To solve the problems of high concentrations of Cr6+, SO42- and H+ in agate dyeing industrial wastewater and heavy pollution and high treating cost, single-factor and orthogonal experiments were conducted to determine the optimum particle size, the ratio of adsorbents dosing and hydraulic retention time based on peanut shells and scrap iron. Experiments, using five dynamic columns filled with the peanut shells, scrap iron and sulfate-reducing bacteria (SRB), were also conducted to determine the effect and mechanism of treating the wastewater. The results show that the best treatment effect was obtained when the diameter of peanut shells was 3 mesh, scrap iron being 60 mesh size, scrap iron and peanut shells with a ratio of 1:2, and hydraulic retention time being 24 h. By the comprehensive comparison of five groups of columns, the treating effect of column 4 was best, in which the removal rate of SO42- and Cr6+ was 30.17% and 88.36% respectively before adding the microorganisms, and 25.34% and 99.31% respectively after adding the microorganisms. The average of chemical oxygen demand (COD) release quantity was 62.11 and 513.75 mg·L−1, and the average effluent pH was 7.09 and 7.93 before and after addition of microorganisms respectively. In conclusion, peanut shells, scrap iron and SRB had a certain synergistic effect on treating agate dyeing wastewater.

INTRODUCTION

A number of rural areas located in Fuxin, Liaoning Province, China, have developed a workshop-style agate dyeing industry. Because most of the untreated dyeing wastewater is discharged directly, the local environment has been seriously polluted. According to the local environmental protection department testing, the main characteristic pollutants of wastewater are the high concentration of hexavalent chromium and sulfate, with a strong acidity. The pollutants in agate dyeing wastewater cause great harm both to human health and aquatic ecosystems (Di et al. 2010). So it is necessary to treat the agate dyeing industrial wastewater in order to achieve harmless emissions.

As a common water treatment method, adsorption has an important position in the wastewater treatment field, and it has also been a research hotspot in recent years (Sud et al. 2008; Wan Ngah & Hanaah 2008). The process involves the removal of one or more substances from water by adsorption onto a porous solid material surface (Peng et al. 2016). Commonly used adsorbent materials are activated carbon, natural inorganic adsorbents, natural organic adsorbents and synthetic adsorbents (Ali & Gupta 2006). Activated carbon (Yahya et al. 2015), natural inorganic adsorbents (Kumar et al. 2010) and synthetic adsorbents (Saini et al. 2015) have very good adsorption abilities and removal effects for contaminants in water, but there are some problems such as high production costs, huge amounts of solid waste and other issues (Mohan et al. 2006). Natural organic adsorbents consist of agricultural biomass materials such as wood fiber, corn stalks (Wang et al. 2016), straw (Sarker & Fakhruddin 2017), wood chips, bark, peanut shells (Wang et al. 2015) and other cellulose and rubber (Rungrodnimitchai & Kotatha 2015); they can remove organic matter and heavy metal ions from the water and have been widely used in water treatment research due to their low cost, non-toxicity, ease of obtaining, etc. (Hokkanen et al. 2016).

In the Fuxin area of Liaoning, peanut shells are agricultural by-products, with advantages of huge annual output, good stability and mechanical strength to treat wastewater. They have good adsorption and removal performance for heavy metal ions (Yan et al. 2011; Wei et al. 2016). However, peanut shells are rarely used alone, because of their limited adsorption capacity, production of organic matter and other secondary pollution problems (Anish et al. 2008). Elemental iron, with strong chemical properties, can reduce strong oxidation ions, compounds and some organic matters (Saber et al. 2014). However, it can form oxide protective films on the iron surface, inhibit electron transfer and hinder the elemental iron redox process in the treatment of heavy metal ions in wastewater (Huang et al. 2013). Sulfate-reducing bacteria (SRB) are used to treat the sulfuric acid wastewater, which contains heavy metal ions, by alienation and reduction. It can remove heavy metals and sulfate and reduce acidity simultaneously (Han et al. 2017). However, SRB are susceptible to heavy metal ions and acidity, so the reaction delay is long and the processing speed is slow (Sandeep et al. 2017).

