Nanostructure of humic acid adsorption layer in the presence of Cs and Sr ions on the surface of waste material obtained from residue after supercritical extraction of hops

Adsorption of ionic molecules at the solid–liquid interface depends on various interactions, particles surface properties as well as the presence of the additional substance in the analyzed systems. The waste material obtained from the supercritical fluid extraction process was used as an adsorbent. The plant-based biosorbent structure was fully characterized. Then the humic acid (constituting the common interfering substance found in the environmental samples) sorption on the hop cones was investigated in the absence and presence of Sr/Cs ions under various pH conditions. Hence, the metal cation valence can influence the interactions between the colloidal system constituents and the adsorption layer structure. Determining the mutual interactions in the complex systems is important due to the potential possibility of using the waste sorbent to remove the radioactive Cs and Sr isotopes from the aquatic environment. Due to the lack of a porous structure of the sorbent and the high surface density of the charge, the metal ions are bound directly on the particles surface. The humic acid (HA) adsorption is greatly pH dependent—the largest removal was achieved under the acidic conditions (Qads = 88.69 mg/g), while the HA sorption process at pH 9 is impeded by a large negative charge deposited at the solid–liquid interface (Qads = 7.06 mg/g). At pH 3, formation of multilayer adsorption structure contributes to the effective removal of organic impurities. The metal ions valence significantly affects the humic acid binding mechanism. Divalent Sr acts like a “bridge” increasing the number of biosorbent active sites (at pH = 3 the HA adsorption increases almost twofold from 88.69 to 156.46 mg/g in the Sr ions), whereas the Cs+ ions leads to the reduction in the HA removal efficiency (Qads. = 46.31 mg/g under the same conditions).


Introduction
Till recent years due to organic and inorganic compounds the environmental pollution has been growing rapidly because of the human activity. Hence, there is a constant need to develop the methods and search for new materials capable of removing some potentially hazardous substances. For this purpose, there has been employed a number of techniques for selective removal of the heavy metal ions, dyes, pesticides and many other substances from the industrial water including coagulation, ion-exchange method, filtration, biological or chemical treatment, oxidation processes, electrolysis, solvent extraction and sorption (Pangeni et al. 2014). A separate but no less important group are substances that can emit potentially harmful ionizing radiation. Therefore, it is necessary to make the attempt to find an effective and economically acceptable method for removing largely soluble radiocontaminants from the environment. Hence, the adsorption of the above mentioned pollutants on the solids surface seems to be one of the most promising methods.
Cesium is an element which belongs to the lithium group. There are many unstable isotopes of this metal, however, the most significant are Cs-134 and Cs-137. Their existence in the natural environment is caused by the nuclear weapon testing or accidents in nuclear plants. Their transformation is often accompanied by the gamma radiation emission (Ashraf et al. 2014;Shaw 2007;Burger and Lichtscheidl 2018). Due to its long half-life equal to 30.17 years and its high solubility in water (and consequently, large environmental mobility), Cs-137 is considered to be one of the principal radiocontaminants in the radioactive wastes. In turn, Sr-90 (halflife of 28.90 years) was released into the environment mainly in a fallout from the nuclear weapons testing and the nuclear disasters due to its production as the fission product. Moreover, Sr is accumulated mostly in bones where partial replacement of the Ca atoms can take place causing bone cancer, cancer of nearby tissues, and leukemia (Sahoo et al. 2016;Larson and Ebner 1958). What is important, the great mobility of the radiocesium and the radiostrontium in the environment as well as in the food chains makes them largerly bioavailable. Their transportability is caused by good solubility in the aqueous media which means that they can be effectively absorbed by plants and animals. This transport is also facilitated by the fact that Cs is chemically similar to K and Sr to Ca which, in turn, are the essential components of the diet of many living organisms (Ashraf et al. 2014). Unstable Sr or Cs radionuclides released into the environment can be accumulated on the plants surface or be transferred to the soil, where Sr forms the colloids analogous to calcium (in this form, it is easily accessible to plants) (Mashkani and Ghazvini 2009;Martell 1959;Munthali et al. 2015).
Adsorption is a highly complex process depending on the various factors such as solution pH, temperature, solid surface functionalities, surface area, chemical character of an adsorbate. It is considered to be the best method among other techniques for the impurities removal even from the diluted aqueous solutions due to its simplicity, economy and large effectiveness. The carbon adsorbents, especially the activated carbons, are commonly used; however, their preparation is quite expensive and sometimes those solids cannot be reused with a similar yield after the regeneration process. The activated carbon surface properties depend largely on the methods of its preparation and further processing leading to the situation that individual batches of these materials may differ significantly from each other (Godlewska et al. 2017). Hence, researchers are looking for new, more accessible materials that could be used in the pollutant removal processes. Interestingly, low-cost adsorbents, often belonging to the agro-based or cellulose-based by-products are an interesting alternative for the commonly used sorbents (Zulfikar et al. 2013;Kamari et al. 2009). Such materials exhibit many obvious advantages including wide availability, biodegradability *and low price. Generally, these sorbents do not require complicated processing and can be reused which makes them eco-friendly. Cellulosebased waste materials have a loose, porous structure with the reactive functional groups (mainly carboxyl and hydroxyl ones) located on the particles surface (Dai et al. 2018;Sulyman et al. 2017;Abdolali et al. 2014). Due to their excellent ability to bind the impurities, e.g. heavy metals, pesticides and dyes, the water purification processes using low-cost adsorbents are gaining in the importance (Demirbas 2008).
The humic substances are very common in the environment causing a serious problem, especially in drinking water tanks imparting the water undesirable colour, taste and odour. They are formed mainly in a process of plant and animal tissues decomposition as constituents of the natural organic matter in soil, water and sediments. Mostly, these substances consist of large particles with a wide range of the molar masses. The humic acids exhibit a strong tendency to form the complexes with the metal ions, especially with calcium, magnesium, sodium and potassium. Moreover, they can react with various oxidizers or disinfectants used in the water purification processes, forming the hazardous compounds with the carcinogenic effect (Zulfikar et al. 2013;Kamari et al. 2009). Therefore, the investigations of the humic acid impact on the low-cost material adsorption properties can be considered as an interesting research issue.
