PULVERIZED RIVER SHELLFISH SHELLS AS A CHEAP ADSORBENT FOR REMOVING OF MALATHION FROM WATER: EXAMINATION OF THE ISOTHERMS, KINETICS, THERMODYNAMICS AND OPTIMIZATION OF THE EXPERIMENTAL CONDITIONS BY THE RESPONSE SURFACE METHOD

Introduction/purpose: In this study, we investigated the possibility of removing the organophosphorus pesticide malathion from water using a new adsorbent based on the biowaste of river shell shards from the Anodonta Sinadonta woodiane family, a material that accumulates in large quantities as waste on the banks of large rivers. Two adsorbents were tested mechanically comminuted river shells (MRM) and mechanosynthetic hydroxyapatite from comminuted river shells (RMHAp). Methods: The obtained adsorbents were characterized and tested for the removal of the organophosphorus pesticide malathion from water. In order to predict the optimal adsorption conditions using the Response Surface Method (RSM), the authors investigated the influence of variable factors (adsorption conditions), pH values, adsorbent doses, contact times, and temperatures on the adsorbent capacity. Results: The best adsorption of malathion was achieved at mean pH values between 6.0 and 7.0. The adsorption data for malathion at 25, 35, and 45 °C were compared using the Langmuir, Freundlich, DubininRadushkevich (DR), and Temkin isothermal models, as well as pseudofirst order, pseudo-second order and Elovic kinetic models for modeling adsorption kinetics. The maximum Langmuir adsorption capacity for MRM and RMHAp at 25 °C was 46,462 mg g-1 and 78,311 mg g-1, respectively. Conclusion: The results have showed that malathion adsorption on both adsorbents follows the pseudo-second kinetic model and the Freundlich isothermal model. The thermodynamic parameters indicate the endothermic, feasible, and spontaneous nature of the adsorption process.


Introduction
The development of technology undoubtedly contributes to the development of society; however, it also causes environmental pollution. A distinct surge in the world's population and an increase in food needs condition the development of intensive agricultural production based on the use of inputs to overcome factors that limit production such as insects, fungi, weeds, and land scarcity (Kamga, 2019). The usage of pesticides is intended to combat animals and plants that are harmful to crops, thus enabling increased yields and ensuring the sustainability of the human population. Non-selective use of pesticides in agricultural activities leads to the pollution of surface and underground water accumulations. Due to its potential danger to health by entering the food chain for humans and animals, pesticide pollution has reached alarming proportions. (Chatterjee et al, 2010) Pesticides are ecologically very important because of their high toxicity to living organisms, including humans; the toxicological profile of this pollutant poses a potential risk to the environment and public health (Kamga, 2019). According to numerous studies, many insecticides such as DDT, deildrin, heptachlor, and aldrin bioaccumulate in blood, milk, and tissues and are also found in food products (Singh et al, 2010).
It has been confirmed that patients with acute organophosphate poisoning suffer from problems such as vomiting, nausea, miosis, excessive salivation, blurred vision, headache, dizziness, and disturbances of consciousness (Singh et al, 2010). In the case of malathion, which is one of widely used organophosphorus pesticides, almost all the observed effects occur due to its active metabolite malacon (Singh et al, 2010) on the nervous system or gives secondary effects to its primary action.
Malathion is slowly absorbed through the skin, but is more rapidly and efficiently absorbed via ingestion. Once they are absorbed, phosphorothioates such as malathion are metabolically activated to the "oxon" forms which have greater toxicity than the parent insecticide. The metabolism of malathion leads to the formation of malathion monocarboxylic acid, malathion dicarboxylic acid, dialkyl phosphate metabolites, and other metabolites (Bouchard et al, 2003).
In recent years, research on the removal of pollutants from water has been intensified, based on the phenomenon of adsorption, among which the removal of pesticides from water occupies a special place. We have a large number of potentially highly effective adsorbents for removing pesticides from water such as activated carbon (Kamga, 2019;Ohno et al, 2008;Hameed et al, 2009), but the high price of activated carbon limits its mass use in many poor countries. Therefore, the attention of researchers is increasingly focused on finding a cheap, environmentally friendly, and highly efficient adsorbent to solve this problem. Various adsorbents such as agricultural by-products, waste materials, cheap minerals and biomass have been used to remove various pollutants from wastewater (Chatterjee et al, 2010;Pantić et al, 2019;Bajić et al, 2013;Stevanović et al, 2020;Karanac et al, 2018;Perendija et al, 2021 Malathion, an organophosphorus highly selective insecticide, is widely used in agriculture worldwide (Chatterjee et al, 2010), primarily in the control of insects, including mosquitoes, aphids, grass insects, and many other parasites of vegetable crops and fruits. Until today, the removal of malathion from wastewater has not been studied in detail and only a few studies are available in this regard (Chatterjee et al, 2010).
The shards of river shells are waste that burdens a large number of beaches and the banks of rivers, seas, and lakes. They also appear as waste after use in human nutrition. The aim of this paper is to apply shellfish as a cheap, widely available, environmentally friendly material for removing organophosphorus pesticides from water. In this way, we get a double benefit: we use waste that burdens the shores of different watercourses to remove pollutants that load the water and cause negative effects on life and health of humans and animals as well as on the environment in general.