In this study, biomass adsorption, chemical redox and microbial degradation methods were combined to construct a peanut shells, scrap iron and SRB system to treat agate dyeing wastewater for the first time. This new treatment method can effectively overcome the defects of using peanut shells, scrap iron and SRB alone. It has the additional advantages of physical, chemical and biological treatment, with low costs and good operability. Firstly, the optimal particle size of peanut shells and scrap iron, optimum ratio of peanut shells to scrap iron and hydraulic retention time of the system were determined by setting up single-factor and orthogonal experiments. Secondly, the effect of the treatment of the agate dyeing wastewater by peanut shells with the assistance of scrap iron was studied by new dynamic experimental devices. Finally, the biomass materials treatment effect was strengthened by adding acclimated SRB sludge, and the effects and mechanisms of the biological and non-biological media synergy of treatment on agate dyeing wastewater are discussed. This study will provide a scientific basis for the treatment of agate dyeing wastewater.

METHODS

Experimental materials and water samples

The peanut shells were taken from local farmland in Fuxin, and the crushed sieved particle sizes were 3, 4, 5, 6 and 7 mesh respectively. Scrap iron was taken from a school training factory, and sieved after grinding particle sizes for 16, 32, 60, 100 and 200 mesh. The concentrations of Cr6+ and were 35–45 and 2,000–3,000 mg·L−1 respectively and the pH value ranged from 2.50 to 3.50 in simulated agate dyeing wastewater.

Experimental apparatus and method

A series of 100 mL conical flasks were used in the single-factor and orthogonal experiments, and the corresponding standard materials and 50 mL simulated wastewater were added to those flasks. The corresponding water quality indexes were measured after sampling.

The dynamic experiments were carried out by using five groups of columns with a diameter of 6 cm and a height of 50 cm. Column 1 was filled with peanut shells, column 2 was filled with scrap iron, column 3 was filled with scrap iron in the lower part and peanut shells in the upper part, column 4 was filled with peanut shells in the lower part and scrap iron in the upper part and column 5 was filled with a mixture of scrap iron and peanut shells. The upper and lower ends of the column were filled with gravel to a depth of 20 mm and 5–10 mm, respectively, as a protective layer. The experiment device was adapted to continuous operation, and the water inlet mode was ‘bottom in and top out’. The flow velocity was controlled with the peristaltic pump and the flow was 50–80 cm/d. On the 10th day, acclimated SRB sludge supernatant and sodium lactate nutrient source were added. After the microorganism had grown in the column completely, the agate dyeing wastewater was added and the dynamic experiment was continued for 20 days to investigate the removal efficiency for pollutants under SRB microbial intensification. At the same time, to ensure that the microorganisms had sufficient nutrients for growth and metabolism, 1 mL sodium lactate solution was injected into the columns at the 10th, 12th, 16th, 19th, 24th and 28th day during the experiment respectively. Regular sampling was taken once per day to determine the corresponding water quality indicators.

Water quality monitoring methods

Cr6+ was measured by diphenylcarbazide spectrophotometry, by barium chromate spectrophotometry, pH with a glass electrode, and chemical oxygen demand (COD) by rapid digestion spectrophotometry.

RESULTS AND DISCUSSION

Single-factor experimental study

Determination of the size of peanut shell particle

Five 100 mL conical flasks of uniform specifications were used. The peanut shells with particle size of 3, 4, 5, 6 and 7 mesh and the scrap iron with particle size of 60 mesh were added to these flasks respectively. The dosing volume ratio of scrap iron to peanut shells was 1:3. Then, 50 mL simulated wastewater was added to the flasks respectively. The hydraulic retention time was set to 24 h, and the changes of Cr6+, , COD and pH were measured to determine the optimum size range of peanut shells. The experiment results are shown in Table 1.

Table 1

Single-factor experiment results of peanut shells particle size

Size/mesh pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
5.81 2.873 0.56 679 
5.81 3.552 0.35 660 
5.92 4.35 0.6 634 
5.5 4.787 0.92 754 
5.46 7.176 1.22 920 
Size/mesh pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
5.81 2.873 0.56 679 
5.81 3.552 0.35 660 
5.92 4.35 0.6 634 
5.5 4.787 0.92 754 
5.46 7.176 1.22 920 

Table 1 shows that the concentration of , Cr6+ and COD in wastewater increased with the decrease of particle size of peanut shells, but the change of pH value was not significant. This could possibly be due to the small size of peanut shells with smaller surface area, and adsorption active sites of pollutant can be less, resulting in poor adsorption capacity, so the concentration of and Cr6+ was high (Zhang et al. 2016a). Small size peanut shells were easily hydrolyzed into organic matter in water, so the content of COD increased significantly. The amplitude of pH value was not large, which showed that the size of peanut shells had no significant effect on pH value. Based on the above four kinds of water quality indexes, the optimal size range of peanut shells was 3, 4 and 5 mesh.

