Study on the Remediation of Cd-contaminated Soil

Yushan WAN, Chunxia JIA, Hao LV, Yanqiu CHEN, Li Huang

College of Environment and Safety Engineering, Changzhou University, Changzhou 213164, China

With the gradually rapid industrialization and urbanization,a lot of chemical enterprises have closed and moved in recent years, these chemical enterprises have left many contaminated sites because of manufacturing chemical products, dumping and discharging wastes and chemical substances,etc.,thereby resulting in a series of serious site soil pollution problems. With the land-use function transformation and secondary development of contaminated sites, contaminated soil has already become “chemical time bomb”,severely endangering human health and environmental safety[1].

Cd is a contaminant with higher hazard in the soil of chemical contaminated sites, its concentration is high,and it is difficult to decompose in the soil and easy to concentrate in vivo,thereby causing great damage to soil ecological environment and human health[1-4].

For the heavy metal contaminated soil, physical, chemical and biological remediation technologies are used for governing. Phytoremediation, as one kind of biological remediation technologies, has the characteristics of green and environmental protection and no secondary pollution,thus it is a good remediation technology with good prospect in recent years. The successful phytoremediation cases[4-5]are: phytoremediation demonstration project of Cu- and Pb-contaminated soil in Lechang of Guangdong, land use regeneration project of Liyang Yanshan Park, heavy metal pollution treatment project of the Big Huanjiang River Basin and contaminated soil phytoremediation of Youtian District in Puyang City.

The soil remediation technology of Cd-contaminated sites was analyzed, and the objective remediation plants with strong enrichment capacity in the soil of contaminated sites were studied, thereby providing some reference for Cd-contaminated soil.

Test and Methods

Experimental soil

The experimental soil was the topsoil (0-40 cm)of paddy fields in a experiment station of Changzhou City,the soil pH was regulated to 6.0-6.5 using quick lime; two weeks after airdrying, the soil was loaded into basins after sifting through 10 meshes, and each basin with 10 cm(diameter)×9 cm(height)had 300 g soil,the saucer was put under the basin. A portion of the soil sample was taken out for freeze drying, and then sifted through 100 meshes after smashing,so as to measure the basic properties. The basic physicochemical properties and pollutant concentrations of the soil were in Table 1.

Table 1 The physical and chemical properties of the experimental soil

The planting of plants

The experimental plant was perennial ryegrass which was screened out from preliminary study of research group and had good tolerance and remediation effect to Cdcontaminated soil, the seeds of the ryegrass were purchased from Jiangsu Academy of Agricultural Sciences.

The soil moisture content was adjusted to about 60% of field capacity,then a certain amount of N(100 mg/kg),P(100 mg/kg)and K(150 mg/kg)fertilizer was added; after a week of balance, the seeds of ryegrass were sown;then after a week of plant germination,the seedlings were thinned out,so as to ensure the same plants in each flowerpot(20 plants/flowerpot),at the same time,the check (CK)treatment was set,and each treatment had 5 repetitions. During pot experiment,the soil and plant samples were collected at 15, 30, 45, 60 and 75 d after the germination.

Sample treatment and testing The pretreatment of soil sample

Soil sample was placed in the refrigerator for cryopreservation, then freezer dryer was used for freeze drying, after that, it was sifted through 100 meshes after smashing using pulverizer,then it was stored in polyethylene valve bag and placed in dry container for measuring.

The analysis of total Cd content.1.000 g soil was weighed and added into digestion tank, then 1 ml H2O2, 3 ml HF, 3 ml HCl and 6 ml HNO3were respectively added, after that, the digestion tank was tightened and put in microwave digestion instrument, and the corresponding soil digestion method was selected for microwave digestion; after the digestion, the digestion tank was taken out for rinsing,and the volume of 50 ml was determinated for detecting.

The analysis of various forms of Cd. Tessier five-step approach[6]was adopted to respectively extract exchangeable form (S1), carbonate bounded form (S2), ferric-manganese oxidation state (S3), organic form(S4)and residual form (S5). Then 1.000 g soil sample was weighed to add into 18 ml 1 mol/L MgCl2(pH=7.0), after that, it was centrifuged for 20 min under the temperature of (22±5)℃after the vibration of 1 h,the supernate was exchangeable form(S1).18 ml 1 mol/L NaAc was added into S1 residue (adjusting to pH=5.0 using HAc) , then it was centrifuged for 20 min under the temperature of (25±5)℃after the vibration of 5 h, the supernate was carbonate bounded form (S2).20 ml 0.04 mol/L NH2OH·HCl was added into S2 residue (adjusting to pH =5.0 using HAc), then it was centrifuged for 20 min under the temperature of(96±3)℃after the vibration of 6 h,the supernate was ferric-manganese oxidation state(S3). 10 ml 0.02 mol/L HNO3and 5 ml 30%H2O2were added into S3 residue,and it was centrifuged for 2 h under the temperature of (85±2) ℃, then 3 ml 30% H2O2was added, and it was centrifuged for 2 h under the temperature of (85±2) ℃, finally, 5 ml 3.2 mol/L NH4Ac and 20% HNO3mixed liquor were added,and it was centrifuged for 20 min under the indoor temperature after the vibration of 0.5 h, the supernate was organic form(S4). The residual form of S4 residue was extracted using total digestion method. All extracting solution was filtrated using quantitative filter papers, and the volume of 50 ml was determinated for detecting.

