Study on Adsorption Model and Influencing Factors of Heavy Metal Cu²⁺ Adsorbed by Magnetic Filler Biofilm

In this study, a biofilm culture device was made to carry out the experiment for testing the biofilm adsorption capacity of heavy metal Cu²⁺ (Fig. 1a). For the culture device, its main component is the glass container, which size was 1200 × 400 × 650 mm (L × W × H). The glass container is divided into 5 water flow channels by 4 glass baffles (Fig. 1b). There were 5 water flow channels are sequentially connected by four flow holes, which are vertically arranged on the staggered edges of the glass diaphragm to facilitate the water flow. The water pump was fixed to pump the culture liquid from the plastic tank into the culture device through the water inlet. The nutrient solution in the glass container overflows back to the plastic tank from the water outlet. In order to maintain a certain concentration of DO in the culture medium, an air pump was designed to pump air directly into each water channel.

When the biofilm was cultivated with the biofilm culture device, the surface sediment of the river course was collected from the river and evenly spread on the bottom of the glass container, with a thickness of 5 cm. The surface sediment of the river contained abundant microorganisms, which were used to inoculate microorganisms when cultivating biofilm.

The biofilm culture solution contained a variety of nutrients (all in mg/L), mainly including: 4.60 of Na₃PO₄·12H₂O; 1.89 of NaH₂PO₄·2H₂O; 17.7 of (NH₄)₂SO₄; 2.05 of MgSO₄·7H₂O; 2.54 of NaCl; 0.74 of CaCl₂·2H₂O; and 1.91 of KCl. The biofilm culture solution was continuously pumped into the glass container from the plastic tank, and then flowed back to the plastic tank through the glass container outlet again by overflow. In order to accelerate the growth of biofilm, glucose was added to the biofilm culture medium to maintain the concentration of chemical oxygen demand (COD) at about 120 mg/L. The flow rate of biofilm nutrient solution in the reactor was set as approximate 0.3 m³/(m² s).

The filler used for cultivating biofilm was three-dimensional magnetic elastic filler. The main components of the magnetic filler were shown in Table 1. Each magnetic filler had a length of 10 cm and a diameter of 10 cm. Five pre weighed magnetic fillers were strung together and assembled into a bunch. These bunches of magnetic filler were strung onto PVC pontoons, and then they were put into the water channels of the biofilm culture device, respectively.

During the biofilm cultured, the DO concentration in the culture solution was controlled at about 4.0 mg/L. With the aid of a time control switch, the biofilm culture solution was aerated for about 10 min every half an hour. The pH of the biofilm culture solution is controlled at 6–8 with the aid of 1 mol/L hydrochloric acid and 1 mol/L sodium hydroxide. The SC100e instrument (American Hach Company) was used to monitor the DO and pH parameters simultaneously. In order to ensure the normal growth of microorganisms in the biofilms, 40% of the culture medium in the plastic tank was replaced every two days to maintain the nutrient contents at a certain level.

By domesticating and culturing, the microorganisms in the culture solution, the biofilms adhered onto the magnetic fillers in the culture device and grew cumulatively. After continuous culturing for 60 days, the biofilms adhered onto the fillers accumulated to a certain amount, when the adsorption and influencing factors experiments of heavy metal Cu²⁺ could be carried out.

On the 60th day of biofilm culture, the magnetic fillers adhered with biofilms were carefully taken out from the culture device, placed them in a 2000 mL beaker, add 1000 mL phosphate buffer (PBS, pH 7.2), which contained 0.092 g L⁻¹ KH₂PO₄, 0.036 g L⁻¹ K₂HPO₄ and 0.493 g L⁻¹ NaCl. Then, the magnetic fillers were stirred vigorously with a glass rod to remove the biofilm from the magnetic fillers. The obtained biofilm suspension was transferred into a 2000 mL volumetric flask. 200 ml PBS was added into the beaker, and the elution operation was repeated again, and the operation was repeated three times, then all the obtained biofilm suspension was transferred into the volumetric flask. Finally, the PBS was fixed into the volumetric flask to 2000 ml for the following experiment.

The total biomass of biofilm in unit volume suspension was determined by measuring the dry weight (DW) of biofilm. The specific operation method was as the following. Firstly put 50 ml uniform biofilm suspension into a crucible [10]. After weighed, the crucible was placed into the electric blast drying oven at 105 °C until it reaches constant weight, and then placed the crucible in a desiccator to cool to room temperature and weighed it again. The total biomass of biofilm in the unit volume suspension was determined by using the mass difference of the two weight. Made three parallel determinations, and took the average of the three mass differences as the final determination result.