Determination of the size of scrap iron particle

Five 100 mL conical flasks of uniform specifications were used. The scrap iron with particle sizes of 16, 32, 60, 100 and 200 mesh and the peanut shells with particle size of 5 mesh were added to these flasks respectively. The dosing volume ratio of scrap iron to peanut shells was 1:3. Then, 50 mL simulated wastewater was added to the flasks respectively. The hydraulic retention time was set to 24 h, and the changes of Cr6+, , COD and pH were measured to determine the optimum size range of scrap iron. The experiment results are shown in Table 2.

Table 2

Single-factor experiment results of scrap iron particle size

Size/mesh pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
16 6.75 2.133 0.5 280 
32 7.01 2.849 0.2 274 
60 7.4 1.339 0.17 217 
100 7.86 3.612 0.45 233 
200 8.15 6.44 0.2 300 
Size/mesh pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
16 6.75 2.133 0.5 280 
32 7.01 2.849 0.2 274 
60 7.4 1.339 0.17 217 
100 7.86 3.612 0.45 233 
200 8.15 6.44 0.2 300 

Table 2 shows that with the decrease of the iron particle size, the concentration of in the wastewater showed a certain upward trend, while Cr6+ showed a declining trend. The pH value significantly increased, but the change of COD was not significant, possibly because scrap iron had a strong reduction performance and could react with H+ in wastewater, which can better improve the pH value. With the decrease of scrap iron particle size, Fe(OH)3 was more likely to be produced in the acidic water environment, resulting in the aqueous solution being weakly alkaline (Deng et al. 2016). At the same time, as the scrap iron particle size decreased, its ability to reduce the anion weakened, so the concentration of significantly increased. The smaller size of the iron powder can enhance the reduction of heavy metal ions and it showed a strong reductive removal ability for Cr6+ (Gheju & Balcu 2011). Because COD in water was mainly produced by peanut shells hydrolysis of macromolecular organic matter, the COD change was not obvious. Therefore, based on the above four kinds of water quality indexes, the optimal particle size range of scrap iron was 16, 32 and 60 mesh.

Determination of the dosing ratio of the scrap iron and the peanut shell

Five 100 mL conical flasks of uniform specifications were used. The scrap iron with particle size of 60 mesh and the peanut shells with particle size of 5 mesh were added to these flasks respectively. The dosing volume ratio of the scrap iron and peanut shells were 1:1, 1:2, 1:3, 2:1 and 3:1 respectively. Then, 50 mL simulated wastewater was added to the flasks respectively. The hydraulic retention time was set to 24 h, and the changes of Cr6+, , COD and pH were measured to determine the optimum dosing ratio range of scrap iron and peanut shells. The experiment results are shown in Table 3.

Table 3

Single-factor experiment results of scrap iron and peanut shells dosing ratio

Dosing ratio pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
1:1 7.87 2,216 0.16 551 
1:2 7.56 2,259 0.31 1,041 
1:3 7.43 1,649 0.35 967 
2:1 7.56 2,276 0.26 443 
3:1 7.66 2,295 0.15 679 
Dosing ratio pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
1:1 7.87 2,216 0.16 551 
1:2 7.56 2,259 0.31 1,041 
1:3 7.43 1,649 0.35 967 
2:1 7.56 2,276 0.26 443 
3:1 7.66 2,295 0.15 679 

Table 3 shows that with the changes of dosing volume ratio of the scrap iron and peanut shells, the pH value could be adjusted to neutral range. With the increase of peanut shells ratio, the concentration of decreased, while the concentration of Cr6+ increased, and release of COD firstly increased and then decreased. With the increase of scrap iron ratio, the concentration of showed a rising trend, while the concentration of Cr6+ showed a decreasing trend, and the release of COD firstly decreased and then increased.

The above shows that both scrap iron and peanut shells had a significant effect on the regulation of pH. Meanwhile, because the scrap iron had a strong negative charge and a poor removal of anion (Nunomura & Sunada 2016), removal of by the peanut shells was dominant, while removal of Cr6+ by the scrap iron was relatively dominant (Liu et al. 2015). So with an increase of the peanut shells ratio, the adsorption ability of peanut shells increased, but the oxide redox ability of scrap iron weakened. In contrast, with an increase of the scrap iron ratio, its oxide redox ability increased, and a large amount of hydroxide precipitation was deposited on the peanut shell surface (Yadav et al. 2012), which would hinder the peanut shells adsorption of . Meanwhile, the oxide redox process of scrap iron would improve the hydrolysis rate of peanut shells in water (Sadiq et al. 2014), so scrap iron would affect the efficiency of release of COD and adsorption of ions by peanut shells.

