Slow-releasing oxygen materials were prepared to overcome some limitations regarding the low dissolved oxygen (DO) concentration and the low efficiency of in-situ purification in groundwater. Tests on slow-releasing oxygen materials that could be used to change the reductive environment in groundwater by slowly releasing oxygen were carried out. Oxygen-releasing laboratory experiments were conducted to monitor changes in DO concentration, pH, and total dissolved solids (TDS) in aqueous solutions. The adsorption of the materials on total Fe and Mn were also analyzed. The experimental results showed that the oxygen-releasing status of materials was stable at 15 mg/L after 20 d for fixed-shape materials. Paraffin wax shells and a KH2PO4 pH regulator facilitated the lowering of pH. The oxygen-releasing process followed the quasi-second-order kinetic model, and the oxygen-releasing rate constant K was 1.28, 1.51, and 1.97 (mg/(L·h)) for silt, silty sand, and fine sand medium, respectively. The larger the seepage medium particles were, the faster the pH and TDS dropped. Adsorption experiment results showed that adsorption on total Fe and Mn were well simulated by the Langmuir nonlinear isothermal adsorption equation. The maximum adsorption capacities of the materials on total Fe and Mn were found to be 0.708 mg/g and 0.438 mg/g, respectively.
Many in-situ purification technologies (e.g., pump and treat, air sparging, bioremediation, and chemical oxidation) have been used to purify contaminated groundwater (Liang et al. 2011; Brunsting & McBean 2014; Lee et al. 2014). When in-situ groundwater purification technologies are used, many excess components are easily removed under oxidation conditions. However, the groundwater environment is usually reductive and the low dissolved oxygen (DO) concentration could significantly reduce the efficiency of in-situ purification, so oxygen is often needed to change the reductive environment and facilitate the efficient removal of excess components (Yeh et al. 2010). However, previous studies focused on the artificial introduction of oxygen and aeration in the underground environment (Luo 2011). The oxygen demand of microorganisms was neglected and the oxygen-releasing rate and their effective utilization were not well managed. This wasted large amounts of oxygen, and encouraged or inhibited the growth of certain microorganisms (Kao et al. 2003). To address anoxia in groundwater, slow-releasing oxygen materials have been used. The materials release oxygen slowly upon contact with water, and improve purification efficiency (Bianchi-Mosquera et al. 1994; Yeh et al. 2010).
Oxygen releasing compounds, such as CaO2, MgO2, and Na2CO3, are the main oxygen sources in slow-releasing oxygen materials and can release oxygen steadily and persistently and cause a significant rise in DO concentration in groundwater (Arienzo 2000; Kao et al. 2001; Gallizia et al. 2004). CaO2, in particular, has exhibited continuous and long-lasting oxygen-releasing properties, although small amounts of slightly soluble Ca(OH)2 are formed. This does not cause secondary pollution for groundwater, and CaO2 is widely used for creating slow-releasing oxygen materials to provide oxygen for sites affected by anoxic groundwater (Arienzo 2000; Nykänen et al. 2012; Lee et al. 2014).
Higher oxygen-releasing rates would enhance the effect of bioremediation and long-lasting DO concentration also accelerates the growth of microbial species (Xu et al. 2015). This promotes the transformation of reductive components in groundwater. Adding inorganic salts such as KH2PO4 is also helpful in maintaining normal metabolism and growth of microorganisms (Prince 2000; Liu et al. 2006). However, high pH values caused by the alkali salt, mainly from the reaction of CaO2, can have negative effects on indigenous microbes and chemical components (Kao et al. 2003). To mitigate this, buffered inorganic salts can be applied to decrease the high pH value (Liu et al. 2006; Yeh et al. 2010).
In this paper, volcanic cinder with high porosity and adsorption was used as a component of slow-releasing oxygen materials. Volcanic cinder increases the contact of the materials with groundwater and facilitates the slow release of oxygen. And volcanic cinder with adsorption was added into the materials to overcome the limitations regarding the low efficiency of in-situ purification in groundwater. Other important advantages of volcanic cinder are that it is minimally affected by the water chemistry, and its cost is low. This paper focuses on oxygen-releasing experiments to investigate the oxygen-releasing effect and the changes of pH in aqueous solution. In addition, the adsorption effect of the slow-releasing oxygen materials on total Fe and Mn was also investigated. This research will have practical significance for the sustainable development and utilization of groundwater.
MATERIALS AND METHODS
Main raw materials
Ca(OH)2 (AR); H2O2 (30%, AR); KH2PO4 (AR); paraffin wax (AR); compound silicon cement; volcanic cinder sieved through a no. 60 mesh, particle size of 0.3 mm (from Jingyu County, Jilin Province, China).