The pretreatment of plant sample

The soil and dirt attached on the plant sample were washed with tap water,then washed with distilled water, and blotted with filter papers. The cleaned plants were divided into two parts,namely root and shoot, and they were cut up and put into an oven to de-enzyme under 105 ℃for 1 h,finally,they were dried under 70 ℃until constant weight. The dry samples of plants were smashed and sifted through 100 meshes for detecting.

The analysis of Cd content in the plant. 1.000 g plant sample was exactly weighed and added into digestion tank, then 2 ml HCl and 5 ml HNO3were respectively added,after that,the digestion tank was tightened and put in microwave digestion instrument, after the digestion, the digestion tank was taken out for rinsing,and the volume of 50 ml was determinated for detecting.

The detection of samples The detection of Cd in the soil and ryegrass.NovAA 300 flame atomic absorption spectrophotometer (the wave length was 228.8 nm, the combustion supporting ratio was 0.15 and the gas flow was 65 NL/h)was used for measuring.

Results and Analyses

The change of Cd content in the soil at different periods As shown in Table 2, with the extension of planting time of ryegrass, total Cd content in the soil showed a gradual decline trend,and the enrichment transformation of Cd at different periods showed different variation trends.

During 0-15 d of planting,Cd content in the soil showed a slowly decreasing trend, because the plant was just at the germination and growth stage, and the biomass was less,thereby causing the low enrichment amount;during 15-60 d,Cd content in the soil showed an obviously decreasing trend, and Cd content in the soil at the 60 d was only 4.64 mg/kg, the re-moval rate reached 54.91% ; during 60-75 d, the decreasing trend of Cd content in the soil was not obvious,because ryegrass had already been mature, the biomass increased slowly or the plant had already reached the maximum biomass, but it still had decreasing trend,these indicated that the plant had certain enrichment effect after maturing, and would become stable at the 75 d. Different forms (exchangeable form, carbonate bounded form, ferric-manganese oxidation state, organic form and residual form)of Cd content in the soil all decreased in different degrees. During 0-75 d,the decreasing amplitude of Cd content of S1 and S2 was the most obvious, and other forms of Cd content all had certain decrease; Cd content of S3 and S4 in the soil was in the stable state, so it was difficult to remove and had certain decrease in the reparative process, this may be caused by the mutual transformation of different heavy metal forms.

The ryegrass effectively reduced Cd content in the soil, and its removal rate to exchangeable form, carbonate bounded form, ferric-manganese oxidation state, organic form and residual form respectively reached 95.28%,84.86%,47.06%,61.17%and 21.49%.

The change of Cd content in the plant at different periods During the period of phytoremediation, different tissues of the ryegrass at different periods were collected and detected,finally, the biomass of the ryegrass was weighed, and the dry weight of ryegrass in each 100 g was about 15.60 g. From Table 3, it was drawn that the aboveground part and root of the ryegrass had significant enrichment effect to Cd over time.

The Cd content of the aboveground part of ryegrass showed a quickly increasing trend during 0-45 d,and became stable after 45 d, because the plant was just at the germination and growth stage during 0-45 d, and the biomass increase was larger,thereby causing the quicker increase of enrichment amount. From the experimental process, it can be seen that the biomass of the plant had an obvious increase,and that at the 45 d was about 2-3 times of that at the 15 d;after the 45 d,although the biomass of the ryegrass had certain increase,the average content had not great change, indicating that the extraction of Cd from the aboveground part of the ryegrass was based on the increase of plant biomass. Cd content of the root had an obviously increasing trend during 0-60 d, and the variation was not great after 60 d, this showed that the enrichment of the ryegrass had already reached saturation at the 60 d,and the enrichment of Cd per unit began to stabilize. Therefore, the remediation of the ryegrass to Cd basically completed during 60-75 d.