Therefore, the concentration changes of and Cr6+ were negatively correlated in the wastewater, while the release of COD was related to the dosing ratio of scrap iron and peanut shells. Therefore, based on the above four kinds of water quality indexes, the optimal dosing ratios of the scrap iron and peanut shells were 1:2, 1:3 and 2:1.

Determination of hydraulic retention time

Five 100 mL conical flasks of uniform specifications were used. The scrap iron with a particle size of 60 mesh and the peanut shells with a particle size of 5 mesh were added to these flasks respectively. Dosing volume ratio of the crap iron and peanut shells was 1:3. Then, 50 mL simulated wastewater was added to the flasks respectively. The hydraulic retention time was set to 12, 24, 36 and 48 h, respectively, and the changes of Cr6+, , COD and pH were measured to determine the optimum range of hydraulic retention time. The experiment results are shown in Table 4.

Table 4

Single-factor experiment results of hydraulic retention time

Time/h pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
12 4.76 256.7 0.46 908 
24 5.92 49.24 0.31 1,061 
36 6.2 51.48 0.28 860 
48 6.48 20.45 0.18 871 
Time/h pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
12 4.76 256.7 0.46 908 
24 5.92 49.24 0.31 1,061 
36 6.2 51.48 0.28 860 
48 6.48 20.45 0.18 871 

Table 4 shows that with an increase of hydraulic retention time, the removal efficiency of and Cr6+ was significant. The release of COD had a decreasing trend, and the pH value increased gradually to neutral. Possibly with the hydraulic retention time extended, peanut shells and scrap iron could fully play the role of adsorption reduction (Di et al. 2015a), which had a better treatment effect on and Cr6+ in wastewater. On the one hand, the adjustment of pH was due to the reaction of scrap iron with H+ to enhance the pH value. On the other hand, Fe2+ formed Fe(OH)3 precipitate with OH in water, which enhanced the alkalinity of the system, so pH value increased significantly. With the extension of time, peanut shells hydrolyzed more fully, and the water content of organic matter increased, so the COD concentration was increased significantly. Therefore, based on the above four kinds of water quality indexes, the optimal range of hydraulic retention time was 24, 36 and 48 h.

Orthogonal experimental study

Based on the above single-factor experiment results, an L9(34) orthogonal experiment was carried out with the factors of peanut shells diameter, scrap iron particle diameter, dosing ratio of scrap iron to peanut shells and hydraulic retention time. Orthogonal experiment settings are shown in Table 5. The experiment results are shown in Table 6.

Table 5

Orthogonal experiment setup table

Serial number Peanut shells diameter/mesh Scrap iron diameter/mesh Scrap iron:peanut shells dosing ratio Hydraulic retention time/h 
60 1:2 24 
32 1:3 36 
16 2:1 48 
60 1:3 48 
32 2:1 24 
16 1:2 36 
60 2:1 36 
32 1:2 48 
16 1:3 24 
Serial number Peanut shells diameter/mesh Scrap iron diameter/mesh Scrap iron:peanut shells dosing ratio Hydraulic retention time/h 
60 1:2 24 
32 1:3 36 
16 2:1 48 
60 1:3 48 
32 2:1 24 
16 1:2 36 
60 2:1 36 
32 1:2 48 
16 1:3 24 
Table 6

Orthogonal experiment results

Serial number pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
Wastewater 3.02 2,511 35.21 – 
7.5 2,363 1.3 508 
6.59 2,006 0.61 952 
8.59 2,211 0.26 409 
6.79 1,517 0.27 815 
7.44 2,377 4.33 102 
6.55 1,295 0.35 744 
6.65 2,505 0.28 390 
6.67 1,994 0.2 691 
7.26 2,258 4.04 829 
Serial number pH SO42−/mg·L1 Cr6+/mg·L1 COD/mg·L1 
Wastewater 3.02 2,511 35.21 – 
7.5 2,363 1.3 508 
6.59 2,006 0.61 952 
8.59 2,211 0.26 409 
6.79 1,517 0.27 815 
7.44 2,377 4.33 102 
6.55 1,295 0.35 744 
6.65 2,505 0.28 390 
6.67 1,994 0.2 691 
7.26 2,258 4.04 829 

Tables 5 and 6 show that based on the above four kinds of water quality indexes, the best results of the orthogonal experiment determined by the relevant mathematical methods corresponded to serial number 1, i.e. particle size of the peanut shells was 3 mesh, particle size of the scrap iron was 60 mesh, dosing ratio of scrap iron and peanut shells was 1:2 and the hydraulic retention time was 24 h.