Materials
A large number of different chemicals were used during the research. Bearing in mind that the properties of the adsorbent, as well as the research results largely depend on the purity of the reagent, high purity chemicals were used: − 5% hydrogen peroxide solution -H2O2 (Sigma-Aldrich, PA), − concentrated nitric acid HNO3 (Fluka, ultrapure) -used for digestion of shells in order to determine the elemental composition and adjust the pH level, − concentrated phosphoric acid H3PO4 (Sigma-Aldrich, PA), − sodium hydroxide -NaOH (Sigma Aldrich, PA) -used to adjust the pH of the solution in the adsorption process and titration during the synthesis of adsorbents, − 96% ethyl alcohol (Sigma Aldrich, PA) -used in the washing of adsorbents, − deionized water (resistance of 18 MΩ cm) -used for sample preparation and dilution of the solution, − malathion, 60% technical solution (Galenika-Fitofarmacija)used for performing an adsorption experiment (as 20 mg dm −3 concentration solution), − biowaste of shellfish from the genus Anodonta Sinadonta woodiane, collected from the banks of the Tisza River.

Synthesis of adsorbents
During the research, two types of adsorbents based on river shell shards were synthesized and tested in the process of adsorption: 1. shells, washed, mechanically ground, sieved, washed, and dried in a vacuum oven -clean shells (MRM), and 2. fish carp scales chemically modified by converting calcium carbonate to hydroxyapatite by mechanosynthesis (RMHAp).
The shards of the Anodonta Sinadonta woodiane river shell were thoroughly washed and rinsed in distilled water, air-dried for 24 h, ground in a steel mill for crushing sediments and sifted into a 0.5 -1 mm granulation powder. The shell powder was washed in vacuo with deionized water, ethyl alcohol and dried in vacuo for 24 h at 110 o C to give the first MRM adsorbent (mechanically prepared river shells).
In the stainless steel vessel of a planetary ball mill (Retsch PM100 CM), 10 g of MRM was mixed with 11.23 ml (18.92 g) of concentrated H3PO4 -CaCO3 ratio: H3PO4 → Ca / P = 1.67; zirconium beads were added in ratio 20:1 of the bead mass to a sample and the mixture was treated in a ball mill for 10 h at 500 grp for further mechanosynthesis. After the treatment in a ball mill, the obtained mixture was washed copiously in vacuo with deionized water to remove unreacted parts of the acid and dried in vacuo for 24 h at 110 o C to obtain a second RMHAp adsorbent (hydroxyapatite).

