Kinetics and Isotherm Modeling for the Treatment of Rubber Processing E ﬄ uent Using Iron (II) Sulphate Waste as a Coagulant

: There is increasing concern to determine an alternative coagulant for treating industrial e ﬄ uent with minimal environmental impact and operational cost. In this study, iron (II) sulphate heptahydrate (FeSO 4 · 7H 2 O) waste, an industrial byproduct from a titanium oxide processing industry, was used as a coagulant for the removal of ammonia (NH 3 ), chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solid (SS) from secondary rubber processing e ﬄ uent (SRPE). The highest percentage removal of BOD, COD, SS, and NH 3 achieved was approximately 97%, 99%, 98%, and 95%, respectively, at pH 5.0, coagulant dose of 1 g / L, coagulation time of 60 min, sedimentation time of 60 min, and at an elevated temperature of 70 ◦ C. The best described adsorption isotherm model was found to be the Brunauer–Emmett–Teller (BET) model, indicated that the FeSO 4 · 7H 2 O adsorption took placed on the surface of iron hydroxide precipitates with multilayer formation and random distribution. The kinetics analysis showed that the adsorption mechanism was well ﬁtted with the pseudo-second-order kinetic model. The ﬁndings of the present study show that the FeSO 4 · 7H 2 O waste has the potential to be used as a coagulant for the treatment of industrial e ﬄ uents, including the secondary rubber processing e ﬄ uent. varying pH, doses, coagulation time, sedimentation time, and temperature. Coagulation–adsorption isotherm models such as BET, Freundlich and Langmuir isotherm models were utilized to determine the amalgamations between FeSO 4 · 7H 2 O and organic particles present in the SPRE. Pseudo-ﬁrst-order and pseudo-second-order kinetic model equations were utilized to determine the adsorption kinetics. The adsorption capacity denoted by q t (mg / mg) at each predetermined time interval was obtained from the kinetic study. The uptake capacity at the equilibrium time interval is represented as q e (mg / mg). The kinetics of the adsorption is used to determine the rate of the uptake of organic particles, which describes the uptake capacity of adsorbate on the surface of the coagulant at each equilibrium contact time. In this study, the pseudo-ﬁrst-order and pseudo-second-order kinetic models were used to analyze the rate of adsorption process by ﬁtting the experimental data to the kinetic models. The plot of ln ( q e - q t ) against t (min) was used to determine the pseudo-ﬁrst-order kinetics for the treatment of SPRE using FeSO 4 · 7H 2 O waste. The values of q e (mg / mg) and k 1 were determined from the slope and intercept of the linearized plot [24]. A slope of 1 q e and the intercept 1 K 2 q e 2 represents the graph of tq t against t (min) for the pseudo-second-order kinetic models. The adsorption kinetics studies for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 · 7H 2 O waste as a coagulant using pseudo-ﬁrst-order kinetics model (a) and pseudo-second-order kinetic model (b) are presented in Figures 9–12, respectively.


Introduction
Natural rubber is a hydrocarbon polymer that is obtained from a milky colloidal suspension of Hevea brasiliensis. Raw rubber processing consumes a large volume of water, and results in the generation of a huge amount of contaminated effluent [1,2]. It has been reported that about 20,500 L of effluent is generated per ton of raw rubber processing [1,3]. The rubber processing effluent contains high total suspended solids (TSS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), nitrate (NO 3 − ), phosphorus (P), and ammonia (NH 3 ) [4]. Therefore, the effluent generated during rubber processing must be treated prior to being discharged into the watercourse to eliminate undesirable eutrophication and death of aquatic organisms living in the water bodies [1,5]. Natural rubber is extensively utilized for various applications and production of latex-based products. It has been estimated that the world consumption of rubber, in 2018, was approximately 29.2 million tonnes [6]. Malaysia is one of the foremost natural rubber (NR) product producers and the world's largest natural rubber (NR) product exporter. It has been estimated that over 1,017,607 tonnes