Dynamic experimental study

According to the orthogonal experiment results and the above experimental scheme, the dynamic columns 1, 2, 3, 4 and 5 were structured respectively. The experimental apparatus is shown in Figure 1. The test results for water quality at all stages and analysis of the mechanism in dynamic experiments are discussed below.
Figure 1

Dynamic experiment device system diagram.

Figure 1

Dynamic experiment device system diagram.

Analysis on the change rules of

The concentration and removal rate of in the five columns are shown in Figure 2.
Figure 2

removal in columns 1, 2, 3, 4 and 5.

Figure 2

removal in columns 1, 2, 3, 4 and 5.

Figure 2 shows that the average removal rates of in columns 1, 2, 3, 4 and 5 were 21.11, 29.91, 26.41, 30.17 and 23.42% respectively in the first 9 days of the experiment. The removal rate of column 1 fluctuated during the first 7 days of the reaction, and the removal rate increased significantly on days 8–9. The possible reason was that the peanut shells had been soaked in water for a long time so that they were evenly dispersed, which increased the contact area of peanut shells and wastewater to improve the adsorption removal efficiency of . The removal rate of in column 2 was on the rise, and was significantly higher than that in column 1. This was because the elemental iron had an active chemical nature, and lower electrode potential, which led to a high reducing ability. It can reduce strong oxidizing ions, compounds and some organic matter. Scrap iron generated Fe2+ and released reduction [H] in the acidic environment, and would be reduced with the formation of metal sulfide precipitation of Fe2+ in water. Therefore, a higher removal rate was achieved than that of the peanut shells single adsorption. The removal rate of in column 3 did not fluctuate in the first 8 days, which may be due to the fact that peanut shells and scrap iron cooperatively adsorbed and reduced to keep the removal rate relatively stable. The removal rate increased significantly on the 9th day, which may be because the hydrolysis of scrap iron in the water generated Fe(OH)3 flocculent colloid (Di et al. 2015b). This could have a certain adsorption of ions in water, so the removal rate had a certain increase. The removal rate of in column 4 showed a certain fluctuation trend, and the whole removal efficiency was better than that of single scrap iron or peanut shells, and it was better than that of column 3 (scrap iron in the lower part, peanut shells in the upper part). The possible reason was that the density of iron chips was much greater than that of peanut shells. The scrap iron were placed on the upper layer, which stopped the peanut shells from floating and allowed full contact with the wastewater, thus improving the removal effect of with peanut shells. The removal rate of in column 5 showed a certain fluctuation, possibly because mixing of peanut shells and scrap iron made scrap iron adhere to the surface of peanut shells. After a period of time, the scrap iron formed metal oxides or hydroxide precipitation, which deposited on the peanut shells' surface, blocked peanut shells' surface and internal pores, reduced adsorption sites of peanut shells in contact with wastewater and reduced the adsorption capacity of peanut shells for (Zhang et al. 2016b), so the removal rate of this column was low.

On the 10th day, the acclimation culture supernatant of SRB sludge was added. The average removal rates of in columns 1, 2, 3, 4 and 5 were 24.42, 25.39, 27.72, 25.34 and 28.55% respectively. On days 10–14, the removal rate of in column 1 showed a downward trend, possibly because the adaptive ability to the environment of initial growth of microorganisms is poor, and the heavy metal ions in the wastewater and the acidic environment inhibited the microbial activity, which was bad for the growth and metabolism of microorganisms.

