Phytoremediation : an innovative technique

Taking Shouguang Facility agriculture in Weifang City as the research object, heavy metals and other pollutants in new and old facility agriculture soil were investigated and analyzed from 2017 to 2019. The content of heavy metals in soil was analyzed, and the environmental quality of heavy metals was evaluated by single-factor pollution index method and Nemerow index method. The correlation between heavy metal content in surface soil and cultivation years, vertical distribution of heavy metal content in soil, frequency distribution of heavy metal in surface soil, and correlation between heavy metals in surface soil and soil nutrients were analyzed. The results showed that the average contents of the eight heavy metals were Cd 0.23 mg•kg−1, Hg 0.05 mg•kg−1, As 8.09 mg•kg−1, Pb 23.31 mg•kg−1, Cr 70.81 mg•kg−1, Cu 38.54 mg•kg−1, Ni 25.87 mg•kg−1 and Zn 147.52 mg•kg−1. Aside from Pb and Ni, the average content of all other heavy metals in the surface soil of facility agriculture was higher than the average of the control and the geochemical background level of soil in Weifang. Compared with the Environmental Quality Evaluation Standard for Farmland of Greenhouse Vegetables Production (HJ333-2006) and the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land (trial) (GB15618-2018), the average content of heavy metals met the standards, with Cd, Hg and Zn content in some samples exceeding the standards. Most surface soil environmental quality is clean or relatively clean, but there is also part of the soil has reached the level of mild or moderate pollution. The content of heavy metals in the soil of the old facility agriculture (cultivation years > 10) was higher than that of the new facility agriculture (cultivation years ≤ 10), and the content of heavy metals in the surface soil (0∼20cm) was higher than that in the deep soil. The quality distribution of heavy metals in the surface soil is affected by exogenous heavy metals, among which Hg and Cd have the largest skewness coefficient and are most affected by exogenous. There was significant positive correlation between heavy metals in surface soil and available phosphorus, hydrolytic nitrogen, available potassium and organic matter. The results indicated that the accumulation of heavy metals in soil was related to the application of organic fertilizer, phosphate fertilizer and compound fertilizer.

The rapid urbanization in China may lead to heavy metal pollution in urban soil, threatening the health of residents. By collecting literature data published in the last 15 years, the characteristics and risks of heavy metals in the urban soils of 52 cities in China were analyzed. The results showed that the average ω(Pb), ω(Cd), ω(Cu) and ω(Zn) in the urban soils of China were 58.5, 0.49, 42.1, and 156.3 mg·kg-1, respectively, and the average Igeo values were ordered as follows Cd(1.10) > Zn(0.36) > Pb(0.28) > Cu(0.13). The high concentrations of heavy metals in the urban soils were mainly found in cities located in coastal economically developed provinces (such as Jiangsu, Zhejiang, etc.) and resource-based provinces (such as Hunan, Henan, Inner Mongolia, etc.). The cities of Kaifeng, Yangzhou, Hohhot, Taiyuan, and Xiangtan had relatively high Igeo values for heavy metals in the soils. The concentrations of heavy metals in soils from industrial areas and roadsides were significantly higher than those from residential areas and parks, suggesting that heavy traffic and developed heavy industry were the main causes of heavy metal accumulation in the urban soils. No significant correlations between the average concentrations of heavy metals in urban soil and urban economic and environmental indicators[such as permanent population, GDP, ρ (PM10), ρ(PM2.5), and SO2 emissions] were found. The concentrations of heavy metals in urban soils showed large spatial heterogeneity, and hence the average concentrations may not reflect the overall accumulation level in a city. The non-carcinogenic risks for children posed by heavy metals in urban soils were generally low, and the main risk contributor was Pb. However, the exposure to heavy metals in soils in cities with developed smelting industries is worthy of attention.