The main objective of this paper is to investigate the possibility of using hop cones derived from the supercritical extraction process as a potential biosorbent. The literature reports many different studies emphasizing the need for removal of potentially hazardous and harmful substances from the aqueous solutions. Although, most researchers are focused mainly on the model systems containing a selected compound and a solid without any interfering substances. Since the existence of the other substances changes the mechanism of the process completely, the determination of the additional substance impact on the adsorption yield is an important topic. According to the above aspects, the discussion over the adsorption abilities toward the potentially harmful Sr or Cs radioisotopes as well as their influence on the organic compound adsorption (including the description of the mutual interactions) provide a novelty element. Moreover, the use of hop cones as the potential biosorbent in the aqueous solution purification processes has never been reported in the literature. The hops cultivation is widespread in the Lublin region. Hence, the mentioned plant material is easily available in the large quantities. What is more, the generated solid has no intended use. Therefore, determining the waste sorbent properties could contribute to its effective use in the future (e.g. in the case of an ecological disaster). This approach is in line with the principles of green chemistry. In addition, the presence of the Sr or Cs ions can greatly influence the humic acid sorption mechanism. The measurements of the systems containing more than one substance are relatively rarely described in the literature. Hence, this approach allows for better understanding the effect of the additional substances presence on the solid adsorption properties, what is of fundamental importance in the context of a better understanding of the processes taking place at the solid-solution interface providing the novelty of the presented paper. The necessity to manage the resulting by-product is also important from the environmental point of view. These measurements are extremely important in the context of the potential release of the cesium and strontium radioisotopes into the natural environment.
The research tasks were accomplished in two stages. First of all, the hop cones particles surface structure was fully characterized. Afterwards, the measurements of the humic acid and metal ions adsorption on the waste biosorbent surface were made providing the model system for the further considerations. Hence, the dependencies between the amount of the humic acid molecules removed from the solution and various factors such as the pH, the adsorbent dosage and the phase contact time were also examined. According to the obtained results, the most probable sorption mechanism at the solid-liquid interface was identified. Furthermore, the influence of the cesium or strontium ions on the humic acid removal process was studied providing a possibility of the mutual interactions description. These measurements are extremely important in the context of the potential release of the cesium and strontium radioisotopes into the natural environment. Characterization of the waste material surface was performed by the low-temperature nitrogen adsorption/ desorption method, the dynamic light scattering, the potentiometric titration and measurements of the electrophoretic mobility. Moreover, the Fourier transform infrared attenuated total reflectance (FTIR-ATR) analysis was applied to confirm the existence of typical functional groups occurring on the sorbent particle surface. In turn, the humic acid concentration after the adsorption (both in the model systems and in the presence of the metal ions) was determined on the basis of the Ultraviolet/Visible Spectroscopy (UV-Vis) spectrophotometry measurements. All tests were conducted at room temperature in the presence of 0.01 M NaCl as a background electrolyte.

Preparation and characteristics of adsorbent
Hop (Humulus lupulus) is a perennial belonging to the hemp family. This plant can be distinguished by characteristic male flowers in the form of inflorescence and female flowers as cones. Hop occurs abundantly in the natural environment, especially in Europe. It is widely known primarily as a substrate in beer industry (Schönberger and Kostelecky 2012;Verheye 2010). Cellulose, hemicellulose and lignin are the three main building components of hop cones. Only a small part of the hop cones constituents is used in beer industry, while the remaining 85% is part of the waste material left over from the process (Fărcaş et al. 2017;Lynch et al. 2016). The hop cones obtained after the bitter acids supercritical extraction process, used as a biosorbent in the experiments, was kindly provided by the Institute of New Chemical Syntheses in Puławy, Poland. Since the hops cultivation is widespread in the Lublin region, large amounts of plant material are treated to obtain bitter acids leading to the generation of significant amounts of waste in the form of the ground hop cones. Thus, the resulting by-product is cheap, locally available in large quantities and, what is more important, has not the final destination. In order to make a comprehensive analysis of the acquired solid, the plant material that has not undergone any technological treatment (Lupuli Flos) as sliced hop cones intended for making the infusions was purchased and used as a reference material.
The hop obtained after the supercritical extraction (designated as hop Puławy; HP) was washed with the distilled water multiple times in order to remove the accompanying impurities. Then the biomaterial was dried at room temperature. The same procedure was applied for the comparative material (designated as Hop_ref). The physicochemical analysis of both biosorbents were made to determine the key surface parameters of the plant material particles. The lowtemperature nitrogen adsorption/desorption method (ASAP 2020, Micromeritics Instruments) provided the information about the specific surface area and the porous structure of the adsorbents. To identify the characteristic functional groups occurring on the waste material particles surface there was used the FTIR technique (Nicolet 8700A, Thermo Scientific) in the registration range from 400 to 4000 cm −1 along with 0.5 cm −1 resolution. The potentiometric titration allowed to identify the hop cones surface charge density. Moreover, in order to make complete characteristics, the electrophoretic mobility was measured (ZetaSizer Nano ZS, Malvern Instruments). All experiments were conducted at room temperature using 0.01 M NaCl (Fluka) as a supporting electrolyte which provided the constant ionic strength. Furthermore, the waste material was analyzed using X-ray fluorescence spectrometry (Spectrometer ED-XRF type Epsilon 5, PANalytical) to identify the main inorganic components of the tested sample. The sample weight taken into consideration was 2 g. This method was also used to determine the contents of Sr and Cs in the samples after the sorption process. The loss of ion concentration in the solution after adsorption was calculated taking into account the appropriate calibration curves. Additionally, a scanning electron microscope (Quanta 3D FEG, FEI, Field Electron and Ion Co.) provided images of the biosorbents surface.

Solutions preparation
In the sorption tests, the solution of the humic acid sodium salt (denoted as HA; purchased from Sigma-Aldrich; the given molar mass was 226.14 g/mol; the density was equal to 1.52 g/cm 3 ) with a concentration of 1000 mg/L was used as the stock solution. In the further experiments concerning the metal ions influence on the anionic HA sorption on the waste material, the solutions of strontium chloride (SrCl 2 ·6H 2 O obtained from Merck) and cesium chloride (CsCl, Acros Organics) were made. Analytical grades of the above mentioned alkali metal chlorides were used to prepare the stock solutions of 1000 mg/L. All stock solutions were prepared by weighing out and dissolving a specific substance in the doubly distilled water. Fresh working solutions were obtained by diluting the proper stock solutions before the use, all the plastic Erlenmeyer flasks were carefully washed with distilled water. Moreover, the solution pH was adjusted using the NaOH and HCl solutions, both with the concentrations of 0.1 M (purchased from POCh, Poland).

Adsorption of humic acid on the biosorbent surface
At first the HA calibration curve in the concentration range from 10 to 200 mg/L in the presence of 0.01 M NaCl as the background electrolyte was prepared. Since slight changes in absorbance as a function of the solution pH were observed, the calibration curves were made for all the tested pH values (equal to 3, 6 and 9, respectively). Moreover, in order to exclude the potential influence of the Cs or Sr ions' presence on the calibration curve course, the appropriate measurements were made for a series of the solutions containing the abovementioned ions in the concentration at the levels used for further determinations.
