Preparation And Effectiveness Of Rice Husk Ash As A Seawater Silicate Fertilizer

Dissolved inorganic nitrogen?DIN?enrichment accompanied by silicate deficiency in coastal waters can stimulate a shift in phytoplankton community from diatom-to dinoflagellate-dominance.Diatom plays an important role in primary productivity,and is usually considered as the prefered food for most secondary producers.It has been tested in natural environments that additional silicate supply,by natural mechanisms,can stimulate growth of benign diatom species and nitrogen consumption.It is thus expected that artificial silicate addition can work as well in other eutrophicated coastal waters without a naturally formed silicate pump.However,such practices in enormous field environments are limited by the lack of an easily available and slow releasing silicate fertilizer.Inspired by silica accumulation capability of graminaceous plants,we here proposed rice husk ash?RHA?as an approporiate silicate fertilizer.Rice husk,on burning,gives ash containing>90%amorphous silica.Optimal condition of RHA preparation was determined following previous research results.Element contents and macro-nutrients releasing capability were measured for RHAs burned under different calcining temperature.Onboard incubation was carried out to investigate its impacts on phytoplankton community when silicate limitation presented,and silicate release rate and duration of RHA was measured in laboratory.The environmental safety was evaluated by determined the temporal variation of heavy metals in RHA soaked in naturel seawater.RHA obtained at temperature ranging from 400–800°C contain 29.18–44.02%of Si and less than 1%of N and P.When soaked in seawater,RHA formed at these temperatures can release phosphate and silicate,but no DIN.SiO₃^2-/PO₄^3-release ratio increased from 37.77 for 400°C to 112.96 for 800°C,while calcining temperature had no clear influence on silicate releasing rate.Based on the energy cost for burning,we proposed that 500°C was the optimum temperature,RHA obtained at this temperature was applicable to alleviate the simultaneous Si-and P-limitation in coastal area,with SiO₃^2-/PO₄^3-release ratio was 52.26.Commercial RHA products gave similar performance during dark incubation,releasing dissolved silicate and phosphate slowly and sustainably but no DIN into seawater.The releasing rates of silicate and phosphate of RHA were 4.76mmol/g/d and 0.23mmol/g/d,and SiO₃^2-/PO₄^3-release ratio was 24.50.During onboard incubation,diatom abundance increased by 20 times in the first 5 days at high-does addition,accompanied by the exhaustion of DIN.Correspondingly,dinoflagellate showed a trend of decreasing at the beginning and then increased.At low-does addition,diatom abundance was significantly lower than control,but diatom/dinoflagellate ratio increased by 9.5 times as dinoflagellate abundance decreased by 60%at first 5 days.Consumption of nitrogen nutrients was also lower than control.A diatom proliferation was induced mainly by Skeletonema costatum,which numerically dominated through the expriment period in all treatments.Numerical increase was also significant in Cylindrotheca Closterium.However,Chaetoceros curvisetus,the second dominant diatom species,decreased in all treatments.The proliferation of dinoflagellate was induced mainly by Protoperidinium bipes,followed by Protoperidinium nipponicum.Soaked in natural seawater for 21 days,Mg content in RHA increased by 1-2times,but only slight changes were observed in heavy metal contents.Specifically,increase was recorded on Cu,Pb and Hg but decrease was observed on Cd.It is suggested that,even though heavy metal absorptivity was outcompeted by other metal ions existing in large amount,RHA can be used as a seawater silicate fertilizer and unlikely intensify environmental pollution.