Experimental and Modeling Column Study of Phosphorus Removal by Permeable Reactive Materials
Abstract
This study evaluates the performance of permeable reactive mate rials for phosphorus removal from water by experimental and model development. A one dimensional solute transport model that describes adsorption process in porous media by mass transfer equation and surface area reduction was developed. Validity of the model was evaluated using several data sets from batch and column experiments. The marble dust, standard sand and volcanic ash were utilized as permeable reactive barriers and porous materials inside packed columns in this research. It was found that the calcium (Ca) content was the most important characteristic of the permeable reactive materials and a factor determining their phosphorus removal efficiency. A high Ca content material showed higher removal capacity of phosphorus. The results of this study demonstrate d that the marble dust sorbent has a high efficiency to remove phosphorus from aqueous solution. Comparing the performances of three packed columns fill ed up with different combinations of the three investigated materials, the differences in permeability played an important role in the treatment residence time and its ensuing effect on the removal efficiencies of phosphorus from water . A combination of 70% marble dust and 30% volcanic ash (as porous packed layers in one column) made a reasonable compromise between high steady phosphorus removal efficiency (~ 80%) and longevity (over 180 days ). A suggestion /recommendation in conclusion was proposed based on these results.
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Introduction
Phosphorus (P) is one of the most essential nutrients of non-renewable resource that can be found in wastewater or surface water. Its presence in water (in forms of orthophosphates, polyphosphates and organic phosphates [1, 2]) originates from different sources such as fertilizers and animal feed run-off (from agriculture wastewater [3]), detergents (from industrial wastewater [4]), and sewage water [5]. Up to now, the main source of phosphorus is mostly obtained from mined rock phosphate. The available rock phosphate reserves are expected to finish within 100 years [6]. However, the estimated amount of phosphorus contained in wastewater and sewage systems corresponds to about 40–50% of phosphorus ore [7]. At the same time the release of phosphorus from wastewater, agricultural areas runoff and landfill leachate into resource water reserves constitutes the main risk for reduced water quality. This excessive release (at phosphate concentrations > 0.1 mg/L [8]) is known to cause environmental problems such as massive algae bloom and eutrophication, which are harmful for the aquatic life and lead to the instability of ecosystem.
For the aforementioned problems, many and different techniques were evolved to remove and recover phosphorus from water bodies. Technologies such as biological phosphorus removal process [9, 10], constructed wetlands, crystallization [11], chemical precipitation and adsorption [12, 13], presented in vast studies [14, 15], offered significant treatment of phosphorus-contaminated water. Because of mud problem of precipitation [16], inefficiency and high cost of reverse osmosis [17] and drop in performance (at low P concentrations) of biological methods, the adsorption technique stands as a prominent and promising method among all other methods. The importance of this method is more clear in every research after research, starting from using natural reactive materials to improve the phosphorus removal efficiency by adsorption, including: limestone, shale, slag, iron rich gravel, zeolite, calcite and other artificial materials [18], then recently bimetallic nanoscale zerovalent iron adsorbed phosphorus with high kinetic rates and adsorption capacity [19], and zerovalent iron was used in column studies supported on sand bed for this purpose [20]. The phosphorus adsorption is controlled by the value of pH and the respective adsorptive surface area [21]. Larger surface area of small particle size can increase phosphorus adsorption rate and capacity. However, small particle size presents usually low permeability, which rapidly leads to the clogging of porous media.
In order to reduce the negative effects of overloading the ecosystems with phosphorus as well as recycling of phosphorus and reducing the high costs of mining and processing of phosphorus, it is necessary to investigate various techniques and materials that could contribute to the removal and recycling of phosphorus. Adam et al. [22] reported that the natural reactive materials of shellsand and filtralite were studied for their adsorption capacity to adsorb phosphorus by conducting batch and column experiments and found that the phosphorus removal rate of 92 % and 91 % was measured in the columns of shellsand and filtralite, respectively. It was also reported that zeolite and pelleted clay, used as natural reactive materials either alone or in combination with soils, alum, calcite and dolomite, was found to improve the phosphorus adsorption capacity [23]. As far as the authors know, materials such as volcanic ash in combination with marble dust were not been used by other researchers in column studies to treat phosphorus.
This study focuses on using a combination of methods, adsorption and precipitation, via constructing different configurations of volcanic ash, marble and sand layers in vertical packed column with clear objectives which are: (i) to evaluate the performance of a mixture of soils and reactive materials used in these vertical columns for removal of phosphorus; and (ii) to gain a phosphorus rich product, which can be recycled by the phosphorus industry and/or may directly be used as a fertilizer. Based on these results, optimum configuration can be selected for an efficient treatment of phosphorus-contaminated water.
Conclusion
This work investigated the adsorption of phosphorus by a mixture of soils with different reactive materials. For this purpose, column experiments were conducted to evaluate the performance of phosphorus adsorption as well as solute transport model was developed to perform column experiments and predict long-term performance of reactive materials for phosphorus removal. The efficiency of phosphorus removal was found affected mainly by the Ca content of reactive materials, a high Ca content material obtained higher adsorption capacity of phosphorus. The present study on the removal of phosphorus recommends and suggests that the marble dust 50% as Ca source mixed with volcanic ash 50% can be used as novel adsorbent for phosphorus removal from aqueous solution, which can maintain reasonable longevity and high steady removal performance. Furthermore, the marble dust as a waste of marble mining areas and marble industry production is not only economic adsorbent that can be used for phosphorus removal, but also at the same time their utilization solves the problem of waste residues from marble mining and industry processes.