Preparation of materials
The slow-releasing oxygen materials were prepared using the following quantities: to prepare the CaO2, 74.0 g of Ca(OH)2 powder was added to 51 mL H2O2 solution (containing 5% KH2PO4) with a mole ratio of 1:1 for the mixture proportion. Then, after stirring for 30 min, volcanic cinder, compound silicon cement, and KH2PO4 (weight ratio of 1:1:1) were added to the mixture. The mixture was prepared in a 500 mL glass beaker, maintained at 15 °C, and stirred at a constant velocity for a further 2 h. The 0.5 mm globular materials were then soaked in liquid paraffin for about 2 h to create a wax layer covering. Finally, the materials were maintained at 40 °C for 1 d in an oven at a constant temperature to dry.
To analyze the oxygen-releasing status of the slow-releasing oxygen materials, static and column experiments were designed, and the changes in DO concentration, pH, and total dissolved solids (TDS) were monitored. In addition, the adsorption characteristics of the materials were investigated. The oxygen-releasing mechanisms of the materials were also investigated and a microstructure analysis of the materials was carried out.
Ca(OH)2 is an alkaline substance, and therefore pH values will increase in the aqueous solution.
Slow-releasing oxygen materials (10.0 g) were added to 200 mL deionized water. The water was treated with N2 and held in glass containers. After sealing the containers, changes in DO and pH were monitored at set intervals in a low temperature environment (10 °C).
Three seepage columns, 30.0 cm high and 4.0 cm in diameter were filled with silt, silty sand, and fine sand (Table 1). After blending slow-releasing oxygen materials with small amounts of sand, the resulting mixture was placed in the seepage columns to 15.0 cm from the top. The mixture in the three seepage columns had a porosity of 0.70–0.75. To reduce the influence of the original oxygen in the seepage column, nitrogen was used to remove the oxygen in each seepage column until all the oxygen was expelled. Then fitted, the experimental apparatus and infused deionized water were inserted into each column from the top to the bottom with a Mariotte bottle, and seepage was maintained at a velocity of 1.35, 1.51, and 1.72 m/d in the seepage columns filled with silt, silty sand, and fine sand, respectively. Then, changes in DO concentration, pH, and TDS were monitored at set intervals at the outlet of each column. The experimental procedure was carried out at a low temperature (10 °C) and in a nitrogen environment.
|Seepage column .||Medium .||Medium quality (g) .||Materials quality (g) .||Velocity (m/d) .|
|Seepage column .||Medium .||Medium quality (g) .||Materials quality (g) .||Velocity (m/d) .|
Oxygen-releasing mechanism and microstructure analysis of the materials
To identify the occurrence of oxygen release and adsorption, a microstructure analysis was carried out using a scanning electron microscope (SEM) and an energy dispersive X-ray (EDX) spectrum analysis.
RESULTS AND DISCUSSION
Static experiments results
Ability of materials to release oxygen
Oxygen-releasing effect of materials with or without paraffin wax shell
From Figure 1, it can be seen that initially materials with a paraffin wax shell released oxygen smoothly and DO concentration rose rapidly. The materials with paraffin wax coating effectively reduced the contact of the materials with water and the oxygen-releasing rate decreased. The oxygen-releasing trend for materials without a paraffin wax shell also clearly dropped. Materials with a paraffin wax shell showed a stable oxygen-releasing state after 5 d with DO concentration maintained at about 15 mg/L in the aqueous solution. This showed the materials with the paraffin wax shell decreased the contact of CaO2 with water, and the oxygen-releasing rate of the materials was controlled effectively.
Oxygen-releasing effect of materials with and without a fixed shape
From Figure 2, it can be seen that the powdered materials released oxygen faster than the fixed shape materials. The DO concentration for the powdered materials dropped continuously to about 11 mg/L after 9 d in aqueous solution, and remained in a slow downward trend. The DO concentration for fixed shape materials dropped at a slower rate than for the powdered materials and the DO concentration was maintained at about 15 mg/L after 20 d. This showed that the slow oxygen-releasing effect of materials with a fixed shape was better than that of powdered materials.
Impact of materials on pH in the aqueous solution
Impact of materials with and without paraffin wax shell on pH
Initially, pH in the aqueous solution containing materials with a paraffin wax shell rose faster than in the solution containing materials without a paraffin wax shell (Figure 3). After the pH reached 11.80, it presented a downward trend and reduced to 10.40 after 18 d in the solution containing materials with a paraffin wax shell. It was clear that the rate of decline in pH for the aqueous solution containing materials with a paraffin wax shell was faster than in the solution containing materials without a paraffin wax shell; the possibility of contact with the water was less for materials with paraffin wax shells, and thus less alkaline substance was generated by reactions with the water. In the later stages of the experiment, KH2PO4 continued to release H+, which lowered the pH of the aqueous solution. Therefore, in terms of pH regulation, the same quantities of KH2PO4 made the pH decrease more apparent in the aqueous solution containing materials with a paraffin wax shell.