Material characterization methods
The synthesized adsorbents were characterized by FTIR, XRD, SEM and EDS techniques. The elemental composition was determined by a chemical elemental analyzer, the content of individual elements was determined by dissolving in acids and measuring the content on a plasma mass spectrometer with a plasma-coupled plasma ICP-MS system Agilent 7500C (Agilent Technologies, Inc.) and an atomic adsorption spectrometer. The concentration of malathion before and after adsorption was determined using a gas chromatograph (GC) equipped with a flame ionization detector (FID) -Varian 3400 with FID operating system. The specific surface area of the adsorbent, the specific pore volume, and the pore diameter were determined by the BET method of adsorption / desorption in a stream of nitrogen at 72.4 K, using a gas sorption analyzer Micromeritics ASAP 2020MP v 1.05 H. The infrared Fourier transform spectrum (FTIR) was recorded in the transformation mode between 400 and 4000 cm-1 at a resolution of 4 cm-1 using an infrared (IR) spectrometer with Fourier transformation (FT) -Nicolet iS 50 manufactured by Thermo Scientific. The adsorbents morphology was observed using Tescan Mira 3 FEG scanning electron field microscopy (FESEM). The morphological structure was determined by x-ray diffraction, XRD, using an ENRAF NONIUS FR590 XRD (Bruker AKSS, MA, USA) diffractometer with Cu Ka 1,2 radiation and a step / scan time regime of 0.05 / 1 s. The pH value of the zero charge point (pHPZC) of adsorbents was determined by the "drift" method (Gao et al, 2009).

Malathion adsorption research
Adsorption experiments were performed in a batch system where the initial concentration of malathion solution was fixed Co = 20.32 mg L -1 , and the adsorbent dose was varied from 100 to 1000 mg L -1 . In order to examine the pH value influence on the adsorption process, the pH value was varied from 4.0 to 10.0. Thermodynamic and kinetic adsorption experiments were performed at temperatures of 25, 35 and 45 o C, and the adsorption process was monitored in a time interval of 10 to 180 minutes. The amount of adsorbed molecules was calculated as the difference between the initial and equilibrium concentration.
The adsorbent capacity was calculated in accordance with Eq. (1): where q is the adsorption capacity in mg g -1 , Ci and Cf are the initial and final malathion concentrations in mg L -1 (μg L -1 ), respectively, V is the volume of the solution in L, and m is the adsorbent mass, expressed in g.

Kinetic studies
The study of kinetics provides an insight into a possible mechanism of adsorption along with the reaction pathways. The adsorption data were analyzed by linear, non-linear least-squares and graphic methods in the form of pseudo-first, pseudo-second-order (Lagergreen) and second order models (Table 1).
Diffusion models such as Weber-Morris, Dunwald-Wagner model, and Homogenous Solid Diffusion Model (HSDM) were used for modeling diffusional processes/limiting step of the overall process (  The activation energy for arsenate adsorption was calculated using Arrhenius (Eq. 8): where k´ (g mg -1 min -1 ) is the pseudo-second order rate adsorption constant, k0 ( g mmol -1 min -1 ) is the temperature independent factor, Ea (kJ mol -1 ) is the activation energy, R (8.314 J mol -1 K -1 ) is the gas constant, and T (K) is the adsorption absolute temperature. A plot of ln K ' versus 1/T gave a straight line with a slope -Ea/R from which the activation energy was calculated.
The Langmuir equation is based on the assumption that the point of maximum adsorption corresponds to a saturated mono-layer of adsorbate molecules on the adsorbent surface -where the energy of adsorption remains constant and no transfer of the adsorbate in the surface plane occurs.
The Freundlich sorption isotherm, widely and reliably utilized as a mathematical determining expression, allows for a calculation encompassing surface heterogeneity and exponential distribution of active sites as well as their respective energies (Karanac et al, 2018).
Temkin conceived this equation for subcritical vapors in micropore solids where the adsorption process follows a pore filling mechanism onto an energetically non-uniform surface.
The Temkin isotherm is based on the assumption that the decline of the heat of sorption as a function of temperature is linear rather than logarithmic. The Dubinin-Radushkevich model states that the adsorption capacity depends on the adsorbed amount on the surface of the material, differently from the Langmuir model." (Karanac et al, 2018).
The equations of adsorption isotherms models are presented in Table 3.

