Study On The Effects Of Reclaiming Substrates For Subsiding Lands And The Phytoremediation Of Heavy Metals

Subsiding wasteland is one of the main degradation lands in China and the main reclaiming method is filled and covered with soils. The lack of covered soil is becoming the main problem in land reclamation. On the other hand, great amount of solid wastes are produced and it lead to some environment problems. So the idea that solid wastes were mixed and used as land reclaiming substrates was put forward. Some substrates that separately mixed with fly ash, furfural residue, sewage sludge and lees were studied. The prophase results showed that the content of heavy metals in the substrates were higher than that of the soils, especially the content of Cd exceeded the standard of Soil Environment Quality StandardsⅡin China. Some measures must be adopted to reduce the content of Cd in the substrates, so the experiments whose purpose was to reduce the content of Cd were made. In the experiments six substrates were successively studied with the potted method and some indexes to evaluate the capability of plants absorbing heavy metal Cd from the substrates were put forward. At the same time the comprehension about hyper-accumulator of heavy metal was advanced.The study had great operation significance in economizing soils, increasing the land area, improving the environment quality around the subsiding lands, utilizing solid wastes as resources. The results can provide new ideas and theoretic evidences to the phytoremediation of heavy metals and the selection of hyperaccumulator. The main results and conclusions were as follows:1) Based on the study of physicochemical properties in the substrates and the effects on the growth of plants, the suitable proportions were 5~20%.2) The substrates had higher porosity, better air permeability and less bulk density than soils. The pH maintained around the range suitable to the growth of plants. The salt content was very high in the early period and after washing by rainwater and irrigational water for a year around, the salt content decrease under 2 g·kg-1. The organic matter and available nutrient content was more than that of soils in the early period and decreased because of the plants absorption and washing of water, but still kept a high-level which could satisfy with the need of plant growth. All the contents of heavy metals in the substrates were higher than soils and only the content of Cd higher than the Soil Environment Quality StandardsⅡ, so some measures must be adopted to reduce the content of heavy metal Cd in order to ensure the substrates to be used in agriculture safely.3)The substrate quality evaluation model was established and the substrates integrated quality was evaluated. The results showed that the substrate quality indexes were accord with the growth of plants.4) Preferable biological effects of planting on substrates had been gotten. The emergence rate of each plant in substrate 1, substrate 2 and substrate 3 was all higher than that of soil comparison plant, and with the increase of planting time, the emergence rate went up. Because of worse physicochemical properties, the plant height was lower than that of soils in germination period, but with the maturing of substrates, the plants growth condition improved gradually. The substrates had not large effect on the chlorophyll content as a whole except ryegrass and fescue were affected a little. For some kinds of vegetables and pasturages, the contents of some heavy metals surpass the relevant quality standards, so when some crops were planted on the substrates, the uneatable species should be selected preferably and if some kinds of vegetables and pasturages must be adopted, the output could not be provided to mankind and also not be used in farming, in order to avoid harming peoples'health.5) The expediency evaluation model of pioneer plant on substrates was established and some plants were studied. The results show that the expediency evaluation of pioneer plants were respectively that clover was expedient to substrate 1, juniper, cotton and fescue were expedient to substrate 2 and juniper, annual bluegrass, fescue, cotton and clover were expedient to substrate 3.6) Some indexes to evaluate the capability of the phytoremediation on heavy metals and the comprehension about hyperaccumulator were put forward. Started from the practice effects of the phytoremediation on heavy metals, a few new indexes such as absorption modulus, remediation age, relative remediation age and relative remediation indexes et al were advanced. In order to weigh the capability of plants absorbing heavy metals from the substrates, the evaluation system whose core was relative remediation age and the evaluation standard were established. Based on the evaluation system, the plants whose relative remediation age was between 0~10 years were named as"Hyperaccumulator"and those between 10 ~ 50 years were named as"Accumulator".7) In the experiments six kinds of substrates were mixed with different solid wastes and the absorption and enrichment of plants on heavy metal Cd was studied. The results showed that relative remediation age was an ideal index to evaluate the capability of the phytoremediation on heavy metals, because it took into account more factors which affected the absorption evaluation effects than others, such as the heavy metal content and the biomass of the over-ground part of plants, the heavy metal content and the whole gross in substrates, harvesting frequency of plants and the environment standard of heavy metal. This made relative remediation age be suitable to the sorption capability evaluation of not only different plants on one heavy metal but also one plant on different heavy metals.8) It can be drawn from the relative remediation age of heavy metal Cd that the absorption capability of different plants was not same. Pakchoi and rape were the ideal plants to all substrates and ryegrass, fescue and clover were not ideal. In addition, for substrate 1, substrate 2 and substrate 3 that utilized for some years, pakchoi, rape and cabbage were suitable to substrate 3 mixed with fly ash and lees and ryegrass and fescue were suitable to substrate 1 mixed with fly ash and furfural residue. For substrate I, substrate II and substrate III that utilized in the first year, because of the differences of their physicochemical properties and chemical constituents, the suitability of the plants didn't appear accordant regularity.9)According to the relative remediation age evaluation criterions of hyperaccumulators ( 0~10 years) and accumulators( 10~50 years), pakchoi, rape and cabbage were ideal accumulators of heavy metal Cd for substrate 1, substrate 2 and substrate 3, but for substrate I, substrate II and substrate III, only pakchoi had this character completely. So the character of"hyperaccumulator"or"accumulator"was relative.10)If considering the dual functions of plant absorption and heavy metal eluviations, under the condition of growing some suitable plants, the heavy metal Cd content of substrates would drop down to the Soil Environment Quality Standards II in 10~15 years and the heavy metal Cd content of root layers could satisfy the needs of the common agricultural production.