In the adsorption tests, the samples sealed in the plastic Erlenmeyer flasks were shaken with 200 rpm for 24 h at room temperature (Unimax 1010 coupled with the thermostat, Heidolph Instruments). In order to find the relationship between the amount of the humic acid bound on the solid surface and the adsorbent dosage, two series of the test solutions were prepared. In each of them, various quantities of the crushed hop cones were added, ranging from 0.01 to 0.12 g to 10 samples containing the HA (at the concentration of 100 mg/L and pH 6). After 24 h of mixing, the sediments were centrifuged (MPW-233e, Med. Instruments). Finally, the humic acid content was determined in the quartz cuvettes at a wavelength of 400 nm (Cary 300 UV-Vis spectrophotometer, Agilent Technologies) using the redistilled water as a reference solution.
The HA adsorption on the hop cones surface as a function of the solution pH was analyzed. In the mentioned tests, the HA concentration was constant and equal to 100 mg/L, the adsorbent dosage was 0.05 g, whereas the pH values were in the range from 2 to 9. In order to assess the adsorption capacity of the hop cones after the supercritical extraction process, identical measurements were made for the plant material without any technological treatment (the sample weight equal to 0.05 g). Subsequently, the influence of the cesium or strontium ions presence on the humic acid removal efficiency with the both analyzed adsorbents was also investigated. In these systems, the concentration of the HA and the corresponding ion was equal to 100 mg/L, and the weight of the used solids was invariably 0.05 g. It is worth mentioning that the HA adsorption was not tested above pH = 9 due to the possible strontium hydroxide precipitation which, in turn, would distort the obtained results. Therefore, in the remaining experiments the pH does not exceed this value. In order to make a comparative analysis, a similar procedure was taken in the systems containing the cesium ions. The contact time of both phases was 24 h.
The results obtained in the aforementioned measurements allow the selection of the optimal parameters for the HA binding at the solid-liquid interface with the use of both biosorbents under the equilibrium conditions. This leads to the determination of the HA adsorption isotherm on the solid particles surface. At the beginning the measurements were made in a model system containing only the background electrolyte (NaCl with a concentration of 0.01 M), 0.02 g of the ground hop cones and the HA in the concentrations range from 20 to 200 mg/L. The prepared suspensions were shaken for 20-24 h achieve the equilibrium state. Subsequently, the samples were centrifuged and the HA concentration was determined spectrophotometrically as stated previously. The humic acid adsorption isotherms were made for the systems at pH = 3, 6 and 9, respectively. As in the previous analyses the measurements were made for both sorbents-the waste material obtained from Puławy (HP) and for the hop cones without any technological treatment (Hop_ref). Moreover, in order to determine the effect of the cesium or strontium presence on the HA binding on the solid particles surface, the adsorption isotherms were obtained in an analogous manner in the systems containing a constant concentration of the inorganic ions (equal to 100 mg/L of the tested ion in each sample) in the solutions at pH 3, 6 or 9. The HA concentration after the sorption process and the removal efficiency were calculated using the following equations: where R (%) is the percentage of the removed HA, Q ads (mg/g) is the amount of the adsorbed HA, C 0 (mg/L) is the initial HA concentration, C e (mg/L) is the equilibrium concentration, V (L) is the volume of solution, M (g) is the dosage of adsorbent. (1)

Measurements of surface charge density
The adsorbent surface charge density measurements were made by means of the potentiometric titration. The sample containing 0.08 g of the biosorbent and 50 mL of NaCl with a concentration of 0.01 M was shaken for 22-24 h to equilibrate the system. After reaching the desired state, the sample was poured into a Teflon vessel connected to a thermostat. Next 0.1 M HCl was added to obtain a pH in the range of 3-3.5. The sample was titrated with the NaOH solution (0.1 M). The process was performed while maintaining the constant temperature (25 °C). The surface charge density calculations were based on the program created by Prof. Władysław Janusz from Maria Curie-Skłodowska University in Poland. The density of surface charges was obtained using the following equation: where σ 0 (mC/cm 2 ) is the surface charges density, ∆V is the difference between the NaOH volume and the supporting electrolyte volume, c (M) is the molar concentration of NaOH, F is the Faraday constant, m (g) is the dosage of adsorbent, S (m 2 /g) is the specific surface of the added biomaterial.

Determination of the zeta electrokinetic potential
At first the suspension containing 0.03 g of the ground hop cones dispersed in 0.01 M NaCl was prepared. Then the sample was sonicated for about 3 min. Subsequently, the obtained suspension was divided into separate beakers and the desired pH value in the range from 2 to 9 was adjusted separately in each sample. An immersion cell with the platinum electrodes was used in the measurements. Before each analysis, the cell was washed several times with the obtained suspension. The zeta potential values were calculated by the computer program based on the recorded electrophoretic mobility of the individual samples using the Smoluchowski equation. The final result was the average obtained from 6 measurements.

The adsorbent characteristics
The surface morphology of the hop cones after the supercritical extraction was analyzed by the scanning electron microscope (Fig. 1). The tested biomaterial is practically nonporous and heterogeneous. The tubes channels visible  Fig. 1 are the plant cellular structures remnants and do not form a regular porous structure of the solid particles. It can be seen that they appear in different shapes and sizes. This is a characteristic feature of the waste sorbents. In the plant-based materials, the building elements belonging to a specific part of the plant are very often observed in the scanning electron microscope (SEM) pictures.
The X-ray fluorescence spectrometry analysis provided the information about the low-cost biomaterial elemental content. After the technological treatment of the hop cones, the obtained results were as follows: SiO 2 -0.644%; P 2 O 5 -1.861%; S-0.735%; Cl-0.188%; K 2 O-39.169%; C a O -1 5 . 2 5 8 % ; T i -0 . 0 0 9 % ; M n -0 . 0 5 1 % ; Fe 2 O 3 -0.108%; Cu-0.056%; Zn-0.028%; W-0.061%. The elements such as Hg, Se, Sr and Cs were not detected. Moreover, the effect of the material washing on the hop cones elemental content was not reported. The remaining 41.832% of the analyzed sample consists of the following elements: C, Na, H, O and N which are not registered by the equipment used for the measurements. The X-ray fluorescence (XRF) measurements of the samples after the adsorption of metal ions or HA do not show any changes in the content of the above-mentioned elements. The analysis of the low temperature nitrogen adsorption/desorption results indicates that the hop cones waste material is characterized by a small specific surface area below 1 m 2 /g and a negligible pore volume of 0.0028 cm 3 /g. Moreover, the absence of the hysteresis loops on the sorption/desorption curves causes that the biomaterial can be considered as practically nonporous. The obtained results are consistent with the previously discussed data and do not exclude the possibility of using the ground hop cones as an effective sorbent in the contaminants removing process from the aqueous solutions. It should be clearly emphasized that the ability to bind the specific substances on the given material particle surface is related not only to the developed porous structure. The functional surface groups presence in the low-cost sorbent structure is a no less important aspect.