Thermodynamic studies
The feasibility of the experimental data obtained from the adsorption studies were analyzed through the thermodynamic investigation. The parameters of free energy change (ΔG°, kJ/mol), enthalpy change (ΔH°, kJ mol -1 ), and entropy change (ΔS°, J mol -1 K -1 ) were calculated using the Van't Hoff equations (13) and (14) (Karanac et al, 2018): The separation factor (RL) is in relation to the Langmuir isotherm and it is used to assess adsorption feasibility on the given adsorbent. It is calculated using the next Eq (15): where C0 (mol dm -3 ) is the initial adsorbate concentration, b (dm 3 mol -1 ) is the Langmuir constant. The value of RL points out to the isotherm type: irreversible (RL = 0), favorable (0 < RL <1), linear (RL = 1), unfavorable (RL > 1).

Statistical analysis of the experimental data
All adsorption experiments were repeated three times and the mean values were taken for further processing and modeling. The obtained results were analyzed using the normalized standard deviation Δq (%) which is calculated using the following equation: where qexp and qcal are the experimental and calculated values of adsorbed malathion, and N is the number of data used in the analysis. The maximum deviation is <3%, which is an experimental error. Standard errors for isothermal, kinetic, and thermodynamic parameters were determined using the commercial software Microcal Origin 8.0 (Pantić et al, 2019). In order to confirm the adsorption model that best corresponds to the experimental data, they were analyzed by the ANOVA variance analysis, using the F value together with the values of the correlation coefficient (R) from the regression analysis. (Pantić et al, 2019;Bajić et al, 2019)

Optimization of the experimental adsorption conditions
The optimization of the adsorption conditions was performed using the RSM (Surface Response Methodology) method. Classical adsorption optimization usually involves examining the impact of each variable separately. However, it is difficult to predict optimal reaction conditions based on such results due to possible interactions between different independent variables involved in adsorption reactions. (Pantić et al, 2019;Bajić et al, 2019) Recently, various statistical programs have been used that are useful to help establish the design of the experiment. Using response surface methodology (RSM) as a mathematical function, it is possible to examine the individual and interactive influences of different variables in relation to different predictors. In that way, we get the optimal conditions that are needed to get the best results. (Pantić et al, 2019) In addition, it has been proven that the central composition design (CCD) and the Bok-Behnken design (BBD) are efficient designs of RSM models in optimizing the adsorption process. (Pantić et al, 2019) In this study, the Z. synthesized modified adsorbent RMHAp was used to remove malathion from water. The adsorption process is optimized by numerical and graphical optimization methods using the Bok-Behnken design. We used the design with four factors (input variables) and three levels of values in which 29 experiments with five replications in the central point were performed. The capacity of the adsorbent was taken as a response. Extremely optimized conditions were confirmed by additional experimental testing. The conditions of the adsorption experiment are given in Table 4. Results and discussion

