = Coefficient of variation; SE = Standard error; ** = significant at the 0.01 level; * = significant at the 0.05 level and n = 9.

6 Figure 2 .
Figure 2. Linear plot of Henry's isotherm model patch.The adsorption energy is generated by the interaction between B-YCW and Hg.

9 of
adsorption surfaces.Adsorbents explain the diverse distribution of adsorption sites as evidence of a shift in isotherm characteristics.

Table 1 .
Linear and non-linear formulas of isotherms models F Ce 1/n Log (Qe) = LogK F + 1/n*LogCe Log Qe vs Log Ce

Table 2 .
Adsorption of Hg 2+ at equilibrium by B-YCW

Table 3 .
Isotherms adsorption of Hg with B-YCW