Physical-Chemical Analyses of SRPE
BOD, COD, SS, and ammonia (NH 3 ) concentration in treated and untreated SRPE were investigated in line with the standard methods for water and wastewater analyses, as reported by the American Public Health Association (APHA) in 2012 [22]. These parameters were chosen in the present study as these are the major parameters for water quality index in Malaysia [23]. Determination of COD was conducted using the reactor digestion method (HACH method, 8000) by employing a HACH DR 2800 spectrometer with high range (HR) COD digestion vials (range 200 to 15,000 mg/L). Next, 2 mL of homogenized sample and 2 mL of deionized water were taken into HR COD digestion vials for samples and blank tests. Then, the vials were taken into the preheated COD reactor at 150 • C for 2 h. Subsequently, the vials were cooled to 120 • C and measured COD (mg/L) with a DRB 200 reactor. The SS was performed following the photometric method (HACH method 8006) and 10 mL of blended sample was taken into a sample. Subsequently, the sample cell was placed in the cell holder and the SS was measured using a spectrophotometer (HACH DR 2800). The determination of BOD in treated and untreated SRPE was conducted at 20 • C for 5 days of incubation following the HACH respirometric method (HACH method 10099). Then, 10 mL of sample was taken into the BOD track sample bottle and filled with deionized water. BOD nutrient buffer pillow and lithium hydroxide powder pillow were added into the bottle and stirred. Then, the sample was incubated into a BOD track incubator at 20 • C for 5 days prior to determining the BOD. The determination of the NH 3 in treated and untreated SRPE was conducted following the salicylate method by using ammonia salicylate reagents powder pillows and ammonia cyanurrate reagents powder pillows. An advanced water quality laboratory spectrophotometer (HACH-DR 900) was utilized for data logging and measurement of the NH 3 (mg/L). Prior to analyses, the pH of the treated and untreated SRPE was warmed and neutralized using concentrated (5 M) NaOH solution. The wavelength of the spectrometer was set to 655 nm and a 10 mL cell riser was inserted into the cell compartment. Then, 0.1 mL of sample and 10 mL of blank sample (distilled water as a control) were taken into the sample cells. Subsequently, ammonia salicylate reagent powder pillow was added into the sample and shaken for 3 min. Later, ammonia cyanurate Water 2020, 12, 1747 4 of 18 reagent powder was added to the same sample cell and shaken for 15 min. The spectrophotometer was then zeroed with blank sample and the sample concentration of NH 3 was measured in mg/L.

Coagulations Experiments Procedure
The initial BOD, COD, SS, and NH 3 concentrations were determined in untreated SRPE and are presented in Table 2. In this study, coagulation experiments were conducted for the removal of BOD, COD, SS, and NH 3 from SRPE in jar tests using FeSO 4 ·7H 2 O waste as a coagulant. A fabricated jar test apparatus was used for the coagulation of SPRE. The jar test apparatus consisted of six paddle rotors to stir six beakers simultaneously. Initially, 500 mL of raw SRPE were taken to each beaker at room temperature (28 ± 1 • C). The coagulation study was conducted with varying temperatures (30 to 80 • C), pH (3 to 8), coagulation times (5 to 90 min), coagulant doses (250 to 2000 mg/L), fixed rapid mixing of 3 min at 200 rpm, slow mixing time at 50 rpm to allow coagulation, and a settling time of 60 min. Immediately after complete settling time was achieved, the residual concentration of COD, BOD, SS, and NH 3 in the treated SRPE supernatant were measured. The percentage removal of BOD, COD, SS, and NH 3 was calculated using the following equation: where C i and C t are the initial and equilibrium concentrations at time 't' of COD, BOD, SS, and NH 3 in untreated and treated SRPE, respectively. The experiments were carried out in triplicate and the recorded values represent mean value ± standard deviation.

Adsorption Isotherm Modeling
Adsorption isotherms were conducted to determine the coagulant-adsorbate amalgamation through the assessment of the removal BOD, COD, SS, and NH 3 from SRPE at various temperatures using Langmuir, Freundlich, and Brunauer-Emmett-Teller (BET) isotherm models. The adsorption experiments were carried out at varying temperatures (30 to 80 • C) as a function of the coagulation time (5 to 90 min) at 1 g/L of coagulant dose, pH 5, coagulation time of 60 min, and sedimentation time of 60 min. The aptness of each isotherm model was assessed via linear regression method by comparing the coefficient of determination (R 2 ) to the experimental data. The adsorption ability was calculated by the following equation: where q e is the adsorption capacity (mg of adsorbate per mg of FeSO 4 ·7H 2 O); V the volume (mL) of SRPE and D the coagulant dose (mg). The BET equation and its liner form are presented in Equations (3) and (4), respectively.  (5) and (6). log where, K f is the Freundlich affinity coefficient (L/mg) and n is the Freundlich exponential constant. The Langmuir isotherm model equation can be written as shown in Equation (7).
The linear form of the Langmuir isotherm model equation can be expressed as below: where a is the Langmuir constant and b the optimal coagulation value for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant.

Kinetics Modeling
Adsorption kinetics has prime importance to elucidate the physical and chemical behavior of adsorbent (physisorption or chemisorption), as well as to determine the adsorbate uptake rate by adsorbent [7,24]. Pseudo-first-order and pseudo-second-order kinetic model equations were utilized to determine the kinetics mechanisms for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant. The experiments were conducted at varying temperature (30 to 80 • C) as a function of the coagulation time (5 to 90 min) at pH 5, coagulant dose of 1 g/L, 60 min of coagulation time, and 60 min of sedimentation time. The pseudo-first-order kinetic model equation can be expressed as below [24]: ln(q e − q t ) = ln q e − k 1 t where q e and q t represents the quantity of COD, BOD, SS, and NH 3 (mg/mg) removed by the FeSO 4 ·7H 2 O waste at equilibrium and at time t (min), respectively, and k 1 represents the pseudo-first-order coagulation rate constant (min −1 ). The pseudo-second-order equation can be written as shown in Equation (10) [24,25]: where k 2 (mg/mg/min) represents the pseudo-second-order coagulation rate constant.