Physical and chemical characterizations of adsorbents
The elemental composition of shellfish shards is given in Table 5. This elemental composition is similar to that of other authors (Vei et al, 2018;Buasri et al, 2013) and indicates that shellfish shards are mostly composed of calcium carbonate-based minerals ( calcite and argonite) and the organic part of chitin that connects calcite structures. The composition of shells also includes various microelements -ions, which replace Ca 2+ in the structure of calcium carbonate and are incorporated into the shell during its formation. The availability and rate of bioaccumulation of these ions is a function of environmental and biological factors. Thus, different habitats, contamination -the presence in the water of different ionic species, different stages of shell development can represent different patterns of metal incorporation. The analyzed shards of the river shell Anodonta Sinadonta woodiane are composed of two polymorphs of calcium carbonate: calcite and aragonite, with calcite being the dominant form. Recent studies have found that the dominant CaCO3 polymorph may be temperature dependent -aragonite deposition is at high temperature and calcite deposition is at low temperature (Kuklinski and Taylor, 2009;Ramajo et al, 2015;Krzeminska et al, 2016). The confirmation of the composition of the shells can also be seen on the spectrum of energy dispersive spectrometry (Figure 1) where the main building blocks are observed before (a) and after the modification of the shells (b). After modification, we notice the presence of phosphorus in the adsorbent, which confirms the transition of calcium carbonate to hydroxyapatite. After mechanosynthesis, the lamellar structure derived from calcium carbonate is lost and we get the granular morphology of hydroxyapatite, presented in Figure 3. Electron scanning microscopy images showed the presence of rounded HAp microparticles in isolated and agglomerated forms. Based on the observations, HAp particles can be considered as microspheres whose crystal size is well below 1 μm.
The physical properties of the adsorbent, the specific surface area, the pore volume and the zero charge point are given in Table 6. The change in the pHPZC value occurred under the influence of the change in the surface properties of the adsorbent due to modification (Table 6). At pH <pHPZC, negatively charged species participate in electrostatic attraction with a positively charged adsorbent surface and vice versa, at pH> pHPZC, electrostatic repulsion is a major factor leading to low adsorption efficiency.   Figure 4 shows the FTIR spectra of both adsorbents (MRM and RMHAp) before and after the adsorption of malathion from aqueous solution. In the spectrum a, the characteristic peaks at 710, 856 and 1460 cm -1 indicate the carbonate group in the sample which confirm that the sample contains CaCO3. In addition, small infrared absorption spectra are shown at ~ 1790, and ~ 2874 cm -1 and have been attributed to regimens of combining different ranges of CO3 2- (Khiri et al, 2016). The spectrum at ~ 1083 cm -1 is related to the C -O tensile vibrations as CO2 adsorbed on the CaO surface (Khiri et al, 2016).
The FTIR spectrum (c) of the RMHAp adsorbent showed pronounced peaks at 560 cm -1 corresponding to the symmetrical bending regime of PO4 3and 1064 cm -1 corresponding to the asymmetric stretching regime of the PO4 3group corresponding to the vibrational structures of hydroxyapatite (Khiri et al, 2016). The large peak in Figure 4 in the spectrum a and a smaller peak in the spectrum c at 1460 cm −1 represent carbonate (CO3), which is more pronounced before  The characteristic vibration peaks of shell dust before and after modification with a comparative review by other researchers are given in Table 7. After sorption of malathion (spectra b and d) on both adsorbents, changes were observed in the appearance of new peaks, decrease in their intensity as well as in their disappearance and displacement.
The diffraction analysis (XRD) results showed that the composition of the river shell (spectrum a) mainly consists of two forms of CaCO3, primarily calcite as shown by the diffraction peak at 2 about 29. 52, 39.56, 43.27, 47.6, and 48.63 (Wei et al, 2018) and argonite (Islam et al, 2013). Other minerals are present in smaller quantities as a consequence of the uptake of these minerals from the water during shell formation. The XRD spectrum of synthesized HAp also shows relatively high intensities and sharp peaks in the range of 23-39 (about 25.80 and 32.90 (Skwarek et al, 2014) corresponding to (hkl) indices) at (002) and (300), and lower peak intensities in the range of 40-39. 60, which is consistent with the formation of the lower crystal structure of HAp.

Influence of the solution pH on adsorption
The influence of the pH value on the system is manifested through surface tension, surface properties, degree of ionization of groups present on the surface of the adsorbent, as well as through the speciation of ions in aqueous solution at a certain pH value.
The pH effect on malathion removal is presented in Figure 6. As mentioned above, malathion retention depends on the nature of the adsorbent. Removal by RMHAp adsorbent is greater than removal by MRM. Similarly to the Saib study (Bouchenafa-Saib et al, 2014), the pH value of 6 appears to be optimal for malathion sorption for both adsorbents. At this pH, H3O + ions attract surface oxygenated adsorbent groups, which could lead to the formation of a bond between H3O + and any doublet without malathion-sulfur. Below and above this pH value, adsorption is lower due to the hydrolysis of malathion at values higher than 8 and lower than 5 and the formation of ionic species with lower affinity for the adsorbent surface, i.e. precipitation contributes to ion removal.

