Yushan WAN, Yanqiu CHEN, Li HUANG, Hui FANG

School of Environmental&Safety Engineering, Changzhou University, Changzhou 213164, China

With rapid development of industrialization and urbanization, closed and relocated chemical plants are increasing in cities or surroundings, because local soils are seriously polluted by the industries in production, storage, discharge, and leakage.With transformation and secondary development of functions of the contaminated sites, polluted soils have become a kind of chemical timebomb, posing threats to human health and environment safety[1].

Pb and Cd are pollutants with high risks in soil environment of chemical plants, whose concentrations are examined high and hardly to be decomposed in soils. The substances usually integrate with PAHs, BTEX,and Alkanes,formulating metal-organic chemicals with higher toxicity, damaging soil eco-environment and human health[1-4].

For soils contaminated by heavy metals and organic matter, physical treatment, chemical treatment and bioremediation are all available. However,phytoremediation is a more economic and effective way of bioremediation, widely applied in soil remediation. For example, it is successively applied in treatment of reconstruction and reformation project of LiYang YanShan Park,contamination of Huan River caused by heavy metals, treatment of contaminated soils in Puyang Oil Farms, and treatment of contaminated soils in Lechang City[4-5].

Therefore, the research analyzed phytoremediation of contaminated chemical plants, selected targeted plants with hyper-accumulation capacity of contaminants and improved the remediation effects by adjusting bio-availability, providing references for soil remediation of the sites with combined pollutions of heavy metals and organic matters.

Phytoremediation Test on Contaminated Soils

Phytoremediation of contaminated soils by Pb and Cd

Toxic soils The test soils were collected from surface soils (0-30 cm)of paddy fields in an experimental station of Changzhou City, and then sifted with a sieve (×10) to detect background values and physicochemicalproperty of soils(Table 1).

The test soils were sifted with a sieve (×4 mm) and added with Pb(CH3COO)2·3H2O and CdCl2·2.5H2O in order to add Pb2+and Cd2+. Specifically, the concentrations were Pb(Cd)50(10),100(25),200(50)and 500(100)mg/kg (Table 2). Subsequently, the contents of Pb and Cd were measured after one week.

Table 1 Physicochemical property of soils

Table 2 Soil toxicity by combined contamination of Pb and Cd mg/kg

Selection, planting and testing of plants for soil remediation Brassica juncea and ryegrass were grown in the site contaminated by Pb and Cd.

At first, soils contaminated by Pb and Cd were put into a plastic basin(￠=15 cm, H=10 cm) where soils at 400 g were loaded and N, P and K fertilizers were applied. After one week, seeds of Brassica juncea and ryegrass were scattered on the basin and sunshine, temperature and humidity should be guaranteed.

Then,seedlings grown for a week should be thinned out to make sure the number of plant in every basin was the same (10-20 plants per basin), and fertilizers should be applied regularly.According to water demands of plants,plants should be watered with deionized water and moisture content of soil should be maintained 60% of waterholding capacity of the field.

Thirdly, plant growth should be recorded every day and plants and soils in basins should be sampled timely until plant ripening.

Finally,the test reagents included concentrated hydrochloric acid, acetic acid,30%H2O2,hydrofluoric acid,ammonium acetate, perchloric acid, concentrated nitric acid,concentrated sulfuric acid, Pb standard solution, and Cd standard solution; the test instruments included NovAA 300 flame atomic absorption spectrometry, FC-204 electronic scale, WD-12 waterbath pressure blowing concentrator,digital display water bath temperature oscillator; analysis of Cd and Pb was conducted with HNO3-HClO4and measured with a flame atomic absorption spectrometry.

Test results and analysis Available Pb and Cd can be effectively enriched by both of ryegrass and leaf mustard(Table 3-Table 4). The higher available Pb concentration, the higher enrichment strength of leaf mustard, and the enrichment coefficient could reach the highest of 2.81.On the other hand,the enrichment of ryegrass and leaf mustard on Cd is also significant. As time went on, the enrichment strength of available Cd by leaf mustard was growing, and enrichment coefficients achieved the peak at 1.60 on the 70th d. However, enrichment coefficient of ryegrass on available Cd was lower compared with leaf mustard. Hence,leaf mustard was chosen as a hyperaccumulator for combined pollution of Pb and Cd in soils.