In order to identify the adsorbent surface functional groups, the FTIR-ATR spectrum was studied (Fig. 2). A wide peak with the maximum at 3290 cm −1 was assigned to the stretching vibrations of O-H bonds present in the cellulose, hemicellulose, lignin and the absorbed water molecules. This intensive absorption band in the 3280-3370 cm −1 range refers to the symmetric and asymmetric N-H stretches in NH 2 groups can be overlapped with the broad absorption band of ν (OH) of the adsorbed water molecules. Moreover, in this range the stretching vibrations of the terminal O-H groups belonging to the SiO 2 moieties located in the biomass-based sorbent structure are also revealed (Tran et al. 2013;Abderrahim et al. 2015;Socrates 2004;Tezcan and Atici 2017). In turn, two bands with the maximum at 2921 cm −1 and 2852 cm −1 can be related to the stretching vibrations of C-H present in the cellulose, hemicellulose and lignin structure (Abderrahim et al. 2015;Tezcan and Atici 2017). The peak from the wavenumber range 1745-1730 cm −1 with the maximum at 1731 cm −1 corresponds to the ester bonds vibrations of the acetyl or uronic groups in the hemicellulose structure (Tezcan and Atici 2017). In addition, this peak could be also ascribed to the stretching vibrations of C=O in the cellulose (Socrates 2004). Furthermore, the peak observed at 1625 cm −1 is assigned to the O-H deformation vibrations of the absorbed water molecules (Socrates 2004;Tezcan and Atici 2017) as well as the band related to the δ (NH 2 ) bending. An absorption band visible in the wavenumber range 1520-1510 cm −1 can be referred to the aromatic rings vibrations present in the lignin structure. Its intensity depends on the amount of the lignin in the analyzed biomaterial. This weakly intense peak reaches the maximum at 1512 cm −1 (Tezcan and Atici 2017). The signals corresponding to the deformation vibrations of the C-H bonds in the lignin structure were observed at 1480-1440 cm −1 . The spectrum shows a peak with the maximum of 1425 cm −1 which could be assigned to the inplane bending vibrations of -CH 2 groups found in the cellulose structure (Goletti et al. 2015). These two vibration bands overlap each other slightly. Another visible signal that can be observed is the peak with the maximum at 1235 cm −1 indicating the Si-O bond vibrations in silica (Oh and Choi 2010). The high-intensity band at 1027 cm −1 is related to the characteristic C-O bonds vibrations of secondary alcohols and ethers present in the main chain of the cellulose macromolecules (Abderrahim et al. 2015). It is worth mentioning that this could be also assigned to the stretching vibrations of the C-O and C-C bonds as well as the C-OH deformation vibrations occurring in the xylan units belonging to the hemicellulose structure (Goletti et al. 2015).
The SiO 2 moieties' presence in the analyzed low-cost material was confirmed by the XRF measurements. In the recorded FTIR-ATR spectrum a peak at 1027 cm −1 can be also assumed as the Si-O-Si stretching vibrations (it should appear in the form of a wide band in the typical range of 1000-1200 cm −1 ). It can be assumed that silica in the tested biomaterial occurs in the form of single grains resulting in the absence of polymerized structures. In the vicinity of 890 cm −1 , a slight change in the absorbance can be seen (resulting from the Si-O-Si bending vibrations absorption band). This probably comes from the Si-O groups vibrations. Its low intensity confirms a small amount of the colloidal silica in comparison with the total amount of C, O, N and H proved by the XRF method. Additionally, the bands related to the silica content in the hop cones can be overlapped by the signals derived from the vibrations of the β-glycosidic bonds occurring in the cellulose structure (Kumar et al. 2014). On the basis of the FTIR spectrum analysis, it was found that the hydroxyl and carboxyl functional groups are the main functional groups in the analyzed waste sorbent. Nevertheless, the presence of silica in the form of small, particulate grains can affect largely the adsorption behaviour of the tested solid.
The presence of the above-mentioned functional groups on the solid particles surface will determine the course of the adsorption process as well as the sorption properties of the waste biosorbent. It should be mentioned that the solution pH plays an important role leading to the formation of charges as a result of dissociation or protonation of appropriate surface groups (depending on the pKa constant value characteristic for a given moiety). At the acidic pH, the carboxyl groups undergo the electrolytic dissociation resulting in the appearance of the negatively charged adsorption centers located on the adsorbent particles surface. Under these conditions, the nitrogen or silicon-containing surface groups can attach a hydrogen ion acquiring the positive charge. As the pH increases, the further acidic groups are dissociated, whereas the nitrogen and silicon containing moieties become deprotonated. On the other hand, the functional groups composed of the hydroxyl group (e.g. secondary alcohols in the cellulose/hemicellulose macromolecules) are able to interact with the adsorbate due to the formation of the hydrogen bonds in a wide range of the solution pH. The particle fragments containing the hydrocarbon chains or the aromatic rings contribute to sorption through the π-π interactions with the adsorbate molecules.
In order to make a more comprehensive analysis of the selected waste sorbent surface properties, the electrokinetic measurements including the surface change density and the zeta potential tests were conducted. The above mentioned results are presented in Fig. 3. As it can be seen, the surface charge density (σ 0 ) values did not change in a wide range, although they are even 1000 times larger compared to those recorded for the oxide materials. This is directly related to the presence of significant amounts of the oxygen and nitrogen surface groups capable of obtaining a charge in the aqueous solution. In the pH range from 3 to 9 the hop cones particles are negatively charged due to the accumulation of negative charges on the biomaterial surface. At pH = 3, the surface charge density value was estimated in the range from − 2 to -3 mC/m 2 . Above pH = 4, the σ 0 value drops significantly which, in turn, is related to the dissociation, among others, of the carboxyl and hydroxyl groups. The pK a of the mentioned functionalities varies in a wide range depending on the structure position and substituent type (Serjeant and Dempsey 1979;Hair and Hertl 1970). In addition, the analyzed waste material contains the silica groups capable of acquiring the surface charge by interactions with the potential determining ions. According to the literature, the pK a values equal to 4.5 and 8.5 were ascribed to the isolated silanol (≡Si−OH) and the germinal silanols (= Si−(OH) 2 ), respectively (Kim et al. 2014;Leung et al. 2009). In this case, the increase in the solution pH proves the silanol groups dissociation resulting in the surface charge density reduction. The previously discussed data are confirmed by the zeta potential measurements.