Effect of Initial pH
The pH plays an effective role in the removal of pollutants from industrial effluent and it influences the interaction of the coagulants in the treatment system. The pH in untreated and treated SPRE was determined using a pH meter (Mettler Toledo F20). The pH was adjusted to the desired pH using concentrate sulphuric acid and sodium hydroxide solutions. However, the initial pH (pH in untreated SPRE) and final pH (pH in treated SPRE) were determined to be pH 7.36 ± 21 and pH 6.81 ± 0.14, respectively. Figure 1 shows the influence of pH on the removal of COD, BOD, SS, and NH 3 from the SRPE using FeSO 4 ·7H 2 O waste as the coagulant. It was found that the removal of COD, BOD, SS, and NH 3 increased with increasing pH from pH 3 to 5, and gradually decreased thereafter. The highest percentage removal of BOD, COD, SS, and NH 3 achieved was 81%, 92%, 82%, and 74%, respectively, at pH 5.0, coagulant dose of 1 g/L, coagulation time of 60 min, sedimentation time 60 min, and at room temperature (28 ± 1 • C). However, the obtained removal of COD, BOD, SS, and NH 3 was 70%, 86%, 59% and 72%, respectively at natural pH (without adjusting the pH). The possible mechanisms of FeSO 4 ·7H 2 O coagulation can be attributed to the precipitation iron (III) hydroxide, as shown in Equations (11) to (13). However, the mechanisms for the removal of NH 3 from SRPE subjected to FeSO 4 ·7H 2 O coagulation can be expressed with the adsorption of ammonium ion (NH 4 + ) on the surface of iron (III) hydroxide. Alternatively, the positively charged NH 4 + could act as bridging material to combine with Fe 2+ and negatively charged organic particles (OP) due to the adsorption cum co-precipitation with sludge, as shown in Equations (14) and (15).
Water 2020, 12, x FOR PEER REVIEW 7 of 19

Effect of coagulant doses
The removal of COD, BOD, SS, and NH3 from SRPE with varying FeSO4·7H2O waste doses was determined, as illustrated in Figure 2. When 250 mg/L of dosage was applied, the lowest coagulation efficiency was achieved for the removal of COD, BOD, SS, and NH3. Therefore, it required higher doses to deal with the high content intractable organic matter present in the SPRE. It was observed that the coagulation efficiency increased with increasing coagulant doses up to 1 g/L, thereafter the FeSO4·7H2O coagulation efficiency declined with increasing doses. At the optimal FeSO4·7H2O doses (1 g/L), the removal of COD, BOD, SS, and NH3 were 81%, 92%, 82%, and 74%, respectively. The increase of the coagulation efficiency with increasing coagulation doses was obtained probably due to the increase of positive charged metal ions concentrations to neutralize negative charge organic particles [27]. Ahead of the optimal coagulant doses, the reversal charge took place on the surface of the coagulant particles with the excess amount coagulant doses, and hence the decline of the increase of coagulation efficiency. A similar observation has been obtained by Zahrim et al. [27] and Hossain et al. [10], during decolorization of highly polluted palm oil mill biogas plant effluent using ferric chloride as a sole coagulant, and treatment of raw palm oil mill effluent using FeSO4·7H2O waste as a coagulant, respectively. The highest coagulation efficiency of FeSO 4 ·7H 2 O at pH 5.0 is attributed to the highest solubility of the iron (III) hydroxide and the optimal charge density in the lower acidic environment, therefore, the organic particles gather in the sediment by adsorption or charge neutralized mechanisms. The decrease of the FeSO 4 ·7H 2 O coagulation efficiency above pH 5.0 could be due to the slower solubility of iron (III) hydroxide, and therefore reduces the COD, BOD, SS and NH 3 removal [25]. The pH 5.0 could be considered as the optimal pH for the removal of BOD, COD, SS, and NH 3 from SPRE effluent using FeSO 4 ·7H 2 O as a coagulant. These findings are in line with the previous findings for the removal of organic pollutants from agroindustrial effluent using inorganic salts as coagulants. For instance, Hossain et al. [10] found the highest recovery of BOD, COD, and TSS from palm oil mill effluent using FeSO 4 ·7H 2 O waste as a coagulant at pH 5.0. Hussain et al. [25] obtained the optimal coagulation efficiency of aluminum-based metal salt at a pH range of pH 5.5 to pH 6.0 for the removal of dissolve organic matter from reservoir water. Loloei et al. [26] obtained the maximum recovery of COD (62%) and turbidity (95%) from dairy effluent at pH 5.0 using FeSO 4 ·7H 2 O as a coagulant.