Table 3 Enrichment coefficients of Pb by plants under pollution of Pb and Cd

Table 4 Enrichment coefficients of Cd by plants under pollution of Pb and Cd

Phytoremediation of soils with PAHs pollution

Soil contamination The test soils were air dried and sifted with a sieve(4mm). Then, B [a]P and 1-OH-P were dissolved in acetone, and the liquids were mixed with soils, followed by drying.It is notable that the quantities of B [a]P and 1-OH-P 0.25, 0.5 and 1.0 mg/kg.

Selection and planting of hyperaccumulators At first,the contaminated soils with B [a]P and 1-OH-P were loaded into a plastic basin (￠=20 cm,H=15 cm),added with fertilizers (N,P and K).After two weeks,seeds of ryegrass, alfalfa and Festuca arundinacea were scattered in the basin and it is necessary to guarantee seed germination[9-12].

Subsequently, seedlings were thinned out to guarantee the same number of plants per basin (10-20 plants per basin). Fertilizers should be applied monthly and deionized water was irrigated occasionally in order to guarantee water contents of 60% in farmlands.

Moreover, plant growth should be recorded every day and plants and soils in basins were sampled regularly until ripening.

The test reagents included benzoapyrene, 1-Pyrenol, acetone, normal hexane, acetonitrile, dichloromethane, methyl alcohol, anhydrous sodium sulfate, and silica gel; the test instruments included an Agilent1200 high performance liquid chromato-graph, a KQ-500TDB digital ultrasonic cleaner, a DHG-9246A electro-thermostatic blast oven, and a hypersil BDSC18 chromatographic column.

Test results and analysis Ryegrass,Festuca arundinacea and alfalfa all had enrichment effects on available PAHs, and enrichment coefficients kept growing upon time, except of alfalfa. After 60 d, enrichment coefficients of ryegrass on PAHs all kept higher compared with Festuca arundinacea and alfalfa in terms of different gradients.After 70 d,however,the coefficients of ryegrass at different gradients all maintained higher than 1,which were higher compared with Festuca arundinacea and alfalfa.Therefore, ryegrass was chosen as a hyperaccumulator for remediation of PAHs.

Phytoremediation of combined pollution of Pb-Cd-PAHs Soil contamination

The test soils were air-dried and sifted with a sieve Subsequently,the materials were added with Pb(CH3COO)2·3H2O and CdCl2·2.5H2O at a proportion and the concentrations were as follows: Pb (Cd) 50 (10), 100(25), 200 (50), and 500 (100) mg/kg(Table 1).Then,B [a]P and 1-OH-P,which were dissolved in acetone,were evenly spread on soils at 0.25,0.5 and 1.0 mg/kg, respectively, and the organic fertilizers were applied,involving P fertilizer at 100 mg/kg, N fertilizer at 100 mg/kg and K fertilizer at 150 mg/kg.

Selection and planting of hyperaccumulator At first, contaminated soils by heavy metals and organic matters were loaded into plastic basins(￠=20 cm, H=15 cm) and added with fertilizers (N, P and K). After two weeks, seeds of ryegrass and Brassica juncea were scattered in basins and the growth condition of seed germination should be guaranteed[13].

Then,seedlings grown for a week should be thinned out to make sure the number of plant in every basin was the same (10-20 plants per basin), and fertilizers should be applied once per month.According to water demands of plants, plants should be watered with deionized water and moisture content of soil should be maintained 60% of water-holding capacity of the field.

Thirdly, plant growth should be recorded every day and plants and soils in basins should be sampled timely until plant ripening.

Test results and analysis Ryegrass and leaf mustard have good enrichment effects on pollutants in soils at different gradients. For example, after 45 d,the enrichment coefficient of ryegrass on available Pb and PAHs kept higher compared with leaf mustard.After 60 d, the enrichments of leaf mustard on Pb, Cd and PAHs started declining and the coefficients showed decreasing. In contrast, the enrichments of ryegrass on Pb and PAHs kept growing. Therefore, with combined pollutions of Pb, Cd and PAHs,the enrichments of ryegrass performed stronger compared with leaf mustard.Finally, ryegrass was chosen as a hyperaccumulator for remediation of soils polluted with Pb,Cd and PAHs.