Impact of powdered materials versus fixed-shape materials on pH
As can be seen in Figure 4, the decrease in pH for the powdered materials in aqueous solution was greater than that of the fixed-shape materials; the KH2PO4 pH regulator in the powdered materials made better contact with water and thus reduced pH more effectively than the fixed-shape materials. However, the fixed-shape materials caused a more obvious decrease in pH at the late observation stage.
Column experiments results
Ability of materials to release oxygen
The fitting equations for oxygen-releasing status in the seepage columns filled with mediums of silt, silty sand, and fine sand using the quasi-second-order kinetic equation were: t/mt = 0.0679t + 0.0036 (R2 = 0.9989); t/mt = 0.0716t + 0.0034 (R2 = 0.9991); and t/mt = 0.0730t + 0.0027 (R2 = 0.9965), respectively. The values of me were 14.73, 13.97, and 13.70 mg/L in the seepage columns filled with silt, silty sand, and fine sand, respectively. The parameter me represents the ability to release oxygen; the higher the value, the greater the ability to release oxygen. The medium particles were small in the seepage column filled with silt and DO was suffocated by the flowing water, resulting in me being maintained at higher equilibrium levels. This showed that the materials in the silt medium had an increased and longer-lasting capacity to release oxygen. Therefore, as particle size increased, the materials' ability to release oxygen gradually attenuated. Values for K were 1.28, 1.51, and 1.97 (mg/(L·h)) in seepage columns filled with silt, silty sand, and fine sand, respectively. The larger the K value, the faster the oxygen-releasing rate. The K value was smallest in the seepage column filled with silt. The small pore spaces in the silt increased the resistance to oxygen-release and thus slowed the oxygen-releasing rate.
Influence of materials on pH and TDS of the aqueous solution
As can be seen in Figure 5, the pH in the aqueous solution initially increased rapidly, peaked after about 10 h, and then decreased to 11.0 after about 150 h. Thus a seepage medium with a good buffering capacity can effectively reduce pH. The permeability coefficient was largest in the seepage column filled with fine sand. This enabled the water to accept more alkaline substances and decrease its pH more quickly. The pH value in the seepage column filled with silty sand showed a similar decrease. However, the larger the seepage medium particles, the faster the pH dropped.
From Figure 6, it can be seen that the TDS initially rose rapidly and peaked after about 12 h, and then decreased rapidly. The Ca2+ in the materials dissolved into the water continuously. But the TDS was almost stable after 30 h. Slightly soluble Ca3(PO4)2 was formed and some Ca2+ was removed by water flow to decrease the TDS. Moreover, the seepage velocity was faster in the fine sand, which had a larger permeability coefficient, and thus more Ca2+ was flushed by the water flow. This showed that the changes in TDS were related to the seepage medium: the larger the seepage medium particles, the faster TDS dropped.
In comparison with previous studies (Cassidy & Irvine 1999; Kao et al. 2003; Liu et al. 2006; Yeh et al. 2010; Lee et al. 2014; Xu et al. 2015), the materials with a paraffin wax shell facilitated maintaining a higher DO concentration. The KH2PO4 pH regulator helped reduce pH. In the column experiments, the materials released more oxygen stably and the slow-releasing oxygen rate was faster. TDS is an important index of groundwater quality standard, and so TDS must be considered in column experiments. After 30 h, the TDS of the aqueous solution reduced to 500 mg/L. This showed adding the materials to the aqueous solution had no negative influence on water quality. However, in previous studies, different quantities of materials were used and the type of slow-releasing oxygen materials may also have been different (Kao et al. 2003; Yeh et al. 2010; Kong et al. 2012; Zhang et al. 2012). Therefore, the optimal amount and type of materials that need to be added to the seepage column still needs to be confirmed.
Adsorption experiments results
The initial concentrations of total Fe and Mn in the simulated groundwater were 0.48, 1.05, 3.12, 4.95, 8.12, and 9.94 mg/L and 0.08, 0.53, 1.08, 2.93, and 4.95 mg/L, respectively. After adsorbing for about 1 h, the isotherm figure of total Fe and Mn between Ce and qe was drawn (Figure 7). The materials' capacity to adsorb Fe and Mn increased with increasing ion concentration.