The problem of heavy metal pollution in soil has become a global environmental problem, and it is very important to predict and manage the heavy metals in the environmental soil in a timely manner. The changes in heavy metal content in soil have characteristics such as nonlinearity and large delay, making it difficult to predict heavy metals in soil using traditional methods. Traditional prediction methods are complex and cumbersome, which can lead to longer treatment time and easy secondary pollution. This article analyzed the Back Propagation neural network (BPNN) in artificial neural networks (ANN) and applied it to the prediction of heavy metals in environmental soils. BPNN has good nonlinear function approximation ability, so it can be well applied to complex problems such as soil heavy metal prediction. The methods of treating soil heavy metals include physical repair method, chemical repair method, microbial repair method, plant repair method, plant microbial combined repair method and so on. The use of BPNN can predict heavy metals in environmental soils through adaptive dynamic learning. However, the training time of the BPNN is relatively long and the convergence speed is relatively slow. Therefore, additional momentum terms were added to adjust the weights and thresholds of the network to improve the BPNN. In the experiment, the prediction performance of the improved BPNN was compared before and after the improvement. This article took 50 monitoring data of heavy metals in the same soil in a certain region in 2021 as sample data and predicted the content of heavy metals in the soil using improved and improved BPNN. Due to time constraints, this article only conducted experimental analysis on heavy metals such as lead and cadmium. In the first experiment, when the soil sample data was 50, the prediction accuracy of the BPNN for cadmium before and after improvement was 75.95% and 89.56%, respectively. In the second experiment, when the soil sample data was 50, the prediction accuracy of the BPNN for cadmium before and after improvement was 77.99% and 89.85%, respectively. The improved BPNN has good prediction accuracy and can effectively predict the status of heavy metals in soil. The analysis in this article can provide scientific basis for the comprehensive prevention and control of heavy metals in regional soil, and also provide reference for the development of pollution-free agriculture and ensuring food safety.

Impact of industrial effluent on growth and yield of rice (Oryza sativa L.) in silty clay loam soil

Heavy metal pollution in agricultural soils poses a direct threat to food safety and human health. It has been shown that the colloids is the carrier of heavy metal transport in the polluted soil by heavy metals, but the sources of heavy metals in the soil and colloids and their interrelations are not transparent at present. This study aims to investigate the distribution characteristics of heavy metals in agricultural soils near mining areas, and reveal the relevance of heavy metal content in colloids with total content in soils and their chemical species in soils. Results showed that the concentrations of Mn, Zn, and Pb in agricultural soils and colloids were higher than those of other heavy metals. The content of heavy metals in colloids was positively correlated with the total content of heavy metals in soil. Heavy metals in soil could be easily combined by humus-like substances and tryptophan-like protein in the colloids. The primary source of heavy metals in soil and colloids was mining activities. This study provides theoretical support for revealing the pollution characteristics and migration of heavy metals in agricultural soils and colloids around mining areas.

Taking a city in Guangdong Province as the research area, the concentration and spatial distribution characteristics of heavy metals in the surface soil were studied to clarify the situation of soil heavy metal pollution and priority control factors, providing basic data for the prevention and control of soil heavy metal pollution in the city. The content characteristics of heavy metals in 221 soil samples in the city were analyzed, and the potential health risk assessment and source analysis were carried out through the Monte Carlo model, the potential health risk assessment (HRA) model, and the PMF receptor model. It was found that heavy metals ω(As), ω(Hg), ω(Cd), ω(Pb), ω(Cr), ω(Cu), ω(Ni), and ω(Zn) in the soil of the city were 18.16, 0.43, 1.46, 68.57, 98.34, 64.19, 26.53, and 257.32 mg·kg-1, respectively, with a moderate to high degree of variation. Except for Ni concentration, the soil concentrations of other heavy metal elements exceeded the background values of soil in Guangdong Province to a certain extent, and the concentrations of Cd and Zn exceeded the national secondary standards, resulting in severe heavy metal pollution; the main sources of heavy metals were industrial sources, and natural parent materials, lead battery manufacturing, transportation, artificial cultivation, and pesticide and fertilizer inputs also had an undeniable impact on the accumulation of heavy metals in the soil. Heavy metals in the soil had a certain degree of tolerable carcinogenic health risk for both children and adults, whereas non-carcinogenic risks could be ignored. The potential health risk of children was greater than that of adults, and the main exposure route was through oral intake. The input sources of pesticides and fertilizers and As should be the main controlling factors for the health risks of heavy metals in the city's soil, followed by mixed sources and Cr. There were differences in the spatial distribution characteristics and relative pollution levels of heavy metals, and it is necessary to deepen zoning monitoring and control, strengthen soil pollution prevention and control, and reduce human input of heavy metals in soil.