Research on Phytoremediation in Contaminated Sites

Planting of hyperaccumulator and influential factors of bio-availability

In the contaminated sites, totaling 27 soil sample sites were arranged and 20 -40 cm soils were sampled from every site. Subsequently, the sampled soils were mixed well and loaded into basins, added with N, P and K fertilizers as per C∶N=1∶150.It is notable that quicklime should be applied to maintain soil pH of 7.0-7.2 in order to guarantee the hyperaccumulators would perform the best in enrichment.

According to the test results of combined pollutions of Pb, Cd and PAHs, ryegrass was chosen as a hyperaccumulator for soils contaminated by heavy metals and organic matter and the selection procedures were as follows:

At first, the sampled soils were loaded into plastic basins(￠=15 cm,H=10 cm)and every basin contained 400 g soils, added with N, P and K fertilizers. It is notable that C ∶N should be kept at 1∶150 and soil pH of 7.0-7.2.A week later, seeds of ryegrass can be scattered in basins to guarantee seed germination in terms of sunshine and temperature.

Then,seedlings grown for a week should be thinned out to make sure the number of plant in every basin was the same (10-20 plants per basin), and fertilizers should be applied once per month.According to water demands of plants, plants should be watered with deionized water and moisture content of soil should be maintained 60% of water-holding capacity of the field.

Thirdly, plant growth should be recorded every day and plants and soils in basins should be sampled timely until the contents of Pb, Cd and PAHs were lower than soil limits.

Remediation progress

The enrichments of ryegrass on Cd reached higher at 6.7 mg/kg in 45-60 d. After 90 d, the content of available Cd was at 4.9 mg/kg, which was lower compared with standard value(Cd≤6.80 mg/kg),suggesting that the remediation effects are satisfied. On the other hand, the enrichments of ryegrass on Pb grew slowly during 15-45 d. After 70 d, the content of available Pb was at 375.26 mg/kg, which was lower compared with standard value (Pb ≤400 mg/kg), suggesting that the remediation effects are satisfied. As for B [a]P, the enrichment of ryegrass reached 0.06 mg/kg on the 110thd, which was lower than soil limit(B[a]P＜0.63 mg/kg),suggesting the remediation is excellent.

Conclusions

In the test of phytoremediation of contaminated soils with Pb and Cd,the higher available Pb concentration, the higher enrichment strength of leaf mustard. As time went on, the enrichment strength of available Cd by leaf mustard was growing, and enrichment coefficients achieved the peak at 1.60 on the 70thd. However,enrichment coefficient of ryegrass on available Cd was lower compared with leaf mustard.

In the test of phytoremediation of contaminated soils with PAHs,after 70 d,the coefficients of ryegrass at different gradients all maintained higher than 1, which were higher compared with Festuca arundinacea and alfalfa.

In the test of phytoremediation of contaminated soils with Pb, Cd and PAHs,after 45 d,the enrichment coefficients of ryegrass on available Pb and PAHs kept higher compared with leaf mustard. After 60 d, the enrich-ments of leaf mustard on Pb, Cd and PAHs started declining and the coefficients showed decreasing.In contrast,the enrichments of ryegrass on Pb and PAHs kept growing.

It can be concluded from the phytoremediation test that after 90 d, the content of available Cd was at 4.9 mg/kg, which was lower compared with standard value(Cd≤6.80 mg/kg),suggesting that the remediation effects are satisfied. On the other hand, the enrichments of ryegrass on Pb grew slowly during 15-45 d. After 70 d, the content of available Pb was at 375.26 mg/kg, which was lower compared with standard value (Pb≤400 mg/kg),suggesting that the remediation effects are satisfied. As for B[a]P, the enrichment of ryegrass reached 0.06 mg/kg on the 110thd, which was lower than soil limit (B[a]P＜0.63 mg/kg), suggesting the remediation is excellent.

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