In the adsorption experiments, nonlinear isothermal adsorption equations were used to evaluate the adsorption process. Nonlinear isothermal adsorption equations take two main forms: the Freundlich (1906) and Langmuir (1918) nonlinear isothermal adsorption equations.
After fitting Equation (5), the graph of 1/Ce versus 1/qe was drawn (Figure 8). The Langmuir nonlinear isothermal adsorption equations for total Fe and Mn were: 1/qe = 7.1994 × 1/Ce + 1.4115 (R2 = 0.9653) and 1/qe = 2.2813 × 1/Ce + 1.0004 (R2 = 0.9868), respectively (Figure 8). Theoretical maximum adsorption amounts on Fe and Mn were 0.708 mg/g and 0.438 mg/g, respectively, which showed that the slow-releasing oxygen materials had certain adsorptions for Fe and Mn.
The volcanic cinder in materials was helpful for adsorption on excess components in aqueous solution. From the above results, it can be seen that slow-releasing oxygen materials were not only helpful in maintaining higher DO concentrations but also had better adsorption effects on total Fe and Mn. Previous studies focused on the supply of oxygen for microorganisms in groundwater (Liu et al. 2006; Zhang et al. 2012; Xu et al. 2015), and lacked an analysis of the adsorption effects on excessive components in groundwater. In northeast China, total Fe and Mn contents are high under natural conditions. Prepared slow-release oxygen materials can adsorb total Fe and Mn and reduce high total Fe and Mn content under natural conditions. This will contribute to the sustainable development and utilization of groundwater resources in northeast China.
Microstructure and EDX spectrum analysis
Microstructure and EDX spectrum analysis of materials before and after contact with water
From Figure 9(a), it can be seen that the original slow-releasing oxygen materials had a relatively regular and smooth surface, which effectively reduced the contact of materials with water so that the chemical reaction of CaO2 with water was reduced. However, the materials' surface had many tiny pores that enabled the materials' contact with water and allowed oxygen to be slowly released. Figure 9(b) shows that, when the materials came into contact with water, the oxygen release affected the surface, and led to the formation of small bumps. Thus, the materials' surface was not as flat as before the experiment. This shows that the microstructure on the surface of the materials was destroyed by the effect of oxygen bubble impacts. In addition, many tiny pores on the damaged materials' surface were still visible. These pores ensured that the materials continued to release oxygen.
From the EDX spectrum (Figure 9(a)), it can be seen that the main elements of the original materials were Ca, P, O, and Si. The materials were mainly composed of CaO, SiO2, and P2O5 and the relative wt% of Ca and O was 37% and 35%, respectively. After contact with water (Figure 9(b)), the main elements were the same as in the original materials, but the relative wt% of Ca and O were less. This shows that O was released from the materials and Ca in the materials was dissolved into solution after contact with water.
Microstructure and EDX spectrum analysis of materials before and after adsorbing total Fe and Mn
As shown in Figure 9(a) and 9(c), before the materials adsorbed total Fe and Mn, their surface was relatively smooth. However, after adsorption, the surface became slightly rough, and small bumps appeared. From Figure 9(c) it can be seen that the slow-releasing oxygen materials were mainly composed of Ca, P, O, Si, Fe, and Mn elements. By comparing Figure 9(a) and 9(c), it can be seen that the adsorption by the materials on total Fe and Mn is very obvious. The characteristic peaks of Fe and Mn changed significantly and the peaks were also higher than pre-experiment values. The relative wt% of Fe and Mn were 0.64% and less than 0.05%, respectively, in the original materials, but after adsorbing Fe and Mn, the relative wt% of Fe and Mn reached 40.28% and 14.52%, respectively. The results show that the adsorption on Fe and Mn significantly changed the microstructure of the materials' surface. These changes allowed slow-releasing oxygen materials to adsorb Fe and Mn effectively.
In this study, slow-releasing oxygen materials were tested for oxygen release in aqueous solution. The DO concentration was maintained at 15 mg/L after 20 d and the reductive environment was significantly altered. The oxygen-releasing process in different mediums followed the quasi-second-order kinetic model. Adding volcanic cinder to the slow-release oxygen materials increased their adsorption effect, and improved the purification efficiency for groundwater. The materials used will have important applications and practical significance for removing excess components in groundwater. Further studies are needed to investigate a suitable environment with an appropriate pH and DO concentration for the growth and reproduction of microorganisms, and to identify possible applications for the slow-releasing oxygen materials in the removal of organic pollutants from groundwater.
We acknowledge the financial support of P.R. China Major Science and Technology Program for Water Pollution Control and Treatment (2014ZX07201-010).