The surface charge density curve course analysis (Fig. 3) indicates that the point of zero charge (pH PZC ) is not observed and only the isoelectric point (pH IEP ) is noted. At this solution pH, the total amounts of the positive and negative charges in the solid particles diffusion layer are Fig. 3 Electrokinetic results (zeta potential and surface charge density) of the hop cones after the technological treatment equal to each other. It is worth mentioning that the amount of the above mentioned charges is closely associated with the charge of the surface functional groups forming the sorbent active sites. As can be seen from the zeta potential data obtained for the low-cost biosorbent, at pH = 2.3 the hop cones particles exhibit the slightly positive electrokinetic potential (equal to 0.29 mV). Therefore, it can be concluded that at pH < 2 the solid particles will also have a positive charge. Analyzing the given dependencies, the presence of the amine groups located on the adsorbent surface cannot be excluded. These surface functionalities, depending on the number of the substituents on the nitrogen atom, can create the positively charged clusters up to pH = 10. According to the fact that the zeta potential and the surface charges density values are the resultant of many factors, the number of the solid surface active groups containing the nitrogen atoms cannot be accurately determined using the mentioned techniques. Hence, on the basis of the measurements, it can be assumed that in the analyzed pH range (from 3 to 9), the tested waste material surface is negatively charged.

Sorption of humic acid on the hop cones surface
The first step of the studies includes the investigations of the waste material sorption properties in relation to the samples containing only the humic acid (HA) molecules. The dependency between the plant adsorbent dosage and the efficiency of the humic acid removal from the aqueous solution is presented in Fig. 4. As can be seen, the increase in the biomaterial mass added to the system leads to the increase in the amount of the organic molecules accumulated at the solid-liquid interface. It should be mentioned that under the measurement conditions (at pH = 6) humic acid belonging to dicarboxylic acids is completely dissociated. The more solid particles are present in the analyzed system, the greater number of the adsorption surface sites becomes accessible to the HA molecules to bind to. Therefore, the observed removal process efficiency is quite high reaching almost 60%, despite the fact that under these pH conditions the plant-based sorbent particles are negatively charged (according to the electrokinetic data). Hence, the obtained results confirm evidently the humic acid adsorption on the hop cones surface. Based on the collected data, it can be also concluded that the analyzed waste biosorbent contains a number of the amine groups. Although the interacting suspension components are negatively charged, the adsorption process takes place. Obviously, one of the factors responsible for the HA molecules binding on the solid surface is the hydrogen bonds formation. Nonetheless, the electrostatic attraction forces occurrence between the positively charged amine moieties and the oppositely charged humic acid molecules are another driving force contributing to such a great organic substance removal efficiency from the aqueous solutions. Figure 5 shows the dependence of the removed HA amount with regard to the samples containing different solid weights, additionally taking into account the comparison between the two plant adsorbents-the hop cones after the technological treatment (HP) and the raw material (Hop_ref). It can be observed that the amounts of humic acid removed from the aqueous solutions are comparable for the samples containing larger adsorbent masses (the HA removal at the level of 60% in the samples with 0.12 g of the analyzed solids). In turn, when the solid mass is smaller, more HA is bound on the particles surface in the systems where the untreated plant material is used. As the weight of the solid increases, both curves begin to overlap. This proves that the commercially available hop cones, which do not undergo any technological processing, contain a greater number of oxygen and nitrogen surface functionalities on their surface in comparison to the material obtained after the supercritical extraction process during which some of the plant constituents were extracted. In the latter case, a smaller number of the surface functional groups can be directly related to the slightly smaller ability of the waste biosorbent to bind the organic acid molecules. Therefore, it can be concluded that for the samples containing the small plant sorbent weight, the raw hops is a more effective adsorbent. On the other hand, when a larger solid mass is added to the measurement system, the adsorption proceeds effectively for both materials. The presented results confirm that the analyzed low-cost material is a promising candidate as an adsorbent for the organic impurities removal, even those negatively charged.
The aqueous solution pH is an essential parameter significantly influencing the adsorption process course. Therefore, the subsequent attempts focus on determining the solution pH impact on the humic acid adsorption on the biomaterial surface. As it was mentioned earlier, the analyzed organic substance belongs to the dicarboxylic acids group and its functional groups can dissociate in the aqueous solutions. Increasing the solution pH induces the rapid growth of the HA molecules dissociation degree. Assuming that the dissociation constant value for the -COOH groups in the humic acid molecule structure is equal to 4.4 (which is a typical value for most carboxyl groups) and using the Henderson-Hasselbalch equation, the HA dissociation degree can be simply calculated. Hence, at pH = 3 the humic acid molecules are dissociated at the level of less than 4%. In turn, at pH = 4.4, the HA dissociation degree is equal to 50% denoting that the dissociated as well as undissociated organic molecules occur simultaneously in the solution. Above pH = 4.4, the previously mentioned parameter increases readily reaching 97.6% at pH = 6. Therefore, the solution pH influences on the humic acid adsorption on the low-cost sorbent as it is shown in Fig. 6. Regarding the solid technological treatment, the greatest adsorption efficiency was recorded under the acidic pH conditions reaching the 90% of the HA removal efficiency at pH 3. A noticeable decrease in the removed HA amount with the solution pH increase can be observed both in the case of the samples containing the by-product after the supercritical extraction process and in the systems, where the commercially available hop cones were added. At pH = 6, only about 30% of the initial HA concentration was bound to the biosorbents surface. This value is three times smaller compared to the data at pH = 3. In addition, such situation can be attributable to the electrostatic repulsion between the negatively charged hop cones surface and the negatively charged HA molecules (Kamari et al. 2009). However, at pH > 7, the curves measured for the both sorbents overlap. Under these conditions, a slightly worse HA binding efficiency is achieved in the systems containing the HP material. Furthermore, at pH = 9 the amount of the humic acid removed from the solution is the smallest of all the investigated systems (13.70% and 23.50% for HP and Hop_ref, respectively). The observed changes in the adsorption behaviour come from the differences in the technological processing of the both biosorbents. It is clearly visible that under these pH conditions, the plant material without an advanced technological treatment is characterized by a higher sorption efficiency. This is related to the presence of the oxygen and amine groups located on the particles surface capable of HA binding. Noteworthy is the fact that in the pH range from 3 to 6, both cellulose-based sorbents achieve quite good adsorption yields as regards the humic acid molecules making them promising candidates for the use in the water purification processes.