Accumulation and bioconcentration of heavy metals in two phases from agricultural soil to plants in Usangu agroecosystem-Tanzania

The contamination of soil and plants by heavy metals is one of the crucial issues to be faced in the world including Pakistan and requires attention because heavy metals above their normal ranges are highly threatened to both plants and animals. To monitor this situation, a survey study was conducted to diagnose the concentration of toxic metals and other chemical properties of wastewater irrigated soils. For this purpose, soil samples were collected from various districts of Punjab, Pakistan and analyzed for heavy metals (cadmium, lead, chromium, nickel) and chemical properties such as electric conductivity, pH, organic matter and macronutrients (Nitrogen, Phosphorus, Potassium and Sodium contents). Result showed that pH of wastewater irrigated surface soil was alkaline in nature. Results also revealed that use of wastewater irrigation improved the O.M content (upto 2%) in soils. Macronutrient dynamics also increased specially build up in total N up to 0.12%, Olsen P (45 mg kg -1 ), Extractable K and Na (608, 1525 mg kg -1 respectively) was also observed in soils of wastewater irrigated areas. Furthermore, results also indicated that the concentration of heavy metal, cadmium was ranged from 0.004-3.44 mg kg -1 , lead 0.05-279.3 mg kg -1 , chromium 0.004-683 mg kg -1 and nickel 0.01-221.7 mg kg -1 in wastewater irrigated areas of Punjab. It was observed that, 28, 19, 15 and 5% samples were found above the permissible limits (35, 100, 0.8 and 85 mg kg -1 ) of Ni, Cr, Cd and Pb, respectively. Based on the results, the order of heavy metal contamination in the wastewater irrigated soil of Punjab occurred as Ni>Cr>Cd>Pb. This study concluded that untreated wastewater affect chemical properties of soil as well as degrades soil quality and at the same time it’s continues use ultimately responsible for buildup of heavy metals in soils.

Human activities (land use) and environmental change (land cover change) affect the concentration of Se and heavy metals in soils. The implementation of the "Return Cropland to Forest (RCF)" ecological project has changed the land use and cover, which has provided an ideal experimental area for studying the effects of land use and cover change on selenium (Se) and heavy metals in the soil. In this study, 91 top soil samples from different land use and land cover types, including dry land, paddy land, natural forest land, and secondary forest land, were collected, and the contents of Se, heavy metals, and soil organic matter (SOM) and pH were analyzed. The results showed that:① the average values of ω(Se) (0.42×10-6), ω(As) (13.0×10-6), and ω(Sb) (1.03×10-6) were higher than the soil background values of western Chongqing. ② The concentrations of Se, Cd, Cr, Ni, Pb, and Zn in soils from secondary forest land were significantly higher than those from dry land soils, suggesting that the Se and heavy metals might have significantly increased in the surface soil after the implementation of the RCF ecological project. ③ The SOM was the key controlling factor for the enrichment and distribution of Se and heavy metals in the top soils. Our results indicated that the land use and land cover change would deeply impact the concentrations of Se and heavy metals in the top soils via influencing the soil properties, especially the SOM.

Heavy metals in soil pose a serious threat through their toxic effect on the human food chain. Phytoremediation is a clean and green potentially cost-effective technology in remediating the heavy metal-contaminated soil. However, the efficiency of phytoextraction is very often limited by low phytoavailability of heavy metals in soil, slow growth, and small biomass production of hyper-accumulator plants. To solve these issues, accumulator plant(s) with high biomass production and amendment(s) which can solubilize metals in soil is required for better phytoextraction. A pot experiment was conducted to assess the efficiency of phytoextraction of sunflower, marigold, and spinach as affected by the incorporation of Sesbania (solubilizer) and addition of gypsum (solubilizer) in nickel (Ni)-, lead (Pb)-, and chromium (Cr)-contaminated soil. A fractionation study was conducted to study the bioavailability of the heavy metals in contaminated soil after growing the accumulator plants and as affected by using soil amendments (Sesbania and gypsum). Results showed that marigold was the most efficient among the three accumulator plants in phytoextraction of the heavy metals in the contaminated soil. Both sunflower and marigold were able to reduce the bioavailability of the heavy metals in the post-harvest soil, which was reflected in their (heavy metals) lower concentration in subsequently grown paddy crop (straw). The fractionation study revealed that carbonate and organically bound fractions of the heavy metals control the bioavailability of the heavy metals in the experimental soil. Both Sesbania and gypsum were not effective in solubilizing the heavy metals in the experimental soil. Therefore, the possibility of using Sesbania and gypsum for solubilizing heavy metals in contaminated soil is ruled out.