Adsorption Model of Heavy Metal Ions by Biofilm. Heavy metal adsorption by biofilm is a complex process, mainly including transmembrane diffusion and transport. For the single component adsorption system of biofilm, there are two classical equilibrium adsorption models. For multicomponent adsorption, an extended adsorption model had been developed based on the single component adsorption system model [11].

Langmuir adsorption mode:

Frenndlich adsorption mode:

In the formula:

\({\text{q}}_{e}\)—the amount of heavy metals adsorbed by the adsorbent per unit mass when the adsorption equilibrium is reached, mg/g; Q—the amount of metal adsorbed by the adsorbent per unit mass, mg/g; b—the adsorption constant, L/g;

\(C_{{\text{e}}}\)—the mass concentration of heavy metals in solution at adsorption equilibrium, mg/L; \(K_{F}\), \(\frac{1}{{\text{n}}}\)—Frenndlich constant.

Determination of Adsorption Isotherm. In the first step, took six 150 ml conical flasks and added 150 ml of Cu²⁺ solutions with the mass concentration (C0) of 10, 20, 30, 40, 50 and 60 mg/L respectively. In the second step, adjusted the pH of the solution to 5.0, and added 10 g biofilm (wet weight) to the conical flask. Put the conical flask into the constant temperature freezer and shaked it for 1 h. The speed of the freezer is 120 r/s. And then, the suspension in every conical flask was filtered using 0.45 μm microporous membrane and fixed the volume to 250 ml. At last, the mass concentrations of Cu element in the treated samples were determined by ICP-AES.

The pH Influencing Factor. Six 250 ml conical flasks numbered from P1to P6 were taken and added 150 ml of Cu²⁺ solution with a mass concentration of 40 mg/L, then 20 g biofilm (wet weight) were added to every conical flask, and adjusted the pH of the solution to 2.0, 3.0, 4.0, 5.0, 6.0. 7.0 and 8.0, respectively. Then, put the conical flask into the constant temperature freezer at 20℃ and shaked them for 1 h. The rotating speed of the freezer is 120 r/s. And then, the suspension in every conical flask was filtered using 0.45 μm microporous membrane and fixed the volume to 250 ml. At last, the mass concentrations of Cu element in the treated samples were determined by ICP-AES.

The Temperature Influencing Factor. Similar to above, six 250 ml conical flasks numbered from W1 to W6 were taken and added 150 ml of Cu²⁺ solution with a mass concentration of 40 mg/L, then 20 g biofilm (wet weight) were added to every conical flask, and adjusted the pH of the solution to 5.0. Secondly, put the conical flask into the constant temperature freezer at a series of temperatures (5, 10, 15, 20, 25, 30 ℃) and shaked them for 1 h, respectively. The rotation speed and subsequent filtering operation and detection were the same as those in subsection.

The concentration of heavy metal Cu²⁺ in the above samples were determined by ICP-AES method. The main working parameters of ICP-AES instrument were as follows: the radio frequency (RF) power is 1.3 kW; the cooling gas flow rate, the spray gas flow rate and the auxiliary gas flow rate were 15.0, 0.8 and 0.2 L/min, respectively; the observation mode was radial or axial, and the solution uptake rate was 15.0 m L/min; the analytical wavelength of heavy metal Cu determined was 327.393 nm (Table 2). The concentration of heavy metal Cu²⁺ in the samples were determined three times in parallel, and the average values were taken. The amounts of heavy metal Cu²⁺ adsorbed were calculated according to the concentrations of Cu²⁺ before and after the adsorption and the corresponding volumes. The adsorption amounts of heavy metal Cu²⁺ in the experiment were calculated according to the concentration values of Cu²⁺ before and after the adsorption and their corresponding volumes.

The adsorption model of heavy metal Cu²⁺ adsorbed by magnetic filler biofilm was studied as an example showed in Fig. 2.