On the basis of the previous measurements, the optimal plant material weight of 0.02 g was used in the humic acid adsorption isotherms tests. The adsorption curves analysis provides valuable information concerning the potential interactions between the system components. Therefore, the amount of the adsorbed humic acid as a function of its concentration in the solution after the adsorption process was investigated. The results obtained for the hop cones after the technological treatment at pH = 6 are presented in Fig. 7. There are many known adsorption isotherms (Henry, Langmuir or BET (Brunauera-Emmetta-Tellera) equations are the most commonly applied), but they are mainly used in the case of the model systems. Practically, for the real and complex samples matching the data to the only one specific type of the isotherm is relatively rare. Analyzing the adsorption results it can be concluded that in the case of the first few points the curve course is typical of the Langmuirshape isotherm while its further course is similar to the BET one. Based on the collected data in the initial concentration range, the formation of a monolayer composed of the HA molecules adsorbed on the solid particles surface can be observed. However, increasing the initial concentration of the organic acid results in the multilayer formation. Therefore, the obtained adsorption isotherm is combination of the two types mentioned before. This phenomenon can be explained by the fact that in the systems containing the small HA concentrations, the organic molecules accumulate freely on the sorbent surface interacting directly with the solid surface groups. This initial concentration range corresponds to the Langmuir isotherm course. However, in the case of higher HA concentrations, only an amount of the molecules can be directly bound on the solid surface mainly due to the limited number of the appropriate surface functional groups. Some of them will be adsorbed on account of the hydrogen bonds formation with the previously bound molecules creating the multilayer structure as evidenced by the lack of a distinct plateau region on the adsorption isotherm curve.
Since the solution pH influences largely the amount of humic acid adsorbed at the low-cost biosorbent-aqueous solution interface, the adsorption isotherms at pH = 3 and pH = 9 were also determined. The results obtained for the HP and Hop_ref sorbents are shown in Figs. 8 and 9. The analysis of the data obtained for the technological by-product (Fig. 8a) indicates the formation of the humic acid multilayer on the biomaterial surface at pH = 3. Taking into account that the adsorption isotherm corresponds to the BET multilayer theory, it can be concluded that in the low HA concentrations range, the adsorption isotherm is Langmuir-shaped (likewise at pH = 6 in the similar concentration range). In this case, the humic acid molecules form a monolayer on the sorbent surface. Increasing the HA content in the tested solutions favours the addition of the subsequent HA molecules to the previously created adsorption layer. Comparing the presented curves, the differences in the humic acid adsorbed amount are also distinctly visible. The highest HA adsorption is reached under the acidic conditions (max. Q ads = 88.69 mg/g) whereas at Fig. 8 Effect of solution pH on humic acid adsorption isotherms on a hop cones after the supercritical extraction (HP); b commercially available hop cones (Hop_ref). Experimental conditions: 0.01 M NaCl, contact time 24 h, adsorbent dosage 0.02 g pH = 9 the adsorption is minimal and the isotherm has the almost flat course (max. Q ads = 7.06 mg/g). According to the collected data the most plausible humic acid adsorption mechanism has been proposed. At pH = 3 the biosorbent surface is negatively charged, although a number of the positively charged groups (e.g., the amino ones) is also present on the solid particles surface. At the same time, virtually all the HA molecules exist in the solution in the undissociated form. Hence, the hydrogen bonds formation between the system constituents is one of the main factors responsible for the sorption process. Moreover, the electrostatic attractions between the HA dissociated carboxyl groups and the solid amine moieties can be also considered as a driving force for the humic acid binding at the solid-liquid interface. It is worth mentioning that under these conditions, the positively charged silicon groups will play an important role in the HA adsorption. Hence, at pH 3 all the mentioned processes will lead initially to the formation of the monolayer composed of the HA molecules adsorbed straightforward to the biosorbent surface. Subsequently, a further increase in the HA concentration in the samples leads to formation of the multilayer structure, in which the adjacent molecules are linked to each other by the hydrogen bonds or interact via the van der Waals forces. As a result, the multilayer formation is considered to be responsible for the high humic acid adsorption on the ground hop cones. Increasing the solution pH induces negative charges formation on the particles surface due to the surface functional groups dissociation. Above pH 6, both the sorbent and the humic acid molecules are therefore negatively charged. Under such conditions, the humic acid binding mechanism on the waste sorbent surface is mainly based on the hydrogen bonds formation. In turn, at pH = 9 the electrostatic repulsion forces between the similarly charged biomaterial surface and the completely dissociated humic acid molecules are predominant contributing to the distinct reduction in the HA adsorption efficiency. It must be emphasized that in the entire studied pH range, the π-π interactions related to the delocalized electrons existence in the HA molecules as well as the solid surface can also occur providing the organic acid binding.
In order to make a comparative analysis of the investigated low-cost material adsorption behaviour, the analogous measurements were made for the commercially available hop cones (Fig. 8b). As it can be seen, the adsorption curves order was maintained-the largest humic acid adsorption was reached at pH = 3 (Q ads. = 81.29 mg/g), whereas the smallest is recorded for the system at pH = 9 (Q ads. = 11.58 mg/g). The analysis of the collected data reveals that at both pH = 3 and pH = 6, initially the adsorption curves can be assumed to have the Langmuir isotherm shape while at higher HA concentrations they correspond to the BET model. Comparing the results shown in Fig. 8a, b, significant differences in the amount of humic acid adsorbed on the surface of two plant sorbents differing in the origin were found. In the case of the material constituting the byproduct after the supercritical extraction process, at pH = 3 as the HA concentration rises, the amount of the removed organic substance also increases. This demonstrates the improved adsorption efficiency under these pH conditions. In turn, the hop cones purchased from the local pharmacy exhibit a little bit smaller affinity for the organic substance binding. The explanation of this phenomenon is related to the origin of both materials. First and foremost, the untreated sorbent can comprise on its surface more functional groups, as regards the nitrogen containing ones. Under the discussed pH conditions, they are positively charged. Hence, the adsorption of the undissociated humic acid molecules can be partially impeded, especially in the areas with their large accumulation. On the other hand, the larger organic matter content can result in fewer silica groups on the adsorbent grains surface. Interestingly, at pH = 6 there was observed the opposite trend-the raw material binds more particles of the already mentioned dicarboxylic acid, particularly in the systems containing the larger HA content compared to the technological by-product (Q ads. = 15.45 mg/g and 18.32 mg/g for HP and Hop_ref, respectively). The reason is a greater number of the positively charged groups (including amine moieties) on the solid surface capable of the electrostatic interactions and favouring the organic acid adsorption. In order to confirm the above conclusions, the adsorption isotherms at pH = 9 for both biosorbents were Fig. 9 Comparison of adsorption isotherms obtained for both biosorbents at pH = 9. Experimental conditions: 0.01 M NaCl, contact time 24 h, adsorbent dosage 0.02 g also investigated. Analyzing the curves course (Fig. 9), the isotherm shape indicates that in the studied concentration range the multilayer adsorption takes place for both the raw hops and waste material. Similarly, at pH = 6 the commercially available hop cones are a more effective adsorbent for the humic acid removal (Q ads. = 7.06 mg/g and 11.58 mg/g for HP and Hop_ref, respectively). This follows from a greater number of amine groups that can potentially bind the HA molecules due to the presence of the electrostatic attraction interactions. The above mentioned processes such as the hydrogen bonds formation, the electrostatic forces as well as the π-π type interactions are responsible for the HA adsorption at pH = 9. The humic acid binding capacity measurements on the biosorbent surface provided promising results. Confirming that the waste material formed as a by-product in the bitter acids supercritical extraction can be successfully used as a sorbent the humic acid removal from the diluted solutions. Therefore, further measurements were made using only the hop cones remaining after the technological treatment.