Table 2 The content change of different Cd forms in the soil at different periods mg/kg

Table 3 The change of Cd content in the plant at different periods mg/kg

Table 4 The change of pollutant content in the soil at different periods mg/kg

Study on the Remediation and Application of Contaminated Sites

Schemes of the remediation and application of contaminated sites

The application site was at the southern districts of Changzhou City in Jiangsu Province,and the area was 1 500 m2. The contaminated site was mainly used to produce and prepare some chemical products, such as aluminium chloride anhydrous, lead sulfate tribasic, nickel oxide, alchlor, lead oxide,manganese sulfate, octanedioic acid, glycollic acid, dimethylbenzene and anisole, etc.,and these mainly include some heavy metals,such as Cd,Cr and Ni, etc., and some organic matters, such as polycyclic aromatic hydrocarbon, benzene and benzene,etc.With the gradual move of industrial enterprises within the area and urban and regional planning and adjusting,the plot would become residential or commercial site. Therefore, it was needed to carry out management and remediation on the heavy metal pollution of the plot before the reuse of the field plot in the later stage,so as to ensure the security of later use.

(1) Combined with the requirements of spot construction units and the actual situation of contaminated site, the block with the area of (5.0×2.0) m2in the contaminated site was selected; then about 0.5-1.5 m depth under the line of elevation in the contaminated site was excavated.

(2) The contaminated soil to be restored was excavated and piled in the pre-determined field (the stacking area had already carried out anti-seepage laying), and the contaminated soilwas isolated and protected, so as to prevent the secondary pollution because of wind and rain erosion, meanwhile the samples were taken out and detected.

(3) The proportion of eluant was mixed,and the test condition was set.

(4) The soil remediation integration devices of construction units, including soil breaking and dosing system,etc.,were used,so as to carry out handbreaking for the soil to be restored, and the eluant was prepared for soil elution.

(5) After the elution, the soil was backfilled to the root position, and the pollutant concentration was detected.

(6) The backfilled soil was compacted,then the ryegrass was planted.

(7) The samples were taken regularly and detected,and the test effect was investigated.Based on the detection and analysis, the pH value of the soil sample in the area was 5.36, the organic matter and available nitrogen contents were 35.64 g/kg and 49.3 mg/kg, respectively, the mean of Cd pollution concentration in the soil was 58.65 mg/kg, and the detection rate was 100%.

Application results

Chemical removal The flushing agent citric acid and β-cyclodextrin were prepared (the concentrations of citric acid and β-cyclodextrin were respectively 0.6 mol/L and 20 g/L,the pH of eluting agent was adjusted to 3,and the temperature of eluting agent was adjusted to 35 ℃using heating module in the integrated equipment). The soil was transmitted into the leaching module of the equipment using conveyor belt, then the eluting agent was added according to the liquid-solid ratio of 10, followed by stirring with the stirring intensity of 320 r/min; there were totally 4 times of elution, and each elution lasted for 3.4 h, then the eluting agent was discharged(to be recycled),the treated soil was put on the impermeable field for stabilizing. 15 d after the stabilization of air drying, the soil was backfilled and laminated. The results of soil sampling inspection showed that the removal rate of Cd was 80.14%.

Phytoremediation The quick lime was added into backfilled soil and the soil pH was adjusted to 6.0 -6.5,meanwhile, the soil humidity was adjusted and the field capacity was adjusted to about 60%;then the ryegrass was sown with the seeding rate of 10 g/m2using spreading way,at the same time, a certain amount of N, P and K fertilizer was added, the sampling analysis was carried out monthly, and the soil pH and humus concentration were adjusted,thereby ensuring better remove the soil pollutant during phytoremediation process.

During phytoremediation process,soil and plant samples were collected every 15 d for detecting, the process lasted until the stabilization of Cd content in the soil. As shown in Table 4,with the extension of planting time of ryegrass, ryegrass had better removal efficiency to Cd in the contaminated soil, and the enrichment and conversion to Cd during different periods of time showed different variation trends.During 0-60 d of ryegrass planting,Cd content in the soil showed a rapidly decreasing trend, and the removal rate reached 89.7%;after 60 d,the enrichment capacity of the plant to Cd gradually weakened with the extension of planting time.

The remediation technology of chemical elution-plant combination had better remediation effect to the contaminated soil;after the elution and remediation of the soil, the removal rate of Cd was 80.14%, moreover, 75 d after phytoremediation,specific pollutant Cd content in the soil greatly reduced, the remediation efficiency was 90.66%.

Conclusions

The remediation technology of chemical elution-plant combination effectively reduced Cd content in the contaminated soil; the removal rate of chemical elution remediation to Cd content in the soil reached 80.14%,and the combination of the two had 90.66% removal rate to Cd content.Phytoremediation not only improved the condition of combined contaminated soil, but also had green effect, thus it was of good economic effect, good ornamental value and wide application future.

The period of phytoremediation was relatively long, but it could be shortened by other remediation technologies in the late-stage study.

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