After the 15th day, the removal rate of began to increase, but the fluctuation was great. This is because the hydrolysis of peanut shells in water could not meet the growth and metabolic demands of microorganisms in the later stage of the reaction. Therefore, sodium lactate was added as a bio-carbon source. On the 10th, 12th, 16th, 19th, 24th and 28th day, 1 mL sodium lactate was added into the columns. With sufficient carbon source, was far greater than 0.67, which can fully meet the microbial metabolic activity, so the removal effect was enhanced. When the microbial metabolism of sodium lactate was completed, because will be lower than the needs of microbial growth and metabolism, the removal rate showed a downward trend (Kiran et al. 2015). Therefore, great fluctuations in the curve occurred. The removal rate of column 2 was lower at days 10–14, and increased significantly after the 14th day. The highest removal rate was 57.76%. The total removal rate of column 2 was higher than that of column 1, which was due to the saturation of peanut shells in the late stage, which did not contribute to the removal of . Most was removed by SRB. In addition to SRB metabolic in the column 2, although there was a part of rust in the scrap iron, the partly corroded scrap iron could still provide the reduction electrons for the microorganisms, and the colloidal suspensions produced by the scrap iron in water also had certain adsorption properties. Therefore, the removal effect of by synergistic scrap iron and SRB was better than that of column 1. Column 3 had a better overall effect than column 2, because the peanut shells in column 3 still had a certain adsorption effect in the later period, and the peanut shells could provide the carbon source for the SRB when the carbon source had a slow-release effect (Lin et al. 2016). The removal rate of column 4 decreased slightly in the initial stage, but the fluctuation increased later. The overall removal effect was slightly worse than that of column 3, possibly because the upper part of column 4 was scrap iron and the lower part of it was peanut shells. SRB sludge supernatant was added from the top of columns, so the scrap iron had more SRB hanging film. However, the upper part of column 3 was the peanut shells, and the lower part was scrap iron, so the main film developed on the upper peanut shells. Peanut shells can provide nutrients for SRB as a bio-sustained-release carbon source. SRB on peanut shells can accept nutrients better, so the removal rate was slightly better than that of column 4. The initial removal rate of column 5 decreased slightly and increased rapidly on the 12th day. The removal rate of the corresponding days was obviously higher than that of the other four columns. Possibly because mixed scrap iron and peanut shells dispersed evenly, when added to SRB sludge, the surfaces of peanut shells and scrap iron were all linked to the film. The microbial nutrient source was sufficient and scrap iron was more convenient for providing electrons to microorganisms. So the removal effect of it was better than that of the layered matrix or a single matrix. In conclusion, microorganisms could strengthen the removal of with scrap iron and peanut shells to a certain extent, and the removal rate of had a certain increase, but the overall removal rate of was still at a low level. Possibly because the adsorption reduction effect of scrap iron and peanut shells on the and other anion was poor, although microbial addition had certain strengthening, the microorganisms were not biologically immobilized, resulting in a certain amount of microorganisms in a free state. The metabolic efficiency for was lower (Bao & Dai 2013), so it was difficult to achieve a high removal effect.

Analysis on the change rules of Cr6+

The concentration and removal rate of Cr6+ in the five columns are shown in Figure 3.
Figure 3

Cr6+ removal in columns 1, 2, 3, 4 and 5.

Figure 3

Cr6+ removal in columns 1, 2, 3, 4 and 5.

This figure shows that the average removal rates of Cr6+ in columns 1, 2, 3, 4 and 5 were 11.76, 96.16, 89.83, 88.36 and 87.21% respectively in the first 9 days of the experiment. The removal rate of column 1 was low and fluctuating, possibly because the adsorption of Cr6+ by single substrate peanut shells was only physical adsorption, which had low adsorption efficiency; meanwhile Cr6+ can be returned to the wastewater after adsorption and the removal effect was poor. The average removal rate of column 2 was much higher than those of the other four columns, possibly because of the strong reduction ability of scrap iron, which can reduce the high-valence toxic heavy metal Cr6+ to low-valence Cr3+ and form precipitates with other ions in water (Gao et al. 2008). So the treatment effect was much better than for a single physical adsorption column. The average removal rate of column 3 was higher in the first 7 days, showing a fluctuating state, possibly because the synergistic effect of peanut shells and scrap iron improved the limitations of the single peanut-shells physical adsorption. One part of Cr6+ was reduced to Cr3+ and the rest was removed by peanut shells adsorption. On the 8th day, the removal rate of Cr6+ decreased significantly, possibly because, on the one hand, peanut shells adsorption saturation led to lower Cr6+ removal effect and, on the other hand, scrap iron rusted in the water to form hydroxide precipitation or metal oxide precipitation, which made it lose a strong reduction ability, and the removal effect decreased. The removal rate curve of column 4 showed a decreasing trend in the first 6 days, possibly because the lower part of column 4 was filled with peanut shells and the upper part was filled with scrap iron, and the water inlet mode was ‘bottom in and top out’. Therefore, in the initial stage, the removal of Cr6+ mainly depended on the adsorption of the lower peanut shells, and the scrap iron did not play a great role at this stage, so the removal rate of Cr6+ decreased with the peanut shells being saturated with Cr6+. When the adsorption of peanut shells became saturated, the Cr6+ in wastewater was mainly removed by scrap iron reduction and the removal was more complete, so the removal effect was obviously enhanced. The removal rate of column 5 was slightly lower than those of column 3 and 4, possibly because scrap iron adhered to the surface of peanut shells by mixing peanut shells and scrap iron. In an acidic environment, the scrap iron formed metal oxides or hydroxide precipitation, which deposited on the peanut shell surface, blocked the peanut shell surface and internal pores, reduced adsorption sites of peanut shells in contact with wastewater and reduced the adsorption capacity of peanut shells for Cr6+. So the removal rate of this column was slightly lower than those of columns 3 and 4.

