Phosphorus recovery through adsorption using iron modified cellulose
Silva Simplicio, Walquiria
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Phosphorus is an important non – renewable resource, widely used as fertiliser to maintain high crop yields. As the global phosphate reserves will be exhausted in 40 – 150 years, there is a need to explore alternatives to phosphate ores. When phosphorus reaches the waterbodies it can result in eutrophication, causing loss of water quality and biodiversity. This study investigated the feasibility of recovering phosphorus from agriculture runoff by adsorption with an adsorbent developed from agricultural by-products. Adsorbents were prepared from cellulose extracted from grass silage and its adsorption capacity was significantly improved using the metal loading method. The resultant adsorbent was characterised systematically and cellulose was shown to be the backbone of the adsorbent. Brunauer – Emmet – Teller analysis showed an increase in the surface area between grass and iron modified cellulose from 18.3 to 41.8 m2/g, which is favourable to the adsorption process. Adsorption capacity of iron modified cellulose was further tested in batch and column experiments. The results indicated that iron-modified cellulose presented a maximum adsorption capacity of up to 20.1 mg P/g adsorbent. The isotherm data were best fitted by the Langmuir model, while kinetic data were satisfactorily described by the pseudo-first-order model. Thermodynamic results revealed that the adsorption was feasible, spontaneous, and endothermic. The solution pH did not affect PO43- uptake in a wide pH the range of 3 – 10. Iron-modified cellulose demonstrated a high affinity for phosphate, which could be observed by a minor decrease in PO43- adsorption when Cl−, NO3-, CO32− and SO42− were tested as competitors for adsorption sites. About 50% desorption efficiency was achieved with a 2.0 M NaOH solution. The column results showed that the highest dynamic adsorption capacity of iron-modified cellulose was 14.6 mg P/g adsorbent. Yoon-Nelson and bed depth service time (BDST) models were most suitable for the description of the column adsorption behaviours. Cost related investigations revealed that iron modified cellulose was produced cheaper than other lab-scale adsorbents made from agricultural waste by-products. However, it is still more expensive than commercially available adsorbents. The iron modified adsorbent can achieve the additional environmental benefits of the use of renewable materials and waste valorisation. Tests of the bioavailability of phosphorus in iron modified cellulose showed that up to 46.7% of adsorbed phosphorus would be bioavailable to crops. The environmental impacts of iron modified cellulose production were assessed with a life cycle assessment perspective. Electricity input was found to be the largest input to the environmental impacts, and among the chemicals used, ferric chloride and sodium chlorite were found to be the most harmful to the environment. Iron-modified cellulose could be used in the land as a barrier to prevent phosphorus from reaching adjacent water bodies, and after could be applied to land, as a slow-release fertiliser. This multipurpose characteristic of the adsorbent is believed to be a major contribution of this research to the field of phosphorus recovery.