Analysis of efficiency of phosphates sorption by different granulation of selected reactive material

In the light of the need to find an effective way to remove phosphorus from wastewater, studies on the suitability of sorption materials in this process should be conducted. The aim of the study was to examine the potential benefits of using selected adsorbents to reduce orthophosphates from the model solution under steady conditions. The study was conducted on a laboratory scale using synthetic wastewater with concentration of P-PO4 in the range of 5-100 mg/dm3. Experiment has shown that fine-grained material M1 (0-2 mm) is highly effective at removal of phosphorus compounds at the level of 97.8% at the highest concentration of P-PO4. The sorption capacity achieved during the investigation was 9.6 mg/g, while the maximum sorption capacity from the Langmuir model could reach up to 256 mg/g. Material M2 (2-8 mm) did not show satisfactory sorption capacity (maximum calculated sorption capacity: 0.36 mg/g) and the effectiveness of phosphate reduction did not exceeded 6% at the lowest concentration of P-PO4.


Introduction
Phosphorus, as a nutrient element, together with the nitrogen compounds enable plants growth.However, these processes require a small amount of nutrients.Their excess is the cause of eutrophication and algae bloom.Phosphorus compounds originate mainly from wastewater discharged into the lakes and other water bodies [1].
The removal of nutrients from wastewater is one of the basic problems of wastewater treatment plants.Most often their level is reduced in biological processes (using phosphorus as a source of energy for microorganisms) and chemical processes (reducing phosphate by salts of ferric or aluminum) of wastewater treatment [2].
Wastewater containing significant amounts of phosphorus compounds may originate both from large wastewater treatment plants and from smaller rural or domestic wastewater treatment plants.Most often the small wastewater treatment plants may discharge to the receivers wastewater purified insufficiently.The solution may be the use of filtration materials in domestic wastewater treatment plants and constructed wetlands [3,4].
In recent years, the subject of research in terms of effectiveness are sorption materials (adsorbents) capable to bind significant amounts of phosphorus compounds.The source of their origin can be natural and anthropogenic.They may consist of silicon, calcium, aluminum, sulfur, iron in various weight ratio [5][6][7][8][9][10][11][12].
Unfortunately, the research methodology for evaluating the effectiveness of adsorbents in process of wastewater treatment is still under discussion.The lack of standardization the guidelines is a cause of difficulties in comparing different materials and their application in practice.In accessible research papers can be found methods differ in the concentration of the model solution, the volume, the amount of added sorption material, way and mixing time of the sample and the time of contact adsorbent with the model solution.Therefore, the scope of the differences in research methodology is significant [1,13].
The aim of this study was an evaluation of selected sorption material in two fractions to compare the influence of grain size on phosphorus removal efficiency from wastewater in steady conditions.Another purpose was the determination of parameters of the Langmuir and Freundlich adsorption isotherms and maximum sorption capacity.Material M1 (Fig. 1) is a fine-grained (0-2 mm) product of thermal treatment of carbonate-siliceous rock with high content of calcium carbonate CaCO3 in temperature 700°C.This fraction if by-product of the M2 material and due to very fine-grain consisting mostly a calcium oxide CaO (above 80 %).

Materials
Material M2 is produced in process of decarbonization of natural carbonate-siliceous rock (opoka) (Fig. 1).Because of high content of elements like calcium, silicon and aluminum, material M2 presents abilities to bind phosphorus compounds from wastewater.Material M2 is also known under the name ROCKFOS ® .The only Polish producer of Rockfos ® is Ceramika KUFEL from Suchynia.
Calcium oxide CaO and silicon oxide SiO2 present in the Rockfos ® chemical composition in the highest concentration (Table 1).[20].

Experiment design
To carry out the laboratory investigation, portions of 10 g of each selected materials was prepared.In order to determine the adsorption isotherms of these materials model solution was used.The model solution was prepared from distilled water and a solution of potassium dihydrogen phosphate KH2PO4 (1 cm 3 -10 mgPO4/dm 3 ).In each of the 5 batch reactors, there was a model solution in a volume of 1.0 dm 3 with a phosphate phosphorus concentration of 5-100 mg/dm 3 (Table 3).
To each of the 5 batch reactors, 10 g of selected material was added.Mixing time lasted 5 minutes and was provided by magnetic stirrer (1000 rpm).Samples were collected after 1 hour of sedimentation.Three series of repetitions were made for each material.All tests were performed at room temperature of approximately 20 o C.