Effect of Coagulant Doses
The removal of COD, BOD, SS, and NH 3 from SRPE with varying FeSO 4 ·7H 2 O waste doses was determined, as illustrated in Figure 2. When 250 mg/L of dosage was applied, the lowest coagulation efficiency was achieved for the removal of COD, BOD, SS, and NH 3 . Therefore, it required higher doses to deal with the high content intractable organic matter present in the SPRE. It was observed that the coagulation efficiency increased with increasing coagulant doses up to 1 g/L, thereafter the FeSO 4 ·7H 2 O coagulation efficiency declined with increasing doses. At the optimal FeSO 4 ·7H 2 O doses (1 g/L), the removal of COD, BOD, SS, and NH 3 were 81%, 92%, 82%, and 74%, respectively. The increase of the coagulation efficiency with increasing coagulation doses was obtained probably due to the increase of positive charged metal ions concentrations to neutralize negative charge organic particles [27]. Ahead of the optimal coagulant doses, the reversal charge took place on the surface of the coagulant particles with the excess amount coagulant doses, and hence the decline of the increase of coagulation efficiency. A similar observation has been obtained by Zahrim et al. [27] and Hossain et al. [10], during decolorization of highly polluted palm oil mill biogas plant effluent using ferric chloride as a sole coagulant, and treatment of raw palm oil mill effluent using FeSO 4 ·7H 2 O waste as a coagulant, respectively.

Effect of coagulant doses
The removal of COD, BOD, SS, and NH3 from SRPE with varying FeSO4·7H2O waste doses was determined, as illustrated in Figure 2. When 250 mg/L of dosage was applied, the lowest coagulation efficiency was achieved for the removal of COD, BOD, SS, and NH3. Therefore, it required higher doses to deal with the high content intractable organic matter present in the SPRE. It was observed that the coagulation efficiency increased with increasing coagulant doses up to 1 g/L, thereafter the FeSO4·7H2O coagulation efficiency declined with increasing doses. At the optimal FeSO4·7H2O doses (1 g/L), the removal of COD, BOD, SS, and NH3 were 81%, 92%, 82%, and 74%, respectively. The increase of the coagulation efficiency with increasing coagulation doses was obtained probably due to the increase of positive charged metal ions concentrations to neutralize negative charge organic particles [27]. Ahead of the optimal coagulant doses, the reversal charge took place on the surface of the coagulant particles with the excess amount coagulant doses, and hence the decline of the increase of coagulation efficiency. A similar observation has been obtained by Zahrim et al. [27] and Hossain et al. [10], during decolorization of highly polluted palm oil mill biogas plant effluent using ferric chloride as a sole coagulant, and treatment of raw palm oil mill effluent using FeSO4·7H2O waste as a coagulant, respectively.

Effect of Coagulation Time
The treatment time plays an important role by increasing the coagulant efficiency for the removal of organic pollutants from wastewater. The influence of the coagulation efficiency on the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste was determined, as presented in Figure 3. From the result obtained, it was observed that the removal of COD, BOD, SS, and NH 3 increased with time, up to 60 min; thereafter the coagulation efficiency for the removal of COD, BOD, SS, and NH 3 was found negligible. At the 5 min treatment time, the removal efficiency obtained for COD, BOD, SS, and NH 3 was 39%, 33%, 26%, and 28%, respectively. The removal efficiency increased to 81%, 92%, 82%, and 74%, respectively, at the 60 min treatment time. The maximum removal obtained for COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant were 82%, 95%, 83%, and 75%, respectively, at 70 min coagulation time for a coagulant dose of 1 g/L, pH 5.0, and sedimentation time of 60 min. The negligible increase in the FeSO 4 ·7H 2 O coagulation efficiency for the removal of COD, BOD, SS, and NH 3 from SRPE which were observed over the 60 min treatment time could be due to the saturation of Fe(OH) 3 with the colloidal organic pollutant over the 60 min treatment time. The coagulation time is crucial for the floc formation. After rapid mixing, the organic particles and coagulant particles require times to induce and make contact for progressively forming larger agglomerates to settling down [24]. At a shorter mixing time, the poor performance of the FeSO 4 ·7H 2 O waste as a coagulant on the COD, BOD, SS, and NH 3 removal was observed due to the low collisions between coagulant and suspended particles which led to lower floc formation for sedimentation [8].
efficiency increased to 81%, 92%, 82%, and 74%, respectively, at the 60 min treatment time. The maximum removal obtained for COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant were 82%, 95%, 83%, and 75%, respectively, at 70 min coagulation time for a coagulant dose of 1 g/L, pH 5.0, and sedimentation time of 60 min. The negligible increase in the FeSO4·7H2O coagulation efficiency for the removal of COD, BOD, SS, and NH3 from SRPE which were observed over the 60 min treatment time could be due to the saturation of Fe(OH)3 with the colloidal organic pollutant over the 60 min treatment time. The coagulation time is crucial for the floc formation. After rapid mixing, the organic particles and coagulant particles require times to induce and make contact for progressively forming larger agglomerates to settling down [24]. At a shorter mixing time, the poor performance of the FeSO4·7H2O waste as a coagulant on the COD, BOD, SS, and NH3 removal was observed due to the low collisions between coagulant and suspended particles which led to lower floc formation for sedimentation [8].