The adsorption isotherm of Cu²⁺ adsorbed by magnetic filler biofilm was performanced as the Fig. 2a. It can be seen from Fig. 2b that the saturation adsorption capacity of the magnetic filler biofilm containing 10% strontium ferrite component for Cu²⁺ is about 0.37 mg/g. The experimental results of the saturated adsorption capacity of the magnetic filler biofilm containing 10% strontium ferrite on heavy metal Cu²⁺ were processed to perform linear fitting of Langmuir model and Freundlich model, and the equilibrium model of the biofilm adsorption of heavy metal Cu²⁺ was discussed, as was shown in Fig. 2b and c. The results showed that the adsorption of heavy metal Cu²⁺ by biofilm is in better agreement with Langmuir model than Freundlich model. The correlation coefficient R² of Langmuir model is 0.982, and the correlation coefficient ruler of Freundlich model is 0.950. Therefore, the adsorption of heavy metal Cu²⁺ can be fitted by Langmuir model.

The pH Influencing Factor. As shown in Fig. 3, the adsorption capacity of the magnetic filler biofilms for heavy metal Cu²⁺ showed a parabolic relationship with the pH value. There is an optimal pH range for adsorbing heavy metal Cu²⁺, and the optimal adsorption pH range is 5–7. When the pH value reached 6.2, the adsorption capacity was the largest. It can be inferred that the adsorption capacity of the magnetic filler biofilm for heavy metal Cu²⁺ decreases when it deviates from the most appropriate pH range. As is the reason that when the acidity increases, the degree of amino protonation in biofilm proteins, which exist in biofilm microorganisms or extracellular polymers, etc., increases, and its coordination ability with metals weakens. Moreover, a large amount of H⁺ and H₃O⁺ in the system will compete for adsorption sites with heavy metal Cu²⁺, which results in a decrease in the adsorption ability of magnetic filler biofilm for heavy metal Cu²⁺. When the pH value is greater than the optimal range, heavy metal Cu²⁺ will hydrolyze to generate corresponding hydroxides and deposit on the surface of the biofilm, which will affect its biosorption active sites to a certain extent, and also lead to the decline of the ability for the magnetic filler biofilm to adsorb heavy metal Cu²⁺.

The pH value can not only have a significant impact on the active sites of heavy metals bound by biofilms, but also have a significant impact on the chemical reactions of heavy metal solutions (such as inorganic coordination, organic complexation, redox, hydrolysis, precipitation, etc.) [12]. The adsorption capacity of biofilm for heavy metal Cu²⁺ increases with the increase of pH value, but there is no linear correlation between the two. At the same time, the heavy metal Cu²⁺ will slightly precipitate when the pH value is 5.5. Thus it can be seen, too high pH value will not be conducive to the heavy metal Cu²⁺ biological adsorption [13].

The Temperature Influencing Factor. It can be seen from Fig. 4 that temperature has a certain effect on Cu²⁺ adsorption by magnetic filler biofilms, but it is not very significant. At 5℃, the Cu²⁺ adsorption rate was 46.3%, and at 30 ℃, it was 57.5%. When the temperature increased by 25 ℃, the Cu²⁺ adsorption rate only increased by 11.2%. Compared with pH value, the effect of temperature on biosorption is relatively small. Generally speaking, biosorption is an exothermic reaction process. Therefore, the adsorption capacity of microorganisms to heavy metal Cu²⁺ increases with the decrease of temperature. However, sometimes biosorption is also an endothermic reaction process. Han et al. [14] found that S Cerevisiae’s adsorption capacity for Cu increases with the increase of temperature (at 293 k, the adsorption capacity for Cu is 0.00809 mmol g⁻¹DW., and when the temperature rises to 323 K, the adsorption capacity continuously increases to 0.0206 mmol g⁻¹DW.), which is an endothermic reaction. However, too high a temperature will not only destroy the active site of the biosorbent, resulting in a decrease in the amount of heavy metal Cu²⁺ adsorbed, but also increase the operating cost [15, 16].

The adsorption of heavy metal Cu²⁺ by magnetic filler biofilm depends on many physical and chemical factors. In addition to pH value and the temperature, other factors such as the adsorption time, the amount of magnetic filler biofilm, microbial activity in the biofilm, the initial concentration of heavy metal Cu²⁺, and other coexisting heavy metal ions will affect the adsorption of heavy metal Cu²⁺ by magnetic filler biofilm. For example, the adsorption capacity of cells with biosorbents for heavy metal ions at the early and late growth stages is stronger than that at the stable growth stage (plateau stage) [17]. Some studies have shown that the magnetotactic bacteria preferentially adsorb Zn²⁺ in the binary competitive adsorption system for the coexistence system of Cu²⁺ and Zn²⁺ (the concentration of both ions is the same), and the adsorption rate is reduced by 36.76 and 24.5% compared with the single system of them, respectively [18].