On the 10th day, the acclimated SRB sludge supernatant was added, and the average removal rates of Cr6+ of columns 1, 2, 3, 4 and 5 were 92.76, 99.35, 99.24, 99.31 and 99.15% respectively. After the addition of microorganisms, the removal effect of column 1 was significantly improved, possibly because the microorganism had a certain metabolic capacity for Cr6+, which can reduce Cr6+ to the state of low-cost non-toxic ions by biological reduction (Zhao et al. 2011), thus improving the lower removal rates of physical adsorption by the single peanut shells system. However, the removal rate of this column was still at a low level. It is because of the lack of scrap iron in the column, which can provide the SRB with the electron pairs required for growth reduction (Han et al. 2016). So the metabolic activity of SRB was inhibited, and the removal rate was low. The removal effect of column 2 was the best. Because the strengthening of scrap iron improved the capacity for SRB growth and metabolism, the removal rate increased significantly. The removal effect of columns 3, 4 and 5 was not much different. It shows that the strengthening effect of microorganisms can effectively make up for the different removal effects caused by the difference between the scrap iron and peanut shells in different orders. Through the removal of Cr6+ by microbial-enhanced scrap iron and peanut shells, the removal rate of all three groups of columns could reach over 99%, meeting the ‘National Comprehensive Wastewater Discharge Standard’ (GB 8978-1996). In summary, the Cr6+ removal effect could be significantly enhanced by microorganisms, which can be used to enhance the removal of Cr6+ by scrap iron and peanut shells.

Analysis on the change rules of COD

The concentration of COD in the five columns is shown in Figure 4.
Figure 4

COD changes of columns 1, 2, 3, 4 and 5.

Figure 4

COD changes of columns 1, 2, 3, 4 and 5.

Figure 4 shows that the average effluent COD concentration of columns 1, 2, 3, 4 and 5 were 258.56, 0, 140.56, 62.11 and 100.89 mg·L−1 respectively in the first 9 days of the experiment. The early effluent COD concentration of column 1 was too high, due to the physical release of small particles or small molecules on the surface of the biomass. Then the release of COD significantly decreased, because peanut shells were gradually consumed in the water. Later the COD value released by the biomass was significantly decreased (Zhu et al. 2014). Column 2 was not filled with peanut shells, so no COD was released. The release amount of COD in columns 3 and 4 was less than that in column 1, which was filled with peanut shells only, because the proportion of added peanut shells in columns 3 and 4 was less than that in column 1. As a result, less COD was released. Compared with columns 3 and 4, the COD release rate of column 3 was higher than that of column 4, possibly because the upper filling of column 3 was peanut shell, and the effluent was in the upper part, which may bring out some small peanut shell debris. Column 5 was filled with a mixture of scrap iron and peanut shells, and its release of COD was higher than that of column 4, but less than that of column 3. Because the mixing made peanut shells and scrap iron evenly distributed in the column, the possibility of small peanut shells being taken out by the water was reduced. However, the possibility still existed compared to column 4 where peanut shells were placed under the scrap iron. So the COD release amount of column 5 was at the intermediate level.

On the 10th day of experiment, the SRB sludge was added after acclimatization. The average effluent COD concentration of columns 1, 2, 3, 4 and 5 were 517, 506.7, 517.9, 513.75 and 531.35 mg·L−1 respectively. The reason for the high COD release was that the hydrolysis of peanut shells in water could not meet the growth and metabolism requirements of microorganisms at the later stage of the reaction. Therefore, the experiment used sodium lactate as a biological carbon source. On the days 10, 12, 16, 19, 24 and 28, 1 mL sodium lactate was added into the columns. The results showed that there was a significant increase of COD release at six times, all on the 10th, 12th, 16th, 19th, 24th and 28th day respectively, and this decreased significantly after being metabolized by microbes. Compared with the above and Cr6+ removal rate curves, when the amount of COD release was large, the nutrient source in the wastewater was sufficient, and the microbial metabolism was strong, so the removal rate of and Cr6+ increased obviously. When the COD release decreased, and Cr6+ removal rate also decreased. It showed that the control of COD on the and Cr6+ removal was significant.