Physical and chemical analysis
Before the designation of the concentration of P-PO4 of synthetic wastewater, the samples were subjected to filtration.The concentration of the phosphorus phosphate was marked by using the cuvette tests HACH Lange and HACH Lange DR 3900 laboratory VIS spectrophotometer with RFID.On the basis of the obtained results, the series of analyses were conducted in order to verify the suitability of selected materials for removing phosphorus compounds by adsorption.The effectiveness of binding P-PO4 by selected materials was calculated by equation (1) [13]: where Graphical interpretation of linear equations ( 4) and ( 6) are shown in figures 2-5, where adsorption isotherms are presented in form (7): where: for Langmuir isotherm: for Freundlich isotherm:

Results and discussion
The basic parameters (q, E) to evaluate the efficiency of phosphorus removal from synthetic wastewater of two selected materials are presented in Table 4.For material M1 efficiency of phosphorus reduction and sorption capacity increased with the increase of initial concentration of P-PO4.At the highest concentration of P-PO4 sorption capacity reached 9.6 mg/g, and the level of reduction of phosphorus compounds was nearly 98 %.Similar results also for fine-grain particles (1-2 mm) were occurred for carbonate-siliceous rock (opoka) heated in temperature 900 °C.Efficiency of phosphorus removal reached to 97 % [2].Those study was proceeded with flow conditions in models of filters with vertical flow.
Material M2 showed much lower efficiency of P-PO4 removal, which decreased along with increasing initial concentration.
For the lowest concentration (5.11 mg/dm 3 ), the effectiveness of binding phosphates by sorption material was only 6 %, while maximum sorption capacity at the highest concentration was equal to 0.3 mg/g.Graphical analysis of the results of laboratory investigation showed good matching for both Langmuir and Freundlich isotherms of adsorption to describe the sorption process for both sorption materials (Fig. 3-6).The coefficient of determination for Langmuir isotherm was equal to 0.99, and for Freundlich isotherm was about 0.97 (Table 5).The linear correlation coefficient was obtain on the level of r = 0.996 for Langmuir isotherm and r = 0.985 for Freundlich isotherm.
Maximum sorption capacity of analyzed materials was obtained from parameters of Langmuir isotherm and equal 294.12 mg/g for material M1, and 0.36 mg/g for material M2.Sorption capacity of both materials is incomparable (Table 5).Comparing the results of current laboratory investigation, material M1 has shown one of the higher sorption capacity.Similar results were obtained in the studies presented in [12], where the sorption material was composed mostly of calcium oxide (70 %), reached maximum sorption capacity over 285 mg/g.Material composed of calcium sulfate, magnesium and iron compounds obtained qmax close to 160 mg/g.Methodology of the research was different from that presented in this paper, as contact time, which lasted 24 hours.According to [6] the highest sorption capacity was achieved for Polonite® (94.32 mg/g), but in this case, contact time was only 15 minutes.Also very high sorption capacity was achieved in the experiment described in [19] at the level of 182 mg/g.The contact time was also 1 hour.Examined materials were different types of opoka heated/treated in temperature 900 °C.The highest sorption capacity was received for opoka with the highest calcium content (420 g/kg).In process of high temperature treatment the conversion of calcium carbonate to calcium oxide occurred and released of carbon dioxide.Increased concentration of calcium oxide was causing the increase of sorption capacity.Also, pH of opoka was rising from 6.8 to over 12.In article [8] various types of heated opoka were analyzed.Obtained results of sorption capacity were around 12-25 mg/g with short contact time which lasts -15 minutes.Again the best results were received for opoka with the highest content of CaO (520.1 g/kg).

of Rockfos ® is over 50 % and size of particles is around 2-8 mm. Disadvantage of this material may be particularly high level of pH (11-12), which results in alkaline solution (Table 2).Table 2 .
Physical characteristic of Rockfos ®

Table 4 .
Sorption capacity and effectiveness of phosphorus removal.

Table 5 .
Parameters of the Langmuir and Freundlich adsorption isotherms of materials M1 and M2.