Influence of strontium and cesium ions on the humic acid adsorption
At first, the measurements including the metal ions sorption on the hop cones surface were performed (Fig. 10). As follows, the amount of adsorbed Sr ions depends insignificantly on the solution pH-the smallest amount of the Sr ions removed from the solution is found at pH = 3, whereas for the samples containing the Cs ions this value remains almost constant (reaching 80%). This is related to the fact that under the acidic conditions, the positively charged functional groups exist on the hops surface. Hence, the electrostatic repulsion forces impede the divalent Sr ions binding. In the pH range from 4 to 9, the solid surface becomes negatively charged leading to the effective metal ions removal. In the case of the adsorption kinetic measurements (Fig. 10b), the Cs/Sr ion removal amount does not change as a function of the phase contact time. This shows that the kinetics of the sorption process is immediately established and the equilibrium state is achieved very quickly which can be explained considering two independent phenomena. First of all, this is related to the large charge density of the solid particles providing the presence of a large number of active sites on their surface. The second reason is the lack of a porous structure which causes sorption only on the particle surface contributing to the faster establishment of the equilibrium time. The sorption isotherms of Cs or Sr ions on the hop cones surface are shown in Fig. 10c. The analysis of the collected data shows that the curves obtained for cesium and strontium have a shape similar to the Henry isotherm. As the metal ions concentration increases, the increase in the sorption amount is observed in the studied concentration range. Differences in the amount of the metal ions adsorbed on the biosorbent surface result from the ion valence. The divalent Sr ion is more difficult to be ion exchanged than the monovalent cesium one. In the case of strontium, more active sites are needed on the sorbent surface in order to bind it.
Subsequently, the solution pH effect on the humic acid removal in the presence of Sr or Cs ions was examined ( Fig. 11a-c). The metal ions concentration was set at 100 mg/L. As it was reported in the previous experiments, the HA sorption efficiency decreases with the increasing solution pH value. Interestingly, throughout the analyzed pH range, the Sr ions addition causes slight improvement in the humic acid adsorption. In turn, the presence of the Cs ions induces the opposite reaction. Such differences are related to the valence of the studied metal cations. Both cesium and strontium in the aqueous solutions exist in the solvated ions form (except for pH > 9 where strontium can precipitate as a hydroxide). It should be also noted that throughout the studied pH conditions, the biomaterial surface acquires a negative charge. Thus, the metal cations, such as Sr and Cs, are able to interact electrostatically with the negatively charged low-cost adsorbent surface. These assumptions are in agreement with the data published elsewhere. Mashkani and Ghazvini (2009) observed that Sr or Cs ions binding on the biosorbent surface was a rapid process reaching the saturation within 60 min which was typical of the metal ions uptake. In the case of the Cs + sorption on the persimmon tannin powder (Gurung et al. 2013), the adsorption equilibrium was attained within a few minutes of the two phases mixing, which was associated with a large number of the polyphenolic groups located on the solid particles. Similar conclusions are published by Ahmadpour et al. (2010). As follows from this paper, short time needed to reach the adsorption equilibrium state can be related to the great accessibility of the sorbent surface functional groups and the electrostatic interactions between the suspension constituents. However, the presence of the additional substance can interfere with the mutual interactions significantly. Therefore, the metal ions impact on the humic acid adsorption on the hop cones surface was investigated.
The divalent Sr ions can combine with the hop cones particles functional groups, and also with the carboxyl groups belonging to the humic acid molecules resulting in formation of the additional active sites on the adsorbent surface. It can also be seen that despite the strontium ions presence, the dependence of the HA removal amount on the solution pH is maintained. As it was mentioned before, in the whole measured pH range the biomaterial surface is negatively charged. In this case, the divalent strontium cations will adsorb to a larger extent on the solid particles due to the electrostatic attraction forces. As a consequence, the lowcost material surface groups are no longer accessible to all humic acid molecules present in the tested solution. Therefore, the decrease in the HA removal with the simultaneous solution pH increase is observed even in the presence of Sr ions. Nevertheless, the favourable effect of Sr influence on the HA binding can be also seen in Fig. 11. However, this differs in the case of Cs ions. A monovalent ion can interact with either the adsorbent surface group or one of the humic acid carboxyl groups separately. It can be concluded that the previously adsorbed cesium ions block the active sites on the analyzed low-cost biosorbent surface contributing to the slightly reduced adsorption efficiency of the organic substance.
The isotherms of the HA adsorption on the waste material surface in the presence and absence of Sr and Cs ions are shown in Fig. 11. As follows, the curves order and shape in relation to the analyzed solution pH values are maintained for both graphs as is the case of the measurements in the metal ions absence. Likewise the previously obtained data, the presented isotherms do not have the typical Langmuir isotherm shape. The plateau is observed indicating the HA molecules monolayer formation to which the subsequent adsorbate layers become attached. Regardless of whether the metal ions are present in the system or not, the isotherms shape resembles the BET curve more closely. The obtained data indicates clearly that the strontium and cesium ions presence influenced largely the humic acid adsorption. Irrespective of the analyzed pH value, there was reported the improvement of the HA removal in the samples containing the divalent Sr cations (compared to the systems where only the organic compound undergoes binding at the analogous pH). In turn, the Cs ions addition contributes to the opposite situation in which the amounts of the humic acid bound on the solid particles are evidently smaller. These differences can be explained by the valence of the both ions. At pH = 3 (Fig. 11a) the undissociated carboxylic groups as well as the hydroxyl ones dominate on the low-cost adsorbent surface. Under these conditions, the divalent Sr ions are able to create various complex combinations, particularly with the carboxyl groups forming the coordination bonds. Moreover, the above mentioned metal ions can interact electrostatically with the negatively charged moieties or the −COO − groups belonging to the HA molecules. In such a system, the alkali metal ion sorption on the biosorbent surface can be considered as a kind of bridging between the solid surface and the humic acid molecules. Such a mechanism results in the HA adsorption efficiency improvement (almost twofold increase in the HA adsorbed amount from 88.69 to 156.46 mg/g in the Sr ions absence and presence, respectively). The opposite situation is observed in the samples containing the cesium ions at pH = 3. These monovalent cations are not able to form such a large number of the complex connections. The larger ion radius (in comparison to Sr) contributes to a larger sorbent area occupancy. It can be assumed that in the mentioned circumstances the competition between the cesium cations and the H + ions as well as the HA molecules for the adsorbent active sites occurs resulting in the reduction of HA adsorption (Q ads. = 46.31 mg/g). Interestingly, the cesium ions binding is more favourable due to the fact that the electrostatic interactions are much stronger than the Fig. 11 Adsorption isotherms of humic acid on hop cones after the supercritical extraction (HP) in the presence and absence of metal ions at a pH = 3; b pH = 6; c pH = 9. Experimental conditions: 0.01 M NaCl, contact time 24 h, adsorbent dosage 0.02 g hydrogen bonds existing between the HA molecules and the waste material surface.