Analysis on the change rules of pH

The pH values in the five columns are shown in Figure 5.
Figure 5

pH changes of columns 1, 2, 3, 4 and 5.

Figure 5

pH changes of columns 1, 2, 3, 4 and 5.

Figure 5 shows that the average effluent pH value of columns 1, 2, 3, 4 and 5 was 5.57, 6.95, 6.61, 7.09 and 6.87 respectively in the first 9 days of the experiment. The amino group, hydroxyl and other active functional groups in the peanut shell surface could accept a large number of H+ in the water, and scrap iron could react with H+ in the wastewater (Alothman et al. 2013). Therefore, the five columns all had a certain ability to regulate pH values. The effluent average pH of column 1 was lower than those of the other four columns, due to the lack of scrap iron for the regulation of pH in column 1. Only peanut shells were available for H+ adsorption, so a limited control was achieved. The improvement ability of pH value in the first 2 days was strong in column 2, and the curve was not volatile largely later on, partly because the small pieces of debris on the surface of scrap iron were easily reduced, and consumed a large number of H+ to improve the pH values of the effluent. As the reaction progressed, the scrap iron rusted in the acidic water environment and an oxide film or a hydroxide precipitate formed on the surface of the scrap iron, which had a certain inhibitory effect on the reaction of elemental iron with H+, so regulation capacity declined. The pH range of column 3 effluent was not great, which may be due to the fact that peanut shell adsorption and iron reduction occurred in conjunction with the treatment of H+, so the process was more stable, and the regulation capacity was more stable, too. The average effluent pH value of column 4 was higher than that of column 3, possibly because column 4 was filled with the scrap iron at the top. As the water directly flowed out after the reduction through the scrap iron, and did not mix with the low pH inflow water, it had a strong effect on improving the wastewater pH value. Column 4 had a strong ability to enhance the improvement in pH in the early stage, but it had a downward trend to a steady state later, possibly because H+ was mainly absorbed by the lower peanut shells in the early stage. The process of adsorption was not complete and with the adsorption saturation, the ability to adjust the pH value decreased. However, due to the main reduction coming from scrap iron, the regulation effect became more stable later. The adjusting ability of column 5 was better than that of column 3, but slightly worse than that of column 4, possibly because mixing peanut shells and scrap iron could be more conducive to their respective advantages and played a complementary role. However, a slight gap existed compared to column 4, which used scrap iron screed to enhance pH before the outflow.

On the 10th day of experiment, the SRB sludge was added after acclimatization. The average effluent pH values of columns 1, 2, 3, 4 and 5 were 7.14, 7.80, 7.76, 7.93 and 7.92 respectively. Compared with before SRB addition, the pH of effluent of each column was improved obviously, and the effluent was nearly neutral. The reason was that SRB at low pH could produce a certain degree of alkalinity (Fang et al. 2010). Therefore, they could increase the pH values of the wastewater. It indicated that SRB had a certain contribution to the regulation of pH values.

CONCLUSIONS

  • The optimum size range of peanut shells was 3, 4 and 5 mesh by setting a single-factor experiment. The optimum size range of scrap iron was 16, 32 and 60 mesh. The best dosing ratios of scrap iron and peanut shells were 1:2, 1:3 and 2:1. The optimum hydraulic retention times were 24, 36 and 48 h.

  • By setting an orthogonal experiment and using the relevant mathematical analysis, the optimal experimental conditions were determined as follows. The diameter of peanut shells was 3 mesh; scrap iron particle size was 60 mesh; the dosing ratio of scrap iron and peanut shells was 1:2; hydraulic retention time was set to 24 h.

  • Through the dynamic experiment study, it was concluded that treatment in column 4 was the best. The lower part was filled with peanut shells, and the upper part with scrap iron. The volume ratio of peanut shells and scrap iron was 2:1. The removal rates of and Cr6+ were 30.17% and 88.36% respectively before the addition of microorganisms, and 25.34% and 99.31% respectively after the addition. The average release of COD was 62.11 and 513.75 mg·L−1 before and after addition of microorganisms respectively. The respective average effluent pH was 7.09 and 7.93, and the effluent was near to neutral. It indicated that the synergistic effect of peanut shells and scrap iron had a certain role in the agate dyeing wastewater treatment. SRB had a certain strengthening effect on the treatment of agate dyeing wastewater.

ACKNOWLEDGEMENTS

This project was funded by the National Natural Science Foundation of China (41672247, 41102157), Liaoning Provincial Natural Science Foundation of China (2015020619) and Liaoning Provincial Department of Education (LJYL031).

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