Effect of Sedimentation Time
The coagulation efficiency is highly dependent on the organic particles settling speeds and floc formation [25]. The influence of sedimentation time on the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant was determined with varying sedimentation time from 30 min to 180 min at pH 5.0, coagulant dose of 1 g/L, and coagulation time of 60 min, as shown in Figure 4. As shown in Figure 4, it was observed that the coagulation efficiency increased with increasing sedimentation time up to 60 min, thereafter the increase was found to be negligible. At 30 min sedimentation time, the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant were 75%, 87%, 69% and 69%, respectively. However, the removal efficiency of COD, BOD, SS, and NH3 increased to 81%, 92%, 82%, and 74%, respectively, at 60 min sedimentation time. Coagulation is an indispensable process to aggregate colloidal organic particles to larger flocs by the neutralization of the surface charges. The degree of aggregation is influenced by the settlement process [28]. In this regard, it results in the clustering of the molecules on the surface of the coagulant. Consequently, it leads to the removal of the pollutant parameters from the wastewater. However, it

Effect of Sedimentation Time
The coagulation efficiency is highly dependent on the organic particles settling speeds and floc formation [25]. The influence of sedimentation time on the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant was determined with varying sedimentation time from 30 min to 180 min at pH 5.0, coagulant dose of 1 g/L, and coagulation time of 60 min, as shown in Figure 4. As shown in Figure 4, it was observed that the coagulation efficiency increased with increasing sedimentation time up to 60 min, thereafter the increase was found to be negligible. At 30 min sedimentation time, the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant were 75%, 87%, 69% and 69%, respectively. However, the removal efficiency of COD, BOD, SS, and NH 3 increased to 81%, 92%, 82%, and 74%, respectively, at 60 min sedimentation time. Coagulation is an indispensable process to aggregate colloidal organic particles to larger flocs by the neutralization of the surface charges. The degree of aggregation is influenced by the settlement process [28]. In this regard, it results in the clustering of the molecules on the surface of the coagulant. Consequently, it leads to the removal of the pollutant parameters from the wastewater. However, it can be assumed that the optimal sedimentation time for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant was 60 min, as per recorded in this study. can be assumed that the optimal sedimentation time for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant was 60 min, as per recorded in this study. The removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste was determined with varying temperatures from room temperature (28 ± 1 °C) to 80 °C, as presented in Figure 5. It was found that temperature potentially influences the coagulation efficiency of FeSO4·7H2O waste. At room temperature, the removal of COD, BOD, SS, and NH3 was found to be 81%, 92%, 82%, and

Effect of Temperature
The removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste was determined with varying temperatures from room temperature (28 ± 1 • C) to 80 • C, as presented in Figure 5. It was found that temperature potentially influences the coagulation efficiency of FeSO 4 ·7H 2 O waste. At room temperature, the removal of COD, BOD, SS, and NH 3 was found to be 81%, 92%, 82%, and 74%, respectively. The removal COD, BOD, SS, and NH 3 rapidly increased with increasing temperature and reached maximum at 70 • C, however, the removal efficiency was decreased with a further increase of temperature above 70 • C. The maximum removal of COD (97%), BOD (99%), SS (98%), and NH 3 (95%) were obtained at 70 • C. The increased coagulation efficiency with increasing temperature which was observed could be due to the reduction of viscosity with an enhancing dispersion rate of the organic particles towards the boundary line of FeSO 4 ·7H 2 O waste. In addition, the kinetics energy of the iron (II) particles could increase with the elevated temperature, which enabled collisions with negatively charged organic particles in SPRE, and therefore increased the coagulation efficiency [10]. However, the decrease in adsorption efficiency with increasing temperature above 70 • C could be due to the weakening of the favorable intermolecular force between the coagulant particles + and organic particles present in SPRE.