Figure 6 -Influence of the pH value of the initial solution on malathion removal Рис. 6 -Влияние значения pH первичного раствора на удаление карбофоса. Слика 6 -Утицај pH вредности почетног раствора на уклањање малатиона
Adsorption largely depends on the solution pH so the process itself is more favorable at medium pH values. Also in the natural environment (water) at pH values lower than 5 and higher than 8, malathion easily hydrolyzes to metabolites that are more toxic than malathion itself (Bouchard et al, 2003), which is another reason why sorption experiments are performed at pH 6.

Adsorption kinetics
The effect of time on malathion adsorption was monitored in the range of 10 to 180 minutes. The final equilibrium was established after 300 minutes but, since the difference in the removal of As (V) ions from 180 to 300 minutes ranged from 3 to 7% in order to speed up the process, we took 180 minutes as the final time.
In order to determine the kinetic model that accompanies adsorption in order to interpret the adsorption mechanism, we used pseudo-first, pseudo-second-order and second-order models (Table 3). The results shown in Table 8, according to the regression coefficient (R 2 ) and the standard error for all model parameters, indicate that the kinetics for all adsorbents is best described using a pseudo-second order model.
The rate constants of diffusion kinetic models, intra-particle diffusion, Weber-Morris, Dunwald-Wagner and homogeneous solid diffusion models for malathion adsorption on MRM and RMHAp adsorbents under the same experimental conditions are presented in Table 9.
The complex nature of the kinetics of adsorption processes can be described by observing the adsorption of all ions adsorbed on the adsorbent as a single step, as described by a pseudo-second order equation, but can also be described by consecutive / competitive steps.
The Weber-Morris model reveals two linear steps that describe the adsorption process: fast kinetics in the first step and slower in the second. The first linear part describes the external mass transfer to the adsorbent surface, while the second part describes the process of material transfer into the porous structure of the adsorbent, and strictly depends on the size and shape of the pores as well as on the density of their network on MRM and RMHAp adsorbents. Intra-particle and film diffusions slow down the transport of adsorbates. In the final phase of the process, adsorption takes place slowly until saturation is achieved on the entire available surface of the adsorbent.

Adsorption activation energy
In relation to the results of the kinetic research performed at temperatures of 298, 308, and 318 K, it is possible to determine the activation energy using the Arrhenius equation (Table 10). The linear form of the Arrhenius equation (19) is: where K' is the reaction rate constant at a certain temperature, Ea shows the activation energy, R is the universal gas constant (8.314), T is the temperature in K and A is the Arenius factor (frequency for a given reaction). Physosorption or physical adsorption generally possesses energy up to 40 kJ mol -1 , while hemisorption requires higher energy and activation energy over 40 kJ mol -1 (Karanac et al, 2018). Based on the obtained results where Ea for MRM is 10.816 kJ mol -1 and for RMHAp 7.711 kJ mol -1 , we can conclude that the main mechanism of adsorption is physical adsorption.

