In recent years, a large volume of literature has been published regarding the removal of phosphorus (P) from wastewater. Various sorbing materials, such as metal oxides and hydroxides, carbonates and hydroxides of calcium (Ca) and magnesium (Mg), hydrotalcite, activated carbon, anion exchange resins, industrial solid wastes and organic solid wastes, have been suggested for P removal. Many of these sorbents are expensive and/or may cause some environmental problems. In contrast, biochar, as an economical and environmentally friendly sorbing material, has received much attention in recent years and has been used as a novel sorbent for the removal of different organic and inorganic pollutants. Biochar is a type of sustainable carbonaceous material that is produced from the thermal treatment of agricultural organic residues and other organic waste streams under oxygen free conditions. This paper reviews the potential use of biochar and the key controlling factors affecting P removal from wastewater. The ability of biochar to remove P from wastewater depends on its physical and chemical properties. Some of the most important physicochemical properties of biochar (structural characteristics, electrical conductivity (EC), mineral composition, pH, zeta potential, cation exchange capacity (CEC) and anion exchange capacity (AEC)) are affected by the feedstock type as well as temperature of pyrolysis and the P sorption capacity is highly dependent on these properties. The P removal is also affected by the water matrix chemistry, such as the presence of competing ions and bulk pH conditions. Finally, several recommendations for future research have been proposed to facilitate and enhance the environmental efficiency of biochar application.

No abstract

Little is known about the effects of drying–rewetting cycles on phosphorus (P) speciation in paddy soils with organic amendments incorporated. In this study, the responses of microbial biomass P (MBP), alkaline phosphatase activity, inorganic pyrophosphatase activity, soluble P, available P (AP), organic P, total P (TP), organic carbon (C), and C:P ratio to two types of the organic amendment (Azolla compost and rice straw) under drying–rewetting cycles were evaluated in a 60-day incubation experiment. The experimental treatments follow: amendment factor at three levels of (i) unamended control (NT), (ii) soil amended with Azolla compost (AC), and (iii) soil amended with rice straw (RS); and irrigation regimes factor at three levels of (i) constant soil moisture in a waterlogged state, (ii) mild drying–rewetting (MDR) (moisture reduced to 100% water-holding capacity (WHC) at alternate periods), and (iii) severe drying–rewetting (SDR) (moisture reduced to 70% WHC at alternate periods). Soil drying–rewetting increased the contents of AP and soluble P and the activities of alkaline phosphatase and inorganic pyrophosphatase, but decreased contents of organic P and organic C. The contents of AP and TP in organic treated soils were significantly higher than those in non-treated soils (P < 0.01). Application of AC and RS amendments increased organic P content (45% and 46%, respectively) and also further intensified the increase in AP content (63% and 37%, respectively), soluble P content (3.7- and 2.7-fold, respectively), and MBP content (6.0- and 1.6-fold, respectively) compared to control. Rice straw addition combined with MDR also increased the C:P ratio 30 days after incubation. The MBP as a microbial factor had the highest correlation with other measured indices. The MDR along with organic matter (RS and AC) caused a continuous increase in the concentration of P species. In short, the best soil quality in terms of P supply in paddy soils was achieved by regulating soil drying–rewetting cycles along with the use of appropriate timing of organic amendments.

No abstract

No abstract