Coagulation Equilibrium Studies
The coagulation equilibrium studies reflect the dependence of the amount of adsorbate particle adsorbed on the surface of the coagulant. In order to determine the nature of the coagulation process, mass balance mathematical equations are required. Although a number of the mathematical models have been utilized in the literature to determine the quantity of adsorbate particles and the nature of adsorption, the Longmuir, Freundlich, and BET models are the mathematical model equations that are used most often. An adsorption study was conducted to model the experimental data obtained from the effect of the process variables for the removal of BOD, COD, SS, and NH3 from SRPE using FeSO4·7H2O as a coagulant. The amalgamations between FeSO4·7H2O and organic particles present in the SPRE were determined using BET, Freundlich ,and Langmuir isotherm models, as shown in Figures 6-8.
The BET model is an empirical version of the Langmuir isotherm model equation [24]. This isotherm model was developed based on several assumptions and can be deduced from either kinetics consideration or thermodynamics of adsorption. The BET model illustrates that coagulation and adsorption occurred by binding the adsorbate onto the surface of the adsorbent in a multilayer formation with random distribution of the adsorbed particles [29]. Figure 6 shows the BET isotherm for the removal of COD, BOD SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant. The constant values such as xm (amount of adsorbate adsorbed by the coagulant to form a monolayer) and A (interaction energy between adsorbate and coagulant surface) were determined, as presented in Table 3. It was found that the A values for the removal of COD, BOD, SS, and NH3 from SRPE were negative, which revealed that the coagulant surface was saturated with the adsorbed organic particles in a multilayer formation. The xm values were determined to be 0.544, 0.187, 0.731, and 0.230 mg/mg for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O as a coagulant. The determined xm values indicated that 0.544 mg/mg COD, 0.187 mg/mg BOD, 0.731 mg/mg SS, and 0.230 mg/mg NH3 were adsorbed by the coagulant to form a monolayer. The Freundlich isotherm model equation is also a modified version of the Langmuir isotherm model. This isotherm model describes the coagulation and adsorption involve adsorbing adsorbent particles on the surface of the coagulant in a multilayer formation with heterogeneous and nonuniform distribution. However, it is a reversible adsorption process and not restricted to the Although the optimal removal of COD, BOD, SS, and NH 3 was gained at 70 • C, increasing the temperature to 70 • C is costly and energy consuming. However, the removal of NH 3 -N from wastewater is challenging using the physicochemical process including coagulation to mitigate stringent discharge limits set by DoE, Malaysia. Therefore, the economic aspect of raising the temperature to 70 • C would be considered in order to preserve the environment and minimize the surface water pollution. Additionally, renewable energy such as solar power could be used to increase the temperature during coagulation in a large-scale water treatment plan.

Coagulation Equilibrium Studies
The coagulation equilibrium studies reflect the dependence of the amount of adsorbate particle adsorbed on the surface of the coagulant. In order to determine the nature of the coagulation process, mass balance mathematical equations are required. Although a number of the mathematical models have been utilized in the literature to determine the quantity of adsorbate particles and the nature of adsorption, the Longmuir, Freundlich, and BET models are the mathematical model equations that are used most often. An adsorption study was conducted to model the experimental data obtained from the effect of the process variables for the removal of BOD, COD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant. The amalgamations between FeSO 4 ·7H 2 O and organic particles present in the SPRE were determined using BET, Freundlich, and Langmuir isotherm models, as shown in Figures 6-8. Water 2020, 12, x FOR PEER REVIEW 11 of 19 exponential constant (Kf) values were between the range of 1 to 10 (Table 1), thus, the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant is favorable [28]. The Langmuir isotherm model describes the surface homogeneity of the coagulation and adsorption with the hypothesis that the coagulation and adsorption occur in monolayer formation of the adsorbate at a specific homogeneous site of the coagulant surface with equally distributed energy levels [25,30]. Figure 8 shows the Langmuir isotherms for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant. It was found that the calculated a (Langmuir constant) values and b (maximum coagulation) values are positive ( Table 3), suggesting that the adsorption of organic particles from SRPE using FeSO4·7H2O waste is favorable.
For the Langmuir isotherm model, the isotherm shape is utilized to predict the favorability of an adsorption process with a specific experimental condition. However, the adsorption behavior of FeSO4·7H2O waste for the removal of COD, BOD, SS, and NH3 from SRPE could be better expressed using the Langmuir isotherm model by the determination of dimensional constant (RL), also called equilibrium parameter, defined as [23]: where Ci is the initial concentration of COD, BOD, SS, and NH3 in mg/L and b is the Langmuir constant. On the basis of the RL value, the adsorption behavior can be classified into four groups, such as favorable (0 ˂ RL ˂ 1), unfavorable (RL > 1), liner (RL = 1), and irreversible (RL = 1). As can be seen in Table 3, the RL values for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste were within the range of 0 ˂ RL ˂ 1, indicating that the adsorption of COD, BOD, SS, and NH3 from   [22]. Hussain et al. [22] found that the BET surface model best described the adsorption behavior of aluminum-based metal salts for the removal of organic contaminates from river and reservoir waters. Similarly, Hossain et al. [10] observed that the BET surface model best fitted with adsorption isotherm for the removal COD, BOD, and TSS from raw palm oil mill effluent using FeSO4·7H2O waste as a coagulant.