Adsorption isotherms
The state of interactions / bonds on the surface of the adsorbate / adsorbent can be observed by fitting the experimental data with different adsorption isotherms. The normalized correlation coefficient and standard deviation were used to estimate the fit of the adsorption data.
The experimental data were compared with the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models already discussed, the parameters of which are shown in Table 11. By analyzing the experimental data on the adsorption of malathion molecules on the tested adsorbents, the best fit for both adsorbents is given by the Freundlich isothermal model. The results of modeling malathion adsorption on the tested adsorbents are given in Table 11.  According to the Freundlich isotherm, the mechanism of ion adsorption on MRM and RMHAp can be described as heterogeneous adsorption, where the adsorbed ions / molecules have different enthalpies and adsorption activation energies. The value of n from the Freundlich isotherm is a measure of adsorption intensity or surface heterogeneity. Values of n near zero indicate a highly heterogeneous surface. Values of n <1 (Table 11)  higher values indicate combined adsorption, e.g. physisorption and hemisorption with different process contributions at different system balancing steps. The values in Table 16 indicate that the adsorption was combined in all cases.
The calculation of the separation factor (RL) according to equation (20) which is based on the parameter b of the Langmir isotherm indicates the feasibility of adsorption on a given adsorbent. It is calculated using the following equation: where C0 (mol L -1 ) is the initial adsorbate concentration and b (L mol -1 ) is the Langmir constant. The value of RL indicates the feasibility of the adsorption process: irreversible (RL = 0), favorable (0 <RL <1), linear (RL = 1), and unfavorable (RL> 1). The RL for adsorption of malathion ions on MRM ranges from 0.023 to 0.204 and for RMHAp from 0.027 to 0.243 indicating that the adsorption process is favorable.
where T is the absolute temperature in K, R is the universal gas constant (8.314 mol -1 K -1 ) and the adsorption constant b is calculated using the Langmir isotherm (Table 12). ΔH 0 and ΔS 0 were calculated from the slope and the sections in the diagram ln(b) -T -1 , assuming that the adsorption kinetics values are stationary. The calculated thermodynamic parameters are shown in Table 12. Negative values of Gibbs free energy (ΔG°) and positive values of entropy (ΔS°) at all temperatures indicate that reactions in the adsorption process take place spontaneously. A decrease in the Gibbs free energy (ΔG°) with an increase in temperature also indicates that the spontaneity of the reaction increases.
Positive values of ΔS 0 indicate a tendency of greater disorder of the MRM and RMHAp surface systems and malathion solution. In Table 12, we can see that the Gibbs free energy values (ΔG°) for both adsorbents are approximate, and the positive entropy values (ΔS°) at all temperatures, while the positive enthalpy values (ΔH 0 ) for MRM and RMHAp are noticeable, which indicates the endothermic process. In general, the exchange of free energy in the case of physisorption is somewhere between -20 and 0 kJ mol -1 , for simultaneous hemisorption and physisorption between -20 and -80 kJ mol -1 , and hemisorption less than -80 kJ mol -1 . The obtained results indicate that in these cases, hemisorption and physisorption are present at the same time.

Optimization of adsorption conditions
The individual interaction and the impact of various variables in relation to different predictors were tested using the response surface methodology (RSM) as a mathematical function by commercial software design Expert 9. The mutual influence of the input variables was analyzed by analyzing the variances of ANOVA using the quadratic model of the equation shown in Table 13. A graph of the optimal conditions with respect to the input variables for removing malathion from water using RMHAp is shown in Figure 7. Conclusion MRM and RMHAP showed excellent malathion removal performance. The results of isothermal, kinetic, and thermodynamic studies suggested simultaneous physisorption and hemisorption between malathion molecules and the surface of MRM and RMHAP adsorbents during the adsorption process. The optimal parameters for the maximum malathion adsorption were: system pH value -6, adsorbent dose -100 mg L -1 , adsorption time -180 minutes, and temperature -45 °C. Adsorption was spontaneous and endothermic as described by thermodynamic parameters. The Bok-Behnken's design within the response surface method has been successfully used in the optimization of experimental adsorption conditions, the goal of optimization being to determine the optimal adsorption conditions with a smaller number of experiments. Optimization methods are maximally harmonized with the principles of environmental protection thus reducing: the number of experiments, the amount of consumed expensive and environmentally harmful chemicals, and the generation of waste. The errors and the predicted response values, derived from a mathematical model, showed acceptable results and confirmed the favorable effect of the studied factors on malathion adsorption using RMHAP. This paper investigates the sustainable use of biowaste for the treatment of water contaminated with organophosphorus pesticides, whereby the biowaste that burdens the banks of rivers is used to remove water pollutants, thus leading to a double benefit for the environment.  (8) Кључне речи: уклањање, адсорбент, кинетика, изотерме, оптимизација, пестициди, вода, речне шкољке.