It can be also seen that the presence of strontium ions results in the increase in the amount of the adsorbed humic acid at pH = 6 and 9 (Fig. 11b, c). However, the process efficiency was smaller compared to the analogous samples at pH = 3. This is related to the dissociation of the humic acid carboxylic groups which are completely dissociated at pH = 6, whereas at the same time the tested plant material surface is negatively charged. Under these conditions, the weak hydrogen bonds and the interactions of delocalized electrons were responsible for the humic acid adsorption. At the same time, the amount of adsorbed HA molecules in the presence of Sr ions is almost three times larger compared to the system in the absence of metal cations (Q ads. = 42.56 mg/g and Q ads. = 15.45 mg/g, respectively). The addition of strontium ions causes the formation of electrostatic connections between the sorbent surface and the HA molecules resulting in better organic substance binding effectiveness. In turn, in the case of the cesium ions presence, the situation is analogous to that at pH = 3 where the HA adsorption is impeded by the competition between the sample constituents (Q ads. = 10.47 mg/g). Based on the above findings it can be concluded that the strontium ions can improve the HA removal regardless of the solution pH reaching the greatest efficiency in the acidic solutions. Meanwhile, the Cs ions preferential binding at the solid-liquid interface and the resulting blocking of the sorbent active sites contribute to the decrease in the humic acid adsorption at pH 3 and 6 as well as the slight adsorption improvement under the alkaline conditions. Further, the impact of the Sr/Cs ions' concentration on the humic acid removal from the tested solutions was examined. The results are shown in Fig. 12. It was found that both curves are characterized by a different course. In the case of the samples containing humic acid and cesium, an almost rectilinear curve was obtained with slight deviations which may have been due to the measurement error. Thus the Cs ion concentration does not influence the amount of the HA adsorbed on the biosorbent surface. Therefore, it can be concluded that the Cs cations binding on the hop cones particles confines the access to the solid active sites causing that only a small number of organic molecules is able to adsorb on such a surface. On the other hand, in the systems with the Sr ions addition the opposite trend was observed. The larger the Sr concentration, the greater the HA adsorption was measured. Initially, there was a slight increase in the HA adsorption amount but after exceeding the strontium concentration equal to 150 mg/L, the effect became more and more evident. The reason is the number of mutual interactions formed between the solution constituents. When the amount of the divalent metal cation present in the sample is small, the number of possible connections between the humic acid molecules and the biosorbent surface covered with Sr ions is also small. It should be mentioned that the measurements were conducted at pH = 6. Under such conditions, the waste sorbent surface is negatively charged and the HA molecules are completely dissociated. As the Sr concentration increases, the amount of bound organic compound also increases due to the fact that the metal ions can act as a "bridge" between the similarly charged biomaterial surface and the HA molecules. It can be also found that the shape of the curve obtained for the strontium-containing system with a characteristic inflection point is similar to the adsorption isotherm noted for the analogous samples. Thus it can be concluded that strontium ions facilitate the humic acid molecules sorption. It is worth mentioning that in the system under consideration the complex connections between the organic compound molecules from the solutions and those previously adsorbed can also appear resulting in formation of multilayer structures. Based on the cesium valence, the above structures would not be formed.
In order to compare the obtained results, there is presented the following summary (Table 1). As follows, the humic acid removal has been extensively studied mainly in the model systems using various solids (Masset et al. 2000;Urík et al. 2014;Salman et al. 2007;Jiang et al. 2014;Capasso et al. 2007;Mal'tseva et al. 2014;Deng et al. 2006). The smallest values were obtained for the materials rich in silica and zeolites, the significantly larger adsorption yields were found for the bacterial origin sorbents or iron oxides. Comparing the presented values of the maximum adsorption, the waste material looks really promising, especially taking into account the price of its preparation and availability.

Conclusions
The main aim of the paper was to characterize the surface structure of the hop cones after the supercritical extraction and to investigate the possibility its application as an adsorbent in the process of the humic acid removal. It was found that the biosorbent particles exhibit a non-porous structure and a heterogeneous surface rich in the oxygen and nitrogen surface groups capable of acquiring a negative charge in the aqueous solutions in the pH range from 3 to 9. The humic acid binding capacity measurements on the biosorbent surface provided promising results. It was shown that the anionic compound is adsorbed with a good efficiency even when a small amount of the solid is used. Furthermore, the HA adsorption is largely pH dependent-the largest removal was achieved under the acidic conditions mainly due to the hydrogen bonds formation and the electrostatic attraction forces. Comparison of the data obtained for the waste byproduct and the raw reference material reveals that the technological treatment does not influence the surface properties of the hop cones particles significantly.
The data obtained for Cs and Sr binding indicate that the ion adsorption is minimally pH dependent and the sorption kinetics is immediately established. The ion sorption isotherms are similar to the Henry type. The humic acid adsorption isotherms on the hop cones surface exhibit a complex course. In the initial concentration range the shape is similar to the Langmuir isotherm, and then being transformed into a BET one. This proves the formation of a multilayer made of the adsorbed HA molecules. The studies on the Sr and Cs ions impact on the humic acid adsorption indicate that the strontium presence in the solution improves the organic compound removal. Strontium ions act as a "link" between the waste sorbent surface and the humic acid molecules increasing the number of active sites. The opposite influence was observed in the systems containing the cesium ions exhibiting smaller sorption effectiveness. The reason is the competition between the metal cations and the organic compound molecules. On the basis of the above conclusions it can be found that the hop cones obtained after the supercritical extraction process can be used as a potential adsorbent to remove contaminants of the organic origin. It is also an interesting alternative to the commercially available solids. The presented low-cost adsorbent is produced in large quantities as a by-product that should be utilized for ecological reasons. Therefore, the application of the hop cones in the adsorption processes can have a positive impact on the natural environment condition ensuring the ecological safety. Concerning the HA removal (e.g., from drinking water tanks) using the analysed biosorbent, the presence of divalent ions can influence the HA adsorption greatly. In the case of the Cs radioisotopes binding, the preferential Cs ions adsorption can be affected.