Adsorption Kinetics
The adsorption kinetics for the removal of COD, BOD, SS, and NH3 from rubber processing effluent using FeSO4·7H2O waste as a coagulant were determined to elucidate adsorption mechanisms. Generally, the mechanisms of adsorption depend on the mass transport process and the chemical characteristics of the adsorbent [31,32]. To determine the adsorption mechanisms for the removal of COD, BOD, SS, and NH3 in FeSO4·7H2O waste and determining the potential rate of mass transfer, the present study utilized pseudo-first-order and pseudo-second-order kinetic models as shown in Equations (9) and (10), respectively.
The adsorption capacity denoted by qt (mg/mg) at each predetermined time interval was obtained from the kinetic study. The uptake capacity at the equilibrium time interval is represented as (mg/mg). The kinetics of the adsorption is used to determine the rate of the uptake of organic particles, which describes the uptake capacity of adsorbate on the surface of the coagulant at each equilibrium contact time. In this study, the pseudo-first-order and pseudo-second-order kinetic models were used to analyze the rate of adsorption process by fitting the experimental data to the kinetic models. The plot of ln (qe-qt) against t (min) was used to determine the pseudo-first-order kinetics for the treatment of SPRE using FeSO4·7H2O waste. The values of (mg/mg) and k1 were determined from the slope and intercept of the linearized plot [24]. A slope of and the The BET model is an empirical version of the Langmuir isotherm model equation [24]. This isotherm model was developed based on several assumptions and can be deduced from either kinetics consideration or thermodynamics of adsorption. The BET model illustrates that coagulation and adsorption occurred by binding the adsorbate onto the surface of the adsorbent in a multilayer formation with random distribution of the adsorbed particles [29]. Figure 6 shows the BET isotherm for the removal of COD, BOD SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant.
The constant values such as x m (amount of adsorbate adsorbed by the coagulant to form a monolayer) and A (interaction energy between adsorbate and coagulant surface) were determined, as presented in Table 3. It was found that the A values for the removal of COD, BOD, SS, and NH 3 from SRPE were negative, which revealed that the coagulant surface was saturated with the adsorbed organic particles in a multilayer formation. The x m values were determined to be 0.544, 0.187, 0.731, and 0.230 mg/mg for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant. The determined x m values indicated that 0.544 mg/mg COD, 0.187 mg/mg BOD, 0.731 mg/mg SS, and 0.230 mg/mg NH 3 were adsorbed by the coagulant to form a monolayer. The Freundlich isotherm model equation is also a modified version of the Langmuir isotherm model. This isotherm model describes the coagulation and adsorption involve adsorbing adsorbent particles on the surface of the coagulant in a multilayer formation with heterogeneous and non-uniform distribution. However, it is a reversible adsorption process and not restricted to the monolayer adsorption formation [29]. The Freundlich isotherms for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant were determined and are presented in Figure 7. The Freundlich affinity constant (K f ) values for the removal of COD, BOD, SS, and NH 3 were found to be 1.603, 0.222, 1.621, and 0.645 L/mg, respectively (Table 3). It was found that the Freundlich exponential constant (K f ) values were between the range of 1 to 10 (Table 1), thus, the removal of COD, BOD, SS, and NH3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant is favorable [28].
The Langmuir isotherm model describes the surface homogeneity of the coagulation and adsorption with the hypothesis that the coagulation and adsorption occur in monolayer formation of the adsorbate at a specific homogeneous site of the coagulant surface with equally distributed energy levels [25,30]. Figure 8 shows the Langmuir isotherms for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant. It was found that the calculated a (Langmuir constant) values and b (maximum coagulation) values are positive ( Table 3), suggesting that the adsorption of organic particles from SRPE using FeSO 4 ·7H 2 O waste is favorable.
For the Langmuir isotherm model, the isotherm shape is utilized to predict the favorability of an adsorption process with a specific experimental condition. However, the adsorption behavior of FeSO 4 ·7H 2 O waste for the removal of COD, BOD, SS, and NH 3 from SRPE could be better expressed using the Langmuir isotherm model by the determination of dimensional constant (R L ), also called equilibrium parameter, defined as [23]: where C i is the initial concentration of COD, BOD, SS, and NH 3 in mg/L and b is the Langmuir constant.
On the basis of the R L value, the adsorption behavior can be classified into four groups, such as favorable (0 < R L < 1), unfavorable (R L > 1), liner (R L = 1), and irreversible (R L = 1). As can be seen in Table 3, the R L values for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste were within the range of 0 < R L < 1, indicating that the adsorption of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste is favorable. The coefficient of determination (R 2 ) values for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO 4 ·7H 2 O waste are displayed in Table 3 [22]. Hussain et al. [22] found that the BET surface model best described the adsorption behavior of aluminum-based metal salts for the removal of organic contaminates from river and reservoir waters. Similarly, Hossain et al. [10] observed that the BET surface model best fitted with adsorption isotherm for the removal COD, BOD, and TSS from raw palm oil mill effluent using FeSO 4 ·7H 2 O waste as a coagulant.