Spatial distribution and ecological risk assessment of soil heavy metals in a typical volcanic area: Influence of parent materials

Geochemical accumulation and source tracing of heavy metals in arable soils from a black shale catchment, southwestern China

Characteristics, correlations and health risks of PCDD/Fs and heavy metals in surface soil near municipal solid waste incineration plants in Southwest China

Lime bordeaux mixture (LBM) and lime sulfur mixture (LSM) are representative environmental friendly organic materials for prevention of insect pests in South Korea. Recently, those have been widely used as an alternative for chemical pesticides in eco-friendly farms. However, South Korea has not established even recommendation of LBM and LSM considering the stability of heavy metals in soil. The aim of this study was to evaluate the accumulation of hazardous heavy metals in soil and plant with long-term application of LBM and LSM. Firstly, we investigated the amount of LBM and LSM used per year in several eco-friendly farms to determine a standard application rate of both materials. The pepper plant was grown on the pot in greenhouse for 14 weeks. Both materials were applied at 0, 1, 3, and 9 times of standard application rates (2.56 and 1.28 L ha-1 of LBM and LSM per year, respectively). Dry matter yield of pepper and heavy metals (As, Cd, Cu, Hg, Ni, Pb, and Zn) concentration in soil and pepper plant were measured after 14 weeks. Yield of pepper plant did not significantly chang with up to application rate of 1 times, thereafter it markedly decreased with more than 3 times. With increasing LBM and LSM application, the concentration of Cu and Zn in soil significantly increased. Especially, Zn concentration in pepper significantly increased with increasing application rates of both materials. This might resulted in significant decrease in dry matter yield of pepper. The concentrations of those heavy metals in soil did not exceed safety levels (150 mg kg-1 for Cu and 300 mg kg-1 for Zn) established by the Korean Soil Environmental Conservation Act as well as concentration of heavy metals in pepper plant by Korean Ministry of Food and Drug Safety. However, particular attention should be paid for heavy metal safety and crop productivity when using LBM and LSM in the organic farm. Concentration of heavy metals in soil amended with different rate of Bordeaux and Sulphur mixtures at harvest time. †Criteria value: Maximum permissible concentration of heavy metal in soil established by Korean Soil Conservation Act.‡Average value: Average concentration of heavy metals in Korean arable soils.§tr: trace.

In southwest China, vegetation restoration is widely used in karst rocky desertification control projects. This technology can effectively fix the easily lost soil, gradually restore the plant community and improve soil fertility. However, the change law of soil heavy metals in the restoration process remains to be further studied. Therefore, in this work, Guizhou Caohai Nature Reserve as a typical karst area was taken as the research object to investigate the influence of vegetation restoration technology on repairing soil heavy metal pollution. The spatial distribution characteristics of soil heavy metals (chromium, nickel, arsenic, zinc, lead) before and after vegetation restoration in karst area were studied by comparative analysis and linear stepwise regression analysis. The main influencing factors and spatial distribution characteristics of heavy metals in karst area were further discussed. The results showed that: (1) heavy metals in karst soils are affected by surface vegetation, root exudates, microorganisms and leaching. Only heavy metals nickel (Ni) and lead (Pb) showed the tendency of surface enrichment and bottom precipitation enrichment in non-karst soils. Path analysis suggested that non-metallic soil factors such as soil bulk density (BD), total nitrogen (TN) and ammonium nitrogen (NH4 +-N) had direct effect on the content of heavy metals in soil. (2) The proportion of 0.25-2 mm aggregates in the surface soil of vegetation restoration belt was more than 40%, and the proportion of surface soil ≤2 mm aggregates in this increased to 83% and 88%, respectively, which could improve the soil structure and properties effectively. (3) Vegetation restoration effectively restored the nutrient elements such as carbon and nitrogen in the soil, and enhanced the soil material circulation. Furthermore the content of heavy metals in the surface soil higher than that in the 10-20 cm soil layer. Plant absorption, biosorption mechanism of microorganisms, coupling of root exudates, dissolution of soil soluble organic carbon and pH make the contents of heavy metals Cr, Ni and Pb in vegetation restoration belt slightly lower than those in karst soil. At the same time, affected by vegetation coverage, residual heavy metals in soil are further leached by surface runoff. Therefore, the content of heavy metals in soil could reduce combined heavy metal enrichment plants for extraction with remediation. This study elucidates the advantages and remedy mechanism of vegetation restoration in the remediation of heavy metal contaminated soils in Caohai area of Guizhou, and this plant activation and enrichment extraction remediation technology would be popularized and applied in the remediation of heavy metal contaminated soils in other karst areas.