Adsorption Kinetics
The adsorption kinetics for the removal of COD, BOD, SS, and NH 3 from rubber processing effluent using FeSO 4 ·7H 2 O waste as a coagulant were determined to elucidate adsorption mechanisms. Generally, the mechanisms of adsorption depend on the mass transport process and the chemical characteristics of the adsorbent [31,32]. To determine the adsorption mechanisms for the removal of COD, BOD, SS, and NH 3 in FeSO 4 ·7H 2 O waste and determining the potential rate of mass transfer, the present study utilized pseudo-first-order and pseudo-second-order kinetic models as shown in Equations (9) and (10), respectively. The adsorption capacity denoted by q t (mg/mg) at each predetermined time interval was obtained from the kinetic study. The uptake capacity at the equilibrium time interval is represented as q e (mg/mg). The kinetics of the adsorption is used to determine the rate of the uptake of organic particles, which describes the uptake capacity of adsorbate on the surface of the coagulant at each equilibrium contact time. In this study, the pseudo-first-order and pseudo-second-order kinetic models were used to analyze the rate of adsorption process by fitting the experimental data to the kinetic models. The plot of ln (q e -q t ) against t (min) was used to determine the pseudo-first-order kinetics for the treatment of SPRE using FeSO 4 ·7H 2 O waste. The values of q e (mg/mg) and k 1 were determined from the slope and intercept of the linearized plot [24]. A slope of 1 q e and the intercept 1 K 2 q e 2 represents the graph of t q t against t (min) for the pseudo-second-order kinetic models. The adsorption kinetics studies for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O waste as a coagulant using pseudo-first-order kinetics model (a) and pseudo-second-order kinetic model (b) are presented in Figures 9-12, respectively.
intercept represents the graph of against (min) for the pseudo-second-order kinetic models. The adsorption kinetics studies for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O waste as a coagulant using pseudo-first-order kinetics model (a) and pseudo-secondorder kinetic model (b) are presented in Figures 9-12, respectively. Table 4 shows the qe values (experimental and calculated values from Figures 9-12), pseudofirst-order and pseudo-second-order rate constant values, and correlation coefficient (R 2 ) values. However, the differences between experimental and predicted qe values and correlation coefficients were utilized to determine the best fitted kinetics model for the removal of COD, BOD, SS, and NH3 from SRPE using FeSO4·7H2O as a coagulant. It was found that the R 2 values for the pseudo-secondorder kinetic model (R 2 > 0.999) were much closer to unity than the pseudo-first-order kinetic model (R 2 ˂ 0.999). Moreover, the experimental qe values for the removal of COD, BOD, SS, and NH3 were more closely matched with the theoretical qe values of pseudo-second-order kinetic model than those from the pseudo-first-order kinetic model. Therefore, we concluded that the adsorption mechanism for the removal of COD, BOD, SS and NH3 from SRPE using FeSO4·7H2O as a coagulant was best described by the pseudo-second-order kinetic model. Thus, we concluded that chemisorption could be the possible adsorption mechanism for the removal of BOD, COD, SS, and NH3 from SRPE using FeSO4·7H2O as a coagulant [28].    Table 4 shows the q e values (experimental and calculated values from Figures 9-12), pseudo-first-order and pseudo-second-order rate constant values, and correlation coefficient (R 2 ) values. However, the differences between experimental and predicted q e values and correlation coefficients were utilized to determine the best fitted kinetics model for the removal of COD, BOD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant. It was found that the R 2 values for the pseudo-second-order kinetic model (R 2 > 0.999) were much closer to unity than the pseudo-first-order kinetic model (R 2 < 0.999). Moreover, the experimental q e values for the removal of COD, BOD, SS, and NH 3 were more closely matched with the theoretical q e values of pseudo-second-order kinetic model than those from the pseudo-first-order kinetic model. Therefore, we concluded that the adsorption mechanism for the removal of COD, BOD, SS and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant was best described by the pseudo-second-order kinetic model. Thus, we concluded that chemisorption could be the possible adsorption mechanism for the removal of BOD, COD, SS, and NH 3 from SRPE using FeSO 4 ·7H 2 O as a coagulant [28].

Conclusions
In this study, the FeSO 4 ·7H 2 O waste byproduct from titanium oxide industry was implemented for the removal of COD, BOD, SS, and NH 3 from the SRPE. We found that the coagulation efficiency of FeSO 4 ·7H 2 O waste was potentially influenced by pH, doses, coagulation time, and temperature. The highest gain in the percentage removal of BOD, COD, SS, and NH 3 was approximately 97%, 99%, 98%, and 95% for the removal of COD, BOD, SS, and NH 3 at pH 5.0, coagulant dose of 1 g/L, coagulation time of 60 min, sedimentation time of 60 min, and at the elevated temperature of 70 • C. The adsorption equilibrium studies revealed that the best adsorption isotherm model was the BET model. This indicates that the FeSO 4 ·7H 2 O adsorption for the removal of COD, BOD, SS, and NH 3 from SPRE happens on the surface of iron hydroxide precipitates with random distribution and multilayer formation. The kinetics analyses show that the adsorption mechanism was best described by the pseudo-second-order kinetic model, which indicates that chemisorption could be a possible coagulation and adsorption mechanism. The finding of the present study revealed that FeSO 4 ·7H 2 O waste obtained from the titanium oxide processing industry has the potential to be used as a coagulant to treat rubber processing wastewater with minimal cost, as the FeSO 4 ·7H 2